Tag: compressor

In order to wrap my head around diagnostic issues it really helps me to engage in thought experiments where I think of more extreme examples of an issue or situation or consider the ideal in order to find the “edges” of a concept. Once I find the edges of the extreme then I can begin to sort down to a more exact conclusion.

So let’s consider compressors and mass flow

First, don’t get overwhelmed by the phrase “mass flow” I’m not going to start in with confusing words and fancy math. As techs we rarely need to do advanced calculations anyway, it’s more about understanding relationships between factors or IFTTT (If this then that). If this thing occurs or I change this what happens to that.

Mass flow just means how much fluid is moving over a a given amount of time. The “stuff” in this case is refrigerant and the mass measurement is generally lbs in the USA and the rate could be minutes, seconds or hours.

Our goal with a compressor is to move as many lbs of refrigerant, as quickly as possibly with the minimal amount of watts in energy used in order to move the greatest # of btus/hr we can … pretty straight forward so far.

The typical single speed, single stage compressor with no unloading capability runs ESSENTIALLY the same speed and with the same volume in the compression chamber (cylinder, scroll etc…) this means that a traditional compressor has a fairly constant volume in the compression chamber and rate of compression. I say “fairly” constant because as the compressor moves greater mass or works against greater pressure the motor will tend to slip more resulting in a slower rotational speed of the rotor.

So let’s imagine an old single speed, single stage reciprocating compressor with no unloading. It’s compressing refrigerant with a constant volume in the cylinder that goes from its largest cylinder capacity at the bottom of the down stroke (suction stroke) to its minimum capacity at the top of the up stroke (discharge stroke). This variation in volume in the cylinder as the pistons actively move and down is what creates the pressure differential between the high side and the low side and this pressure difference is what allows the refrigerant to move through the circuit.

So you may think to yourself (as I have in the past)

“If pressure differential is what causes the refrigerant to move then don’t we want a big pressure difference between the compressor discharge and suction so that more refrigerant will move?”

The answer is absolutely NO

We actually want the minimum pressure differential we can get away with while still accomplishing the task of maintaining an evaporator (or evaporators in the case of multi-circuit systems) at the desired temperature and (nearly) full of boiling refrigerant.

The reason we want lower pressure differential has to do with mass flow rate, if the compressor has a fixed volume in the cylinders and the pistons are pumping away at the same speed then that part of the equation is fairly fixed. The only way to increase the amount of refrigerant being moved by the compressor is to

#1 – Increase the density of the refrigerant

#2 – Reduce the amount or re-expansion waste in the cylinder

#3 – Reduce the pressure to overcome in the discharge

The first part of that equation is simple, when suction gas is higher pressure it is also higher density, when the suction pressure entering the compressor drops the density also drops. When then density of the refrigerant drops entering the compressor the compressor moves less refrigerant because there is just less there for it move.

Think of this like an old PSC blower motor on undersized duct work. When the static pressure on the return increases the amount of air being moved decreases because the density of the air is decreased. The blower is still spinning the same speed (on a PSC), heck, it may even be spinning faster due to the motor experiencing less resistance, but the airflow decreases. This happens not becasue the motor is doing anything different, it moves less air mass becasue the air is less dense entering the blower and therefore you are moving less air.

When you drop the suction pressure entering a typical compressor you drop the mass flow rate becasue the mass entering the compressor is reduced, lower mass flow rate means moving fewer lbs of refrigerant which (by itself) means lower capacity.

Now let’s move to the second part which is re-expansion and this one applies more to reciprocating compressors where there is a clear compression and expansion stroke vs. a scroll , rotary or screw where the compression is essentially a continuous cycle.

Imagine a compressor sitting in a room with no tubing connected just pumping air. The compressor would be pulling from 14.7 PSIA and discharging into 14.7 PSIA (atmospheric air pressure at sea level). When the piston draws down it would pull in air and fill up and then as the piston pushes up it would start to discharge air out of the cylinder really quickly in the up stroke because the only thing pushing against the discharge is 14.7 PSIA and therefore the highest pressure that would build up inside the compressor is slightly more than 14.7 for it to overcome the pressure of the discharge valve and push out into the air.

If that same compressor were pumped into a chamber where the pressure built up to 200 PSIA what would change?

The compressor would move less air even if  the suction was still left open to atmosphere (and therefore the same air density) because now the discharge valves wouldn’t open until the pressure in the cylinder went above 200 psi meaning that the effective stroke would be reduced due to the pressure being pushed against (#3 on the list above). It would also need to pull down further to re-expand the gas left over in the cylinder to below 14.7 PSIA for more air to enter the cylinder again.

In a scroll, rotary or screw there isn’t valves and cylinders in the same way but the amount of refrigerant being moved is still impacted by changes in suction density (suction pressure) and the pressure exiting the compressor… in other words the COMPRESSION RATIO.

Have you ever noticed that the BTU ratings on compressors have dropped over the last 10 years as units become more efficient? Where a 3-ton unit may have previously had a compressor with a 36 in nomenclature for a nominal three tons you may now find it has closer to 30 or even less.

You may also notice that high efficiency systems often have larger condensing coils and larger evaporators which bring the head pressure and therefore the condensing temperature closer to the outdoor temp and the evaporators are also running a higher temperature bringing up the suction pressure. Manufacturers are increasing how much refrigerant the compressor can move (mass flow rate) by bringing the design head pressure down and the design suction suction pressure up. They can then afford to downsize the compressor achieving the same capacity with less input watts also known as greater energy efficiency.

Let’s give some real world examples of altering mass flow rate by impacting these factors in the field –

  1. Dirty Condenser Coil – Decreases mass flow rate and system capacity because the head pressure and compression ratio go up
  2. Low Indoor Airflow – Decreases mass flow rate because refrigerant density goes down entering the compressor and compression ratio goes up (to a degree). Keep in mind that when there is low air flow or low load head pressure will also tend to drop as the mass flow rate drops. It is held up by the outdoor temperature as a limitation on how low the condensing temperature will drop however.
  3. Overcharge – The impact of overcharge on mass flow rate will vary depending on the metering device and how overcharged the system is. On a TXV / EEV system it will always result in lower mass flow becasue the head pressure will increase. On a fixed orifice it may result in a slight increase in mass  flow initially as suction pressure increases.
  4. High Indoor (Evaporator) Load – Increases mass flow unless there is some control preventing it from doing so like a CPR (compressor pressure regulator). Increased heat entering the evaporator will increase the pressure and density of the refrigerant returning to the compressor, this will increase the mass flow rate, system capacity and head pressure if all else remains the same.

What happens if we change compressor capacity on the fly?

For years in residential and light commercial we’ve been used to fairly fixed compressor volume flow rates but nowadays we see many different types of multi-stage and variable capacity technologies from a simple dual capacity unloading scroll to a digital scroll all the way to variable frequency, variable speed scroll compressors. These compressors have their “rated” capacity which is the state at which they are tested for bench-marking against other units. They can then reduce their capacity below their rating and some can every produce a higher capacity then their rating.

In all of these cases the compressor is altering the amount of refrigerant it is moving by making a change within the compressor itself resulting in lower mass flow when the compressor stages or ramps down and higher mass flow when it ramps up.

Lets imagine a theoretical 4-ton rated unit with a compressor that can ramp down to 2-tons or it ramp up to 5-tons.

What that means in practice is that the compressor is capable of moving an amount refrigerant consistent with two tons of capacity up to a mass flow that can produce 5-tons of capacity at the same rated conditions.

So here is what you would see change when that compressor changes mass flow in comparison to rated capacity if everything else remained the same –

Low Stage (2-ton)

High Suction Pressure

Low Head Pressure

Low Subcool

High Superheat (potentially)

Low Evaporator Delta T

Poor Dehumidification due to high coil temperature

Low compressor amps

Low Compression Ratio

Low Discharge Temperature

Low Approach (liquid line temperature above outdoor temperature)

High Efficiency (EER / SEER)

 

High Stage (5-ton)

Low Suction Pressure

High Head Pressure

High Subcool

Low Superheat (potentially)

High Evaporator Delta T

Strong Dehumidification due to lower coil temperature

High compressor amps

High Compression Ratio

High Discharge Temperature

High Approach (liquid line temperature above outdoor temperature)

Low Efficiency (EER/SEER)

 

Now think about how a system responds when the compressor isn’t pumping properly. It is almost exactly the same as the low stage / low mass flow example listed above with the exception of the efficiency.  When we have lower mass flow than rated these are symptoms we will see whether it is by design or due to a failure.

In practice these variable capacity systems will often be matched with a variable speed blower and a wide range TXV or EEV so that the coil temperature and feeding can adjust with the change in mass flow to help mitigate some of the negative effects of staging down.

There are come interesting things that can be done with modern controls and variable mass flow compressors. One example is Bosch branded condensing units that vary the compressor mass flow to set a fixed evaporator temperature, effectively adjusting the capacity to match the load on the evaporator coil. Another is Carrier Greenspeed heat pumps that ramp the compressors up during heat mode to drive up the pressure on both coils to increase the heat produced inside and reduce defrost requirements.

— Bryan

 


I walked into my first real job interview in the HVAC trade. The manager was a guy named Ernie and he walked me out to the warehouse.

Quick warning.. guys named Ernie are tough. Don’t mess with a dude named Ernie.

He walked up to a box, snatched a pen out of his shirt pocket and scribbled a circle, 3 dots, and three numbers on it while grunting “which is common, start and run”

I was in luck…

While I may have had almost zero practical knowledge of air conditioning, this was one thing I HAD actually learned in school.

I marked the terminals and I got the job.

Now, of course, this only applies to single-phase compressors and this leg to leg reading is helpful for identifying terminals but tells you very little about the condition of the windings unless you know the resistance in the first place or have historical readings or another identical compressor to compare to.

Before you say that this information is useless let me stop you.

It isn’t useless. It may not be something you use every day, but I have needed to ohm out a motor or compressor a handful of times and it got me out of a pinch.

So here it goes –

The lowest ohm reading is between Common and Run

The middle ohm reading is between Common and Start

The highest ohm reading is between Start and Run

Common is just a point between Start and Run and therefore the Common to Start and Run to Start readings will add up to the run to start reading. Many will tout this as a diagnostic reading you should check. it’s more a mathematical fact than something useful to check. If you did see a higher reading Start to Common + Run to Common vs. Run to Start it could really only indicate an increased resistance through the motor thermal overload that breaks common.

Here is how I remember which winding resistance is which (let the mockery begin)

Starting is hard… so it has the highest resistance

Running is hard also… but not as hard as starting, so it has a resistance less than Start.

Common is easy… being common requires the lowest resistance

So common to run is the least and start to run is the most.

Also…

The orientation when read like a book (top left to bottom right) is usually… if not always Common, Start then Run. Many techs remember that with the phrase “Can She Run”.

Understanding common, run and start is uncommon… so it requires a lot of resistance… so start… knowing it

OK, I’m done.

— Bryan

When I was younger I used to play and watch golf quite often. My Father enjoys golf and he would take me with him on many of his Saturday morning rounds

As a kid it was all about DISTANCE, how far I could whack the ball with a driver and ESPECIALLY when I could outdrive my friends.

When I was 15 I watched a young Tiger Woods win the Masters by 12 strokes while hitting the ball farther than anyone had ever seen and it made golf even more appealing to my teenage brain.

At 16 I even took a part time job at a local golf course, picking the range and washing the carts in exchange for minimum wage and FREE GOLF.

If you know anything about golf you know it can be mentally and emotionally infuriating it can be. The more I practiced and played the further I learned to hit the ball…. and the WORSE I SEEMED TO GET.

I never shot a good a score at the golf course where I worked, NEVER! Every time I would get frustrated and just swung harder and harder to compensate.

This proved two things to my teenage mind

#1 Golf is stupid

#2 I was no Tiger Woods

Now as an adult I almost never play, I could still outscore my teenage self and I understand something that I never did then regardless of all my practice.

You just need to hit your spots

Nowadays, both Tiger and I are balding and have been kicked in the teeth by life enough to know that you “drive for show and putt for dough” as my Dad used to say.

I saw a documentary recently where Tiger was working with a new golf club manaufacturer and he was talking about how his focus now is never optimizing distance but rather to optimize consistency and repeatability.

It doesn’t matter as much if that 9-iron carries 160 or 140 yards, it matters much more if it gives the same repeatable result time and time again with the same swing.

The HVAC business is like that as well

My son came home the other day and proudly mentioned “Sam and I did a compressor today and it only took 3 hrs”

I remember a time when beating my previous time on a compressor or a changeout was a big deal to me as well and there is nothing wrong with that.

However…

Those of us who have done this a while know that the really important part is less about if a compressor can be installed in 3 or 5 hours and more about;

  • Getting your diagnosis right
  • Reconfirming the diagnosis of others
  • Confirming it’s the correct part
  • Looking for and correcting the underlying causes of failure
  • Doing the job properly
  • Lifting and working in a way that prevents injury
  • Testing everything completely once the system is running
  • Communicating well with the customer so that they are thrilled with the service and feel they got an excellent value
  • Complete the paperwork
  • Returning everything to the work van in an organized fashion
  • Dumping your garbage and tagging turning in any warranty parts or cores

This is one example of many tasks a tech needs to accomplish well in a work day that may not have any flash and may result in the job taking a bit longer.

An experienced tech knows, you don’t just make your money by working fast and collecting checks. You keep your money by eliminating costly mistakes that result in

  • Callbacks
  • Warranty calls
  • Upset customers
  • Failed inspections
  • Driving complaints
  • Injury
  • Conflicts with other employees

All of these mistakes happen when you are rushing around focused on “driving the ball long” rather than “hitting your spots”

We work in a precision, cognitive trade where understanding and detail are at a premium over brute force.

We are like golfers or marksmen where precision, focus and patience are at a premium rather than brute strength or natural talent.

We can learn to discipline ourselves for better results over time by elliminating small mistakes and getting more and more consistent in hitting our spots.

Like golf, we will NEVER be perfect but our results will continue to improve the more we focus on a sound strategy rather than “gripping it and ripping it”.

The MINDSET of hitting our spots really matters to our results and supporting ourselves with tools and processes to help prevent gaps and mistakes can make all the difference.

So…

Drop the ego, calm down, focus and hit your spots to maximize your results and earning potential.

— Bryan

PS – I will be in the SanFrancisco Bay Area on 7/17 and maybe 7/18. I would be interested in setting up a training class or lunch and learn and possibly doing a meetup dinner that evening. If there are any wholesalers, contractors, reps, manufacturers willing to host, sponsor or help organize something please reply to this email and let me know. Thanks!

This article is written by regular contributor, experienced rack refrigeration tech and RSES CM Jeremy Smith. Thanks Jeremy. Also… There is a podcast out about what kills compressors HERE


A technique that you can use to diagnose compressor problems and to help differentiate them from other possible issues is the use of compressor performance analysis.

Manufacturers do extensive testing of their compressors before they sell them, and a part of that testing is available to you as a troubleshooting tool. The compressor performance chart. I’ll primarily refer to Copeland compressors as they are what I service most, but I’ve been able to find charts and data from other manufacturers through their websites and tech support lines.

Let’s look at a real-world example. I went to do a follow-up check after a major leak and recharge on a set of freezers. On arrival, the cases, which had been running now for 14 or 15 hours since having been repaired, weren’t as cold as expected. Checking the unit, here is what I found:

Copeland compressor
2DA3-060L-TFC
R404A
27# suction
185# discharge
209v (average of all 3 legs)
13.9A draw.
Unit at 18-20°F

The suction line was cool to the touch and the sight glass had a thin ‘river’ of refrigerant in it. The high suction pressure really jumped out at me here as worthy of more consideration.

Now, a high suction pressure in this instance can be caused by high load (note the high unit temperature) or it can be caused by a compressor problem. Looking over the data here, I was concerned about the health of the compressor and its ability to pump properly. I did a quick “pump down test” and found it inconclusive. The compressor pulled to 24” Hg easily and held there. Still, I wasn’t happy with this, so I pulled out my smartphone and opened the Copeland Mobile app.

A quick note on pump down ‘tests’. They really aren’t effective on most modern compressors. I performed the test and included the results here to illustrate exactly that fact.

Entering the model of the compressor leads you to select the application (R502 low temp which is closest to R404a low temp). Selecting the “Diagnostics” tab brings you to a screen where you and input pertinent data and the app then outputs both the expected amperage at your conditions and the percent deviation
from the norm.

In this case, my expected amperage was significantly higher than my observed amperage, so the high suction was definitely caused by a compressor problem.
I recovered the refrigerant from the machine and removed the compressor head and valve plate for internal evaluation.

Finding a single broken suction reed, the rest of the internals were intact making this a good candidate for a new valve plate. I Installed new valve plates, evacuated and restarted the machine and re-evaluated operation.

Had this been a hermetically sealed compressor, I would have had no choice but to condemn and replace the compressor. This time, the amperage was within 5% of specifications (sorry, didn’t get a screenshot) and I continued to monitor unit operation until equipment reached 0° F, verified and completed proper
charging of the unit and called it a day.

Why not use RLA (Rated Load Amps) (? Or use LRA÷6 (Or is it 8?) to diagnose?

The simple answer is that they just aren’t sufficiently accurate enough for me dealing with high stakes, high dollar equipment and they shouldn’t be accurate enough for you, either.

Let’s return to my real-world example…

The compressor has a listed RLA of 25.8 and an LRA (Locked Rotor Amps) of 161.0. Now look back at the original screenshot of the app. It calls for an amp draw of 17.09A at that set of conditions. If we compared that to the RLA, even the correct amperage looks low. If we use common LRA divisors 161 ÷ 6 gives us 26.83A and 161 ÷ 8 gives us 20.125A. Maybe a little better than the RLA method but still off by a significant amount. Enough to cause concern and possibly lead to an incorrect diagnosis.

Not one of these methods gives us an accurate expected amperage for this machine. That inaccuracy can lead us to draw a bad conclusion and potentially wasting time and money pursuing a “bad” compressor that is in fact, working exactly as it should.

Like most things in HVAC/R using a fixed operational target without considering the specific conditions can lead to misdiagnosis and a lot of wasted time. You would be surprised what is available within manufacturer specs if you take the time look.

— Jeremy Smith, CM


This is a basic overview of the refrigeration circuit and how it works. It isn’t a COMPLETE description by any means, but it is designed to assist a new technician or HVAC/R apprentice in understanding the fundamentals.

First, let’s address some areas of possible confusion 

  1. The Word “Condenser” Can Mean two Different Things Many in the industry will refer to the outside unit on a split air conditioner, heat pump or refrigeration unit as a “condenser” even though it will often contain the condenser, compressor, and other parts. It’s better to call the outside component the “condensing unit” or simply the “outside unit” to reduce confusion.
  2. Cold and Hot are Relative terms Cold and Hot are both an experience, a description, a comparison or an emotion. Cold is a way to describe the absence of heat in the same way that dark describes the absence of light. We will often use the words cold and hot to compare two things “Today is colder than yesterday” or to communicate comfort “It feels hot in here”. These are useful communication tools, but they are comparisons not measurements.
  3. Heat and Absolute Zero Can be Measured We can measure heat in BTUs and light in lumens, we cannot measure cold or dark. Absolute cold is the absence of all heat.  -460°F(-273.3°C) (cold) is known as absolute zero, -460°F(-273.3°C) is the temperature at which all molecular movement stops. Any temperature above that has a measurable level of heat. While this is a known point at which all molecular movement stops, it has not (and likely cannot) be achieved.
  4. Boiling Isn’t Always Hot When we say it’s “boiling outside” we mean it’s hot outside. This is because when we think of boiling we immediately think of water boiling in a pot at 212°F (100°C) at atmospheric pressure, which is 14.7 PSI (Pounds Per Square Inch)(1.01 bar) at sea level. Boiling is actually just a change of state from liquid to vapor, and the temperature that occurs varies greatly based on the substance being boiled and the pressure around the substance. In an air conditioner or a refrigeration system, refrigerant is designed to boil at a low temperature that corresponds to the design of the system. On an average air conditioning system running under normal conditions with a 75°(23.88°C) indoor temperature, the evaporator coil will contain refrigerant boiling at around 40°F(4.44°C). In air conditioning and refrigeration when we refer to “boiling”, “flashing” or “evaporating of refrigerant” we are talking about the process of absorbing heat, otherwise known as cooling.
  5. Cooling and Heating Cannot be “Created” We are not in the business of making heat or creating cool; it cannot be done. We simply move heat from one place to another or change it from one form to another. When we “cool” a room with an air conditioner, we are simply absorbing heat from the air into an evaporator and then moving that heat outside to the condenser where it is “rejected” or moved to the outdoors.
  6. Heat and Temperature Aren’t the Same  Imagine a shot glass of water boiling away at 212°F(100°C). Now imagine an entire lake sitting at 50°F(10°C). Which has a higher (hotter) temperature? That answer is obvious-I just told you the shot glass had 212°F(100°C water in it so it is CLEARLY hotter. But, which contains more heat?  The answer is the lake. You see, heat is simply energy and energy at its basic form is movement. When we measure heat we are measuring molecular movement; the movement of molecules–atoms stuck together to make water or oxygen or nitrogen. When molecules move FASTER they have a HIGHER temperature and when they move SLOWER they have a LOWER temperature. Temperature is the average speed (velocity) of molecules in a substance, while heat is the total amount of molecular movement in a substance. The lake has more heat because the lake has more water (molecules).
  7. Compressing Something Makes it Get Hotter (Rise in Temperature) When you take something and put pressure on it, it will begin to get hotter. As you pack those molecules that make up whatever you are compressing, they get closer together and they start moving faster. If you drop the pressure the molecules will have more space and will move slower causing the temperature to go down.
  8. Changing the State of Matter Moves Heat Without Changing Temperature  When you boil pure water at atmospheric pressure it will always boil at  212°F(100°C). You can add more heat by turning up the burner, but as long as it is changing state (boiling), it will stay at 212°F(100°C). The energy is changing the water from liquid (water) to vapor (steam) and the temperature remains the same. This pressure and temperature at which a substance changes state instead of changing temperature is called its “boiling point”, “condensing temperature” or more generally “saturation” point.
  9. Superheat, Subcool, Boiling, and Saturation Aren’t Complicated  If water is boiling at sea level it will be 212°F(100°C). If water is 211°F(99.44°C) at sea level we know it is fully liquid and it is 1°F(-17.22°C) subcooled. If water is 213°F(100.55°C) at sea level we know it is vapor and superheated. If something is fully liquid it will be subcooled, if it is fully vapor it will superheated, and if it is in the process of change (boiling or condensing) it is at saturation.

Where to start 

Take a look at the diagram at the top of this piece and start at the bottom left. Are you looking at the part at the bottom left? OK, now read this next line OUT LOUD:

Compressor > Discharge line > Condenser > Liquid Line > Metering Device > Expansion Line > Evaporator > Suction line and then back to the Compressor

When I first started in HVAC/R trade school this was the first thing my instructor forced me to LITERALLY memorize forward and backwards before he would allow me to proceed.

While I am not always a huge fan of rote memorization as a learning technique, in this case, I agree with committing this to memory in the proper order.

These four refrigerant components and four lines listed above make up the basic circuit that every compression refrigeration system follows. Many more parts and controls may be added, but these basics are the cornerstone on which everything else you will learn is based. Once you have these memorized we can move on to describing each.

Compressor

The compressor is the heart of the refrigerant circuit. It is the only mechanical component in a basic refrigeration system. The compressor is like the heart that pumps the blood in the body or like the sun that provides the earth its energy. Without the compressor to move the refrigerant through compression, no work would be done and no heat would be moved.

The compressor creates a pressure differential, resulting in high pressure on the high side (discharge line, condenser & liquid line) and low pressure on the low side (suction line, evaporator and expansion line).

There are many different types of compressors, but you will most likely see Scroll and Reciprocating type compressors most often. A reciprocating type compressor uses pistons, valves, and a crankshaft. Reciprocating compressors operate much like car engines; pulling in suction vapor on the down-stroke and compressing that vapor on the up-stroke. A scroll compressor does not have any up-down motion like a reciprocating compressor. A scroll compressor uses an oscillating motion to compress the low-pressure vapor into high-pressure vapor.

The compressor pressurizes low-pressure vapor into high-pressure vapor, but it also causes the temperature of the gas to increase. As stated in the gas laws, an increase in pressure causes an increase in temperature and a decrease in volume. In the case of refrigerant cooled compressors, heat is also added to the refrigerant off of the kinetic (bearings, valves, pistons) and electrical (motor windings) mechanisms of the compressor. Compressors require lubrication; this is accomplished through oil that is in the compressor crankcase, as well as oil that is carried with the refrigerant. Liquid entering the compressor through the suction line is a very serious problem. It can cause liquid slugging, which is liquid refrigerant entering the compression portion of the compressor. Liquid slugging will most likely cause damage to the compressor instantly. Another problem is bearing washout or “flooding”. This occurs when liquid refrigerant dilutes the oil in the compressor crankcase and creates foaming, and it will greatly reduce the life of the compressor because it will not receive proper lubrication and too much oil will be carried out of the compressor and into other parts of the system. The compressor also (generally) relies on the cool suction gas from the evaporator to cool the compressor properly, so it’s a delicate balance to keep a compressor from being flooded and also keep it cool.

Condenser

Condensers come in all different types, shapes, and sizes. Regardless, they all perform the same function: rejecting heat from the refrigerant. The refrigerant entering the condenser was just compressed by the compressor, and this process increased the temperature by packing the molecules together which added heat to the vapor refrigerant due to the motor and mechanical workings of the compressor. This process in the compressor also greatly increased the pressure from a low-pressure in the suction line entering the compressor, to a high-pressure vapor leaving the compressor.

The condenser has three jobs:

  1. Desuperheat the refrigerant (Drop the temperature down to the condensing temperature)
  2. Condense (saturate) the refrigerant (Reject heat until all the refrigerant turns to liquid)
  3. Subcool the refrigerant (Drop the temperature of the refrigerant below the condensing / saturation temperature)

The condenser’s job is to reject heat (drop the temperature) of the refrigerant to its condensing (saturation) temperature, then to further reject heat until the refrigerant fully turns to liquid. The reason it must fully turn to liquid is that, in order for the refrigerant to boil in the evaporator, it must first have liquid to boil.

The way in which the condenser removes the heat from the refrigerant varies. Most modern condensers flow air over the tubing where the refrigerant is flowing. The heat transfers out of the refrigerant and into the air. The cooling medium can also be water. In the case of a water source system, water is circulated across the refrigerant in a heat exchanger.

In either case, the condenser relies on the removal of heat to another substance (air, water, glycol etc..). For instance, if you turned off the condenser fan so that no air was flowing over the condenser coil, the condenser would get hotter and hotter. This would cause the pressures to get higher and higher. If it kept going that way it would trip the internal overload on the compressor or cause other damage.

The hot vapor from the compressor enters the condenser and the superheat  (temperature above condensing temperature) is then removed. The refrigerant then begins to change state from vapor to liquid (Condense). The refrigerant maintains a constant temperature until every molecule of vapor is condensed. The temperature of the liquid again starts to fall. This is known as subcooling. When we measure subcooling we are measuring degrees of temperature rejected once the refrigerant has turned completely to liquid.

Temperature above the saturation temperature is called superheat. Temperature below the saturation temperature is called subcool or subcooling. So when something is fully vapor (like the air around us) it will be superheated, and when it is fully liquid (like the water in a lake) it is subcooled. 

Metering Device

The metering device is a pressure differential device that creates a pressure drop to facilitate refrigerant boiling in the evaporator coil.

The metering device is located between the liquid line and the evaporator. The liquid line is full of high-pressure liquid refrigerant. When the high-pressure liquid hits the small restrictor in the metering device, the pressure is immediately reduced. This drops the pressure of the refrigerant to such a degree, that the saturation temperature is lower than the temperature of the air surrounding the tubing that the refrigerant is in. This causes the refrigerant to start changing from liquid to vapor. This is called “boiling” or “flashing”. This “flashing” brings the refrigerant down from the liquid line temperature to the boiling (saturation) temperature in the evaporator, and in this process a percentage of the refrigerant is immediately changed from liquid to vapor. The percentage of the refrigerant that changes during flashing depends on how great the difference is. A larger difference between the liquid line temperature and the evaporator boiling temperature results in more liquid lost to flashing and reduces the efficiency of operation.

There are a few different types of metering devices. The most common ones being the Thermostatic Expansion Valve (TXV/ TEV) and the Fixed Orifice (often called a piston)– as well as electronic expansion valves, capillary tubes, and others.

Evaporator

The evaporator is also known as the cooling coil, because the purpose of the evaporator is to absorb heat. It accomplishes this through the refrigerant changing from liquid to vapor (boiling). This boiling process begins as soon as the refrigerant leaves the metering device, and it continues until the refrigerant has absorbed enough heat to completely finish the change from liquid to vapor. As long as the refrigerant is boiling it will remain at a constant temperature; this temperature is referred to as saturation temperature or evaporator temperature. As soon as the refrigerant is done boiling, the temperature starts to rise. This temperature increase is known as superheat.

When the indoor air temperature or the air flow going over the coil is higher, the evaporator pressure and temperature will also be higher because more heat is being absorbed into the coil. When the air temperature or airflow over the coil is lower, it will have lower pressure and temperature in the coil due to less heat being absorbed in the coil.

The refrigerant leaves the evaporator, travels down the suction line and heads back to the compressor where the cycle starts all over again.

Refrigerant Lines 

Suction Line = Line Between the Evaporator and the Compressor

The suction line should contain low-pressure superheated suction vapor. Cool to the touch on an air conditioning system, and cold to the touch in refrigeration.

Discharge Line = Line Between the Compressor and the Condenser 

The discharge line should contain high temperature, high pressure superheated vapor

Liquid Line = Line between the Condenser and the Metering Device

The liquid line should be high pressure, slightly above outdoor temperature subcooled liquid

Expansion Line (When applicable) = Line Between the Metering Device and the Evaporator

On most systems, the metering device will be mounted directly to the evaporator making the expansion line a non-factor. Some ductless mini-split units will mount the metering device in the outside unit making the second, smaller line and expansion line. The expansion line is full of mixed vapor/liquid flash gas.

Yes, this was long, but more than anything else just keep repeating over and over: compressor>discharge line>condenser>liquid line>metering device>expansion line> evaporator>suction line and on and on and on…

Jim Bergmann did a great whiteboard video on the MeasureQuick YouTube page that explains the basic refrigerant circuit

-Bryan


First off I want to thank Ulises Palacios for taking these photos. He is in the habit of cutting open the compressors he replaces to see why they failed (when possible). I think that’s pretty boss.

So why would the compressor have copper plating on the inside? They certainly aren’t manufactured that way.

The short answer is that acids inside the system can eat away at the copper and brass components in the system. The copper is then deposited in the high pressure/temperature environment of the compressor.

Why does this happen?

The presence of any acids in the system can cause this to occur but the most likely causes are the combination of air and moisture reacting with the refrigerant oil (most prevalently POE) to create an environment in which the copper is dissolved internally and redeposited on the steel in the compressor.

The result inside the compressor is reduced clearances and ultimately locking, overheating and even short circuits if the mechanical failure results in winding damage as is fairly common.

So for a technician, what we can do in ensure we are properly evacuating the system and installing appropriate filter driers to reduce or eliminate the presence of air and moisture.

— Bryan

P.S. – For an in-depth analysis of a study on copper plating in compressors you can read here

Basic Compressor Functions

The job of the compressor is to circulate refrigerant through the system by means of vapor compression, similar to the way your heart moves blood through your circulatory system.

Refrigerant circulation is measured in lbs/min or lbs/hour; called mass flow rate. The mass flow rate changes depending on the density of the refrigerant and the compression ratio.

The denser (higher the pressure) the refrigerant is coming back from the evaporator the greater the mass flow rate and the lower the suction pressure the lower the mass flow rate.

The ability of the compressor to move refrigerant efficiently is often measured in volumetric efficiency. This is a measure of how much refrigerant enters the suction line vs. how much leaves the outlet of the compressor in the discharge line. The difference between the two is loss or waste to re-expansion of the gas in the compressor cylinder (in a reciprocating compressor).

The greater the compression ratio (absolute head pressure divided by absolute suction) the lower the mass flow rate will also be and lower the volumetric efficiency will be . In other words, low suction with high head pressure are the worst case scenario for mass flow rate and volumetric effeciency when the compressor is working as it should.

Proper lbs/min or lbs/ hour of refrigerant circulation is vital to the capacity of the evaporator, condenser and metering device as well as the cooling of th compressor if it is refrigerant cooled.

The Compressor size (pumping ability) controls the system’s lbs/min or lbs/hour mass flow rate.

Compressor pumping action also performs two other functions.

  1. It maintains the evaporator pressure: when the compressor runs, it lowers evaporator pressure. This sets evaporator pressure, operating TD, and BTUH capacity.

2. It increases condenser pressure: when a compressor runs, it pumps heat into the condenser, this causes condensing temp and TD to go up until heat can flow out of condenser as fast as it enters.

As evaporator heat load and temp increase, compressor heat output increases and drives condenser TD even higher to increase condenser heat rejection.

Compressor response to changing Evaporator heat loads

Here is a way of thinking about load and how it impacts mass flow rate, compression ratio and volumetric efficiency.

Higher heat loads produce vapor faster than compressor can remove it from the evaporator. When this occurs the evaporator pressure and temperature go up with the increased heat load.

The compressor’s flow in lbs/min or lbs/hr increases as the suction pressure increases and compressor draws more amps due to pumping more refrigerant.

Lower evaporator heat loads produce vapor slower than compressor is removing it from the evaporator. Evaporator pressure and temperature go down with the reduced heat load. Compressor’s flow in lbs/min or lbs/hour goes down. The Compressor draws fewer amps due to pumping less refrigerant.

Compressor’s Volumetric Efficiency

The goal is to keep the Volumetric Efficiency as high as possible. With a higher VE, a compressor produces more lbs/min or lbs/hour of refrigerant flow
Systems operating conditions, evaporating and condensing pressures, directly affects compressor pumping ability VE Ratio of Condenser pressure to evaporator pressure is called compression ratio. To calculate compression ratio, convert pressures to absolute values (add 14.7 to existing pressure) then divide condenser pressure by evaporator pressure

Volumetric Efficiency Charts

VE (Volumetric Efficiency) Charts show the effect of compression ratio on Volumetric Efficiency: As CR goes up, VE goes down. As CR goes down, VE goes up. Our goal is to keep volumetric efficiency of the compressor as high as possible for capacity, energy usage and compressor longevity.

Factors that determine system CR

System compression ratio is based on a few factors, primarily desired space temp and temperature of the cooling medium. Corresponding evaporator and condenser pressure establish the compression ratio the compressor must work against. Refer to the compression ratio chart for each compressor as a guide.

Keeping Volumetric Efficiency Up

In order to improve VE, you must keep the compression ratio low. You can do this by keeping condenser pressure low, maintaining clean condenser and supply it with a cool condensing medium (proper temperature and flow of air or water across the condenser coil or condenser HX). You must also keep the evaporator pressure up, don’t run the evaporator pressure any lower than needed to do the job. Lower compression ratio allows the compressor to pump more lbs/min or lbs/hour through the system. Higher compression ratios reduce the compressor’s ability to maintain the desired mass flow rate.

Compressor Approved Application Range (operating range) 

Hermetic and semi-hermetic compressors are designed for specific evaporator temperature ranges. The range of evaporating temps varies by manufacturer and model and you will need to do some reading to be sure you have it right. Evaporator temperatures above maximum approved temperature results in motor overload; drawing excess amps and overheating. An evaporator temperature below the minimum approved application temperature will result in poor motor cooling due to a low lbs_hour flow rate.

Compressor Data Sheets

Data sheets show compressor performance in its approved application range. Data may be shown in a table or as performance curves, these tables or curves will show : capacity  mass flow rate, power and current. This can be used for design, proper commissioning and system diagnosis. Just keep in mind that the compressor when working properly is still at the mercy of system conditions, it is up to us to set it up for success.

Compressor Amp Ratings

Compressor amps change as the evaporator and condenser temperatures change. Under load conditions, the compressor could draw more than rated load amps and not necessarily be in any danger of motor overload. As long as the motor amperage drawn is well below trip amperage. Most compressors will run at less than rated load amps during normal conditions but may run high under heavy evaporator load. All of this can be found by looking carefully at the compressor charts or curves.

– Louie Molenda

 

 

 

fusite_plug

This tip will be like an episode of Columbo, we will start with the what and who and then get to the why.

  1. Don’t pump down a scroll into a vacuum
  2. Don’t run a scroll in a vacuum
  3. Don’t run a high voltage megohmmeter or Hi-pot test on a scroll (As a general rule don’t go over double the rated running volts)
  4. Don’t do any megohmmeter test with a scroll under vacuum

These points have been confirmed with Copeland (Emerson) as being on the naughty list this Christmas.


Resistance / Megohm Testing

A scroll is like any other compressor in that it has a motor and a compression chamber “hermetically” sealed inside the shell. There are many differences between a scroll and a reciprocating compressor but let’s focus on the few that are pertinent to this conversation (or at least the pertinent ones I can think of).

  • In a scroll, the motor is located on the bottom, this means that the motor is immersed in refrigerant and oil. When the compressor has been off and is cold, there can even be some liquid refrigerant in the compressor.
  • A scroll is more compact and balanced design as there is no need for “suspension” like a reciprocating compressor. This results in closer tolerances/distances between the electrical components and the other metal parts.

The motor being located at the bottom is the biggest thing. Copeland states in bulletin AE4-1294 that megohm readings as low as 0.5 megohms to ground are acceptable. Besides that fact that this makes a scroll difficult to successfully meg (essentially impossible with a tool like the Supco M500 because it only reads down to 20 Mohms) it is a clear indication that a scroll compressor is running tighter resistance tolerances and a higher risk of internal arcing due to many factors. Another thing to consider is the scroll will read lower ohms to ground when it is cold than when it is running due to higher refrigerant/oil density at lower temperature and of course you are generally doing a meg test when a scroll has been off…. so that makes it tricky.

Some of the factors that can decrease resistance further and lead to problems are:

  • Moisture contamination
  • Free metallic particles due to copper leaching (acids), small metal pieces left from copper fabrication or metal from compressor breakdown due to other issues like overheating, flooding and improper lubrication.
  • Other contaminants

All of this to point out that tolerances are tight in a scroll to begin with.. add in some extra nastiness and you are at risk.


Pump Down 

First, many scroll compressors won’t even allow you to pump them down into a vacuum. Either they are equipped with a low pressure cut out or some sort of low pressure / low compression bypass like shown in this USPTO drawing

vacuum_prevention

For example, in Copeland AE4-1303 it states “Copeland Scroll compressors incorporate internal low vacuum protection and will stop pumping (unload) when the pressure ratio exceeds approximately 10:1. There is an audible increase in sound when the scrolls start unloading.’ This is to prevent the compressor from pulling down into a vacuum.

In addition to that, there are lots of threats and warnings about running a scroll while it is in a vacuum, as in if you had just evacuated the system and then accidentally turned the system on. Which is a bad idea on any compressor, but worse on a scroll.

Why?

The totally obvious reason is that the compressor itself isn’t designed to run in a vacuum and it will overheat as well as fail to lubricate properly, but that isn’t the only reason or even the primary reason. All of the literature mentions arcing and I spoke to more than one tech rep who mentioned the “fusite” plug arcing or being damaged.

First, Fusite is a brand name and one of the companies in the Emerson family. So when we say “fusite” we are using a ubiquitous term for a sealed glass to metal compressor terminal feed through. There are many different types and designs of Fusite terminal just as there are many different types and designs of compressor. There are scroll compressors that use them, there are reciprocating compressors that use them, the ice cream truck that plays that obnoxious music driving through your neighborhood probably has one…. on the refrigeration compressor. Do certain fusite terminals short out more easily than others? I’m sure some are more susceptible than others. Is that what is going in here… maybe.. but if so it’s only part of the story.

What we do know about a scroll is the electrical tolerances are tighter… and when electrical tolerances are tighter there is a greater likelihood of arcing.

It’s about to get really nerdy here so if you don’t care just stop reading and go back to the very beginning, memorize the 4 points and move on with your life.

I can’t do that… because I’m broken.


Why is vacuum an issue? Isn’t vacuum the absence of matter and isn’t matter required for electrons to arc from one surface (cathode) to the other surface (anode)?

The answer is not really simple AT ALL but the summary is that under certain circumstances vacuum increases the likelihood of arcing and scroll compressor terminals inside the compressor happen to be one of those circumstances.

First thing to remember is that while electrons do travel through matter, electromagnetic fields do not require matter to exist and in either case.. we are incapable of achieving a perfect vacuum so no matter how deep we pull a vacuum, some molecules are still present.

I’ve heard some techs attribute this to the corona discharge effect which can occur due to the ionization of particles around a high voltage conductor. I really don’t see this as being the answer both because the voltages applied are not THAT high and corona discharge is not an arc or a short in the traditional sense, just a “loss” to the environment around the conductor and a pretty cool looking light (as well a decent Mexican beer).

My opinion (and this is an opinion, not a proven fact) is that the arcing is due to something called field electron emissions which can result in insulator breakdown in vacuum conditions (NASA has to deal with it all the time in space because space is a vacuum ).

The conclusion is that while this phenomenon can happen in ANY compressor, it is made more likely in a scroll due to tighter tolerence and “motor down” configuration. This means that doing a high voltage meg test, or any running/meg testing under vacuum is a bad idea.

If you want to read more about Fusite, Copeland scroll compressors and a great overall guide that includes evacuation procedures just click the links.

Nerd rant over.

— Bryan

 

 

Now is the part where we get specific about Start capacitors and inrush. If you haven’t read the first three parts please do so before reading this one or it may not make sense.

I’m going to come out and say it so you keep reading.

What you were taught about hard start kits decreasing inrush amperage is wrong.

Look at the oscilloscope image above. It’s a 3 ton reciprocation compressor with equalized pressures and 230V applied. Now keep in mind the voltage value shown here is RMS and the amperage is PEAK so if the amps seem high in these charts that is why.

So this compressor starts up at locked rotor (on the run winding remember) and it gets up to speed at around 180 milliseconds of run time. That is pretty typical of a good, equalized compressor under normal conditions.

Sometimes compressors do struggle to start and this can be due to.

  • Low input voltage (usually due to voltage drop)
  • Starting unequalized due to short cycling and or non-bleed (Hard Shut Off) expansion valves
  • Long line lengths
  • Refrigerant migration into the crankcase
  • Compressor wear

Here we show a system with low voltage applied (187v) and it HAS a hard start in place (not necessarily the same compressor as the last so don’t try to compare apples to apples)

In this case, the compressor doesn’t get up to speed until about 550 milliseconds and at that point both the run and start winding amperage drops.

This proves that the hard start kit is working in that case.

You can tell this because the only amperage that can enter the start winding is dictated by the run and start capacitors and like we said before… the capacity of a capacitor is dictated completely by the capacitance (mfd) and the voltage across it.

For the amperage of the start winding to go UP at 580 milliseconds, it can only be due to an increase in back EMF as that motor gets up to speed. Then at 600 milliseconds, the potential relay removes the start capacitor and the amperage drops down to the run capacitor only level.

Conclusion #1

Fixing a voltage drop issue does more good than adding a hard start kit.

A valuable test is measuring the voltage feeding your compressor with it under load and rectifying poor line voltage connections or undersized conductors.

Look at these two identical compressors, one with a hard start kit and one without.

Techs are often taught that measuring inrush on the compressor common wire is a way to show how a hard start kit decreases startup amps.

This isn’t what we are measuring when we see a big difference in inrush amps.

What we are measuring is how QUICKLY the compressor starts not the true inrush amps at start. I’ve looked at the specs on several high-end ammeters that measure inrush and they read at 100 milliseconds.

If you look at the two examples above the compressor with a hard start will read lower at 100 milliseconds because it is already almost at full speed and the LRA has nearly passed.

LRA (locked rotor amps) is essentially the amperage the compressor runs when the run winding is functioning as a heater rather than a motor. The quicker the motor starts turning the quicker it gets out of the LRA range.

Now consider the start winding. Take a look at the start winding with no hard start…. it stays the same even though the compressor is essentially locked all the way up to 600 milliseconds. So even though the compressor is locked the current through the start winding is limited to what the run capacitor can hold and release.

With the hard start in place, the start winding amperage peaks right up until the potential relay takes it out at 100 milliseconds.

The problem is that we use tools that measure at 100 milliseconds when that may be before or after the motor has hit that 80% speed

 

Conclusion #2

Locked rotor amps are what they are… that’s why the manufacturer can publish it on the data tag. It’s the amperage that motor will draw when it’s locked at the rated voltage.

When we measure inrush with a meter we are really just taking a snapshot at a particular point in time that may or may not line up with what the manufacturer published depending on how close the motor to full speed at that time.

Conclusion #3

Hard start kits don’t decrease starting amps at the moment of start, they can’t.

What they can do is reduce the time it takes to get the motor started, So in a time-averaged sense, a hard start kit very well may reduce amperage and wattage.

This is why a hard start is often specified for long line applications and non-bleed TXVs. It’s also why Techs have found that adding a hard start can reduce light dimming complaints by speeding up the amount of time the compressor remains at LRA.

Conclusion #4

Hard start kits do increase the current on the start winding, in fact, that’s essentially all they do differently than a run capacitor. They both provide a phase shifted current to the start winding, a start capacitor just does MORE of it.

The argument that hard start kits reduce wear and stress on the start winding is false. With no hard start the load on the start winding is constant and very low. Adding in a hard start adds in more current at start and the possibility of potential relay sticking that can definitely cause start winding stress.

This isn’t to say start kits are a problem when sized properly. They can and do reduce LRA on the run winding by adding more phase shifted current to start.

Conclusion #5

Start capacitor sizing and potential relay voltage ratings are really important and should be selected to do the job of starting the compressor quickly without staying in too long or providing more start winding current than needed.

— Bryan

Before we get into the parts that will ruffle some feathers lets talk a bit about what a “Start” capacitor is and what it does.

First, let’s review that both start and run capacitors connect between the leg of power opposite of compressor common and the start winding.

Even though it seems like a run capacitor should connect to the run winding, it doesn’t… it connects to the start winding just like a start capacitor

Most start capacitors have a much higher MFD (Microfarad) rating than the run capacitor meaning they can store and release much more current. They are also generally electrolytic capacitors instead of oil filled metal film type like a run capacitor.

All this adds up to a start capacitor being able to store and release a lot of current into the start winding but it only stays in the circuit a short period of time without damaging itself because it cannot dissipate heat easily like the metal film run capacitor.

The start capacitor is wired in parallel with the run capacitor as shown in the image above. On startup, the potential relay contacts are closed which means the capacitor is in the circuit with all that electron storage capacity. When the compressor contactor closes a large amount of current can move into that start winding because there is a larger “membrane” (see part 1 of this series) that can store and release energy. This extra current moving through the start winding helps get the compressor started more quickly,

But the start capacitor must be pulled out of the circuit very quickly to avoid overheating itself or damaging the compressor start winding

Single phase compressor start windings are not designed to carry high continuous current like the run winding. If the start capacitor were to stay in the circuit too long the current on the start winding will stay high and will risk damaging the start winding.

There are various types of relays and controls that can remove a start capacitor from the circuit but the most common is a potential relay. The potential relay coil is either connected between start and common or start and run and it is sized to open up when the motor reaches about 80% of it’s full speed.

The potential relay opens based on an effect called “Back EMF” which leads us to our next thought experiment.

Thought Experiment #6 – Where Does Back EMF Come from?

Next time you check a dual capacitor, measure voltage (Safely) between the C and HERM terminals on the capacitor, Now measure between L1 and L2 at the contactor (line in). You will notice that the voltage at the capacitor is significantly higher than the input voltage when the compressor is running.

This has led many techs to conclude that the capacitor somehow “boosts” voltage like a transformer… that is not what’s happening at all.

That increased voltage is actually being generated by the compressor motor and we that power “back EMF”. When we spin a motor using magnetism (which is how we spin a motor) the motor also acts like a generator as the magnetic fields from the rotor (the part that spins) interacts with the stator (the part that stays still with the windings in it). When the motor is still it generates no back EMF or Inductive reactance (magnetic resistance) and this is why at startup the motor draws high amps and produces no back EMF.

As the motor begins spinning faster and faster the back EMF and inductive reactance increases, this causes the amperage to drop on the run winding and the back EMF to increase. Now you generally won’t see the back EMF when you measure between Run and Common because they are connected to the line power which dissipates this returned energy very quickly. You do measure it between start and common and run and start because the start winding is only connected between the lines capacitively.

When use this back EMF to our advantage to open up the relay contacts on the potential relay and get that start capacitor out of the start winding circuit as soon as the motor approaches full speed.

Let’s do a bit of recap…

  • The start winding and run windings are not the same and do not function in the same way in single phase air conditioning compressors
  • Common is a point not a winding
  • A capacitor functions like a membrane or storage tank for current
  • The current that can move in and out of the start winding is dictated by the voltage across the capacitor and the size of the capacitor and has nothing to do with the load on the compressor, LRA or anything else.
  • Locked Rotor amps occur on the run winding not on the start winding. When we measure LRA on common we are seeing the combination of start and run but without a start capacitor, the vast majority of the amperage will be on the run winding.
  • If your capacitors are failed open you can have no current through the start winding
  • A start capacitor increases the amount of current that can move through the start winding for the first few hundred milliseconds after startup
  • The Back EMF we measure at the capacitor is generated by the motor and increases the faster the motor spins

We wrap it all up in the next and final article in the series.

— Bryan

 

 

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