Tag: refrigeration

Why Defrost?

Let’s start with the basics and move on from there. Defrost is necessary when the coil temperature drops below 32°F. Defrost can be as simple as turning the compressor off for a period of time or as elaborate as reversing the flow of refrigerant for the whole system or for just parts of the system.

 

As we were all taught in school, frost buildup is an insulator and prevents heat transfer, also airflow through a coil is a big factor. If the coil is iced up, the fans can’t move any air and without air movement, the equipment can’t do its job. This applies to all equipment with defrost, really.

Fin spacing

For refrigeration techs, this isn’t surprising, but A/C coils ice over a lot faster than refrigeration coils do. Why? Because the fins on a refrigeration coil are much more widely spaced than those on an A/C coil. So, when an A/C coil starts to get cold and that little bit of frost starts to build on the tube surface and the fin, it affects airflow through the coil much faster than it would if the fins were spaced more widely apart.

Moderate fin spacing medium temp coil with 6 fins per inch

Wide fin spacing on a freezer coil four fins per inch

If we had refrigeration coils with fin spacing like an A/C unit, it would ice up too quickly, and we couldn’t get anything done. The wider fin spacing illustrated shows how refrigeration equipment can run longer between defrost cycles. The evaporator coils are built in such a way to accept a certain amount of frost before the performance starts to degrade.

 

So, how do we get the job of defrosting done?

 

The most basic defrost is one we all probably remember Granny taking everything out of the “icebox,” unplugging it and going after it with a screwdriver, hair dryer or an ice pick. Simple, right?

 

But there has to be a better way, doesn’t there?

 

One of the simplest and most common automatic defrost control strategies is commonly referred to as a “cold control,” more properly called a coil temperature sensing thermostat. You’ll sometimes hear it called a “constant cut in” control.

 

With either a little-coiled bulb on the end of the sensing tube or a tube that kind of embeds in the evaporator, this senses the temperature of the evaporator coil and cycles the compressor based on that. The sequence of events runs like this. Coil temperature rises above cut in which is typically in the upper 30s. I like to see about 37°F at the lowest. This setting is nonadjustable, hence the name “constant cut in.” Control closes bringing the compressor on. As the coil temperature drops, the control eventually reaches its cut out point. I’ve seen this as low as 9°F. The cut out is what you’re adjusting when you adjust the control.

See what’s happening? Every single time it cycles off, the coil temperature has to rise above freezing by enough to ensure a good, complete defrost.

You’ll see this type of control on stuff like prep tables and smaller, under counter type refrigerator units.

 

Simple and easy.

 

A similar method for defrost control uses a pressure control to cycle the compressor. With this type of system, you set the cut in of the control to a saturation pressure equal to the same 37°F to 40°F, remembering this is saturation temp, not air temp, and adjust the cut out to maintain the temperature desired.

 

The big drawbacks of these controls are that they aren’t always predictable in that the defrost happens when the unit cycles rather than at a specific time (or times) every day and that the temperature can fluctuate over a pretty wide range. For some products, especially fresh meat, wide temperature swings are detrimental to product quality.

 

Taking a step up from the idea that every off cycle is a defrost, we’re going to just add a timer to the circuit. Now, we can set that timer up to shut the refrigeration off at regular intervals for a specific period. The interval and duration will be situation dependent as we’ll discuss.

Looking at this mechanical timer, the silver screws in the outer timer ring initiate defrost when they rotate past the pointer at the top left. The defrost ends when the copper-colored pointer on the inner ring rotates past the same pointer.

This digital timer has little black bars on the display indicating both time and duration of defrost. In that picture, the time of day isn’t indicated in the photo. In its simplest form, this timer just opens the control circuit to the compressor or the control valve for the set duration of the defrost.

 

So, what’s happening? As far as the system is concerned, the same thing is happening here that was happening before when we used a cold control or a low-pressure switch. We’re shutting the refrigeration off and allowing the frost to melt naturally off of the coil. The biggest difference is that now, with a timer, instead of being subject to the unknown of when the system will cycle off and how long it will take to melt the frost, assuming the time of day is set correctly, you can reliably predict the defrost times. Now, you can say that it defrost at, 6 AM and 6 PM for 45 minutes and the customer can note that and account for it when checking temps on their equipment.

 

Let’s talk for a minute about how long a defrost needs to last… obviously, until the coil is completely clear of frost and ice, but we need to know when that is….

 

In most cases, the manufacturer will give guidelines to set your defrost control system up. It will spell out frequency or interval (time between defrosts) and duration of the defrosts. Because we’re trying to maintain proper product temperatures and we got away from the cold controls and low-pressure controls because they were fluctuating over a wide range of temperatures, we need to look for a way to limit that fluctuation.

 

For years this was only used on defrosts that added heat to the evaporator coil (which we will look at later) but in recent years with more stringent product temperatures requirements and temperature expectations from the customer, combined with government efficiency mandates, trimming even a couple minutes off of a defrost cycles improves both product holding quality and unit efficiency.

 

How does it work? The manufacturer will typically install either a thermostat or a temperature sensor on the coil or in the air stream leaving the coil. After experimentation in their labs, they determine just how warm that spot has to be to ensure the coil is free of frost. So, in the middle of summer in a hot, humid kitchen the defrost runs longer than it does in the middle of winter on outside access only cooler box. Why?

 

We all learned about sensible and latent heat in school, right? Well, melting frost is just latent heat added to change the state, right? So, since we’ll have more frost on a coil with a higher humidity than on one in a lower humidity environment, the coil with a higher frost buildup is going to take longer to melt off of that coil which means that it will take longer to reach that set temperature.

 

In practice, here’s how that timer handles defrost. Time of day initiates a defrost, so say 6 AM, the timer switches to defrost mode. Internally, that means that the contacts in the timer open to de-energize either the control valve or the compressor. For simple off cycle defrost, the fans continue to run to keep moving air across the coil and accelerate heat transfer. The defrost ends, in the simplest form, when the timer reaches the duration pin, switching the timer contacts back to closed and energizing the load. If we have a termination control, it’s a normally OPEN contact that closes on the rise of temperature. So, when that temperature reaches the termination point determined by the manufacturer, the contact closes energizing a small solenoid in the timer to push the contacts back to normal position regardless of the timer position. In an electronic control, this is just another signal input, either digital (NO\NC contact) or analog (sensor) that tells the software in the controller to switch the relay back to refrigeration. A coil thermostat or sensor might be set as low as 34°F while an air sensing control will typically be set between 48 and 55°F.

 

Electric Defrost

Since some refrigeration equipment runs at temps significantly colder than 32°F sometimes, we’re going to need to add some heat because there simply isn’t enough heat in the refrigerated space to get the frost melted without causing significant damage to the product. The simplest way to add this heat is usually with an electric heater. Let’s take a look at how this adds some complexity to the defrost control system.

 

The basic timer type defrost initiation control doesn’t change. The same type of timer is used and when defrost initiates, the refrigeration circuit de-energizes the same as before. The big difference now is that, at the same time, we’re energizing a heater that is going to add heat to melt frost of the evaporator coil. In the case of most pieces of equipment, we’re also going to de-energize the evaporator fan circuit. This is to keep the heat concentrated where it is needed to do the job in as little time as possible. We also don’t want to blow hot, humid air around the refrigerated space.

 

Defrost termination is really the standard for this type of system. Almost all electric defrost systems will have a type of defrost termination built in. The most common are referred to as a DTFD control (Defrost Termination Fan Delay) or 3 wire control. This dual-purpose control handles both termination of defrost obviously and post-defrost fan delay which we’ll get to in a minute or two. The DTFD is normally attached to one side of the evaporator coil in a position that takes the longest to get warm during defrost. This way, the coil gets the best possible defrost.

 

Refrigeration is off; heaters are on, frost is melting away. All is well. Once our DTFD control sees it’s high event temperature, usually about 55°F, it closes the part of the circuit to terminate defrost, same as before. Refrigeration machine starts back up and we’re moving heat again, but wait…. What about the fans? They aren’t running. Quick get a meter and a ladder…

This is the other half of the DTFD control. We’ve terminated defrost (DT) now we have to wait a couple of minutes until the coil temperature drops below freezing. We have to remember that coil was just 55°F and there is some humid air still trapped in that sheet metal box up there. Slam the fans on right now, and you’ll have a wintery Wonderland in your freezer with icicles and snow all over in a week or so. Wait a minute or two and the coil will freeze that last bit of moisture. When the coil temp drops to around 30°F at the control, our fans will restart.

 

Gas defrost, particularly for large refrigeration systems is going to require an entire article in and of itself to cover in any depth. I’m going to try to summarize it in a paragraph or two and give it a more thorough treatment in the future.

 

These, like all other defrost, operate on a time basis. The systems where this is more common aren’t single system but multiplex systems with multiple evaporators operating on different schedules. When one goes in defrost the rest continue to run in refrigeration.

 

When the timer initiates a defrost, a few things happen all at once. A differential valve de-energizes to create a pressure differential to allow flow in reverse. To create a section of reversed gas flow, two valves actuate. One that stops suction gas flow to the compressor and another that dumps hot discharge gas into that suction line, sending superheated discharge gas out to the evaporator where it rejects its heat to the frost on the lines and is condensed just like in a heat pump. It returns to the system through a check valve piped around the TEV, same as with a heat pump. Without the pressure differential, the hot gas cannot flow properly through the check valve.

 

Once either the time limit is reached, or the termination temperature is reached, all of those valves return to their normal positions, and the refrigeration cycle resumes normally.

 

— Jeremy Smith CMS

 

Illustration Courtesy of Emerson

CO2 is a pretty nice refrigerant.

It has zero ODP (Ozone depletion potential) and a GWP (global warming potential) of 1. CO2  has been used as a refrigerant almost from the very beginning of refrigerants and its been making a big comeback in market refrigeration (especially in colder climates).

CO2 (R744) is naturally better suited for lower temperature refrigeration applications because of its low temperature saturated state at atmospheric pressure (-109.3F). You will notice I said “saturated state” because CO2 does not “boil” at atmospheric pressure. At any pressure below 60 psig CO2 goes straight from solid (dry ice) right to a vapor, This is why 60 psig is known as the “triple point” or the point that could be either solid, liquid or vapor.

Now go to the top of the range with CO2, when you apply 1055 psig the saturation temperature is 87.8F but go up even 1 more degree and CO2 CANNOT be liquified, this is known as the critical point of the substance. Whenever a substance is forced beyond it’s critical point it becomes what is known as a supercritical fluid and has properties that are unique to this state but it is certainly not a liquid. You can see more in this natural refrigerants PT chart.

In a transcritical (trans means beyond or through so transcritical means “beyond critical”) booster refrigeration system the low temp portion of the system operates using it’s own compressors that “boost” the refrigerant from the low temp side and discharge into the suction of the medium temp side. The high stage compressors then pressurize the CO2 (R744) above its critical pressure / temperature.

What is traditionally called a condenser becomes a gas cooler and decreases the temperature (rejects heat from) of the discharge without actually condensing it into liquid. The cooled supercritical fluid goes through a pressure reducing valve, where some of it condenses into liquid and the rest remains as gas. Liquid and gas are separated in a flash tank (receiver). Pressure in this tank are usually controlled to around 450 to 500psig.

It’s super critical that you understand all of this…

See what I did there.

— Bryan

 

 

Do you know how a solenoid valve works?

 

Really?

 

On the surface, I think we all understand how a solenoid valve works.  The Coil energizes creating an electromagnet.   That temporary magnetism lifts an iron plunger within the valve itself allowing refrigerant to flow.

 

But…  is it really that simple?

 

Turns out, the answer isn’t as straightforward as you’d expect.

 

The simplest type of solenoid valves are direct acting solenoid valves.   These are exactly what is described above.   The iron plunger directly controls the flow of refrigerant through the valve. Every single solenoid valve you see incorporates a direct acting valve, but there is more than what meets the eye.

Courtesy of Sporlan

Direct acting solenoid valves have an inherent limitation.   If the force created by the fluid flowing through the valve that is acting on the iron plunger is enough to lift that plunger, then it isn’t going to close regardless of what the electromagnetic coil tries to tell it to do.   What this means is that direct acting solenoid valves are limited in size, and that size is pretty small.

 

 

So, how can we control the fluid flow in larger lines with solenoid valves?

 

We start to use the pressure within the system to actually force the valve closed.

 

Say what???

 

These are called pilot operated or pilot actuated valves   The direct acting solenoid doesn’t try to control the entire flow, it only acts to control a small portion of the fluid which acts on a diaphragm or other device to open and close the valve.

Courtesy of Sporlan

 

Let’s see if we can start to understand how these valves work in practice.

 

First, a few basics.

 

  1. Solenoids, like most valves, are directional. If you install it backwards, it isn’t going to work correctly.    This is why.
  2. Solenoids must be sized properly. You can’t just go buy a ½” solenoid valve and expect it to work because your line is ½”.   This is to ensure a small pressure drop across the valve which is what actually makes the valve work.

 

Ok.   Refrigerant flowing through an energized solenoid.   Now, the coil de-energizes causing the iron plunger to drop and seal a tiny port.  What this does it stop a small amount of flow from inlet to outlet, preventing that small flow from leaving the valve body.    That small port being blocked causes pressure to build on top of the diaphragm or valve seat disc, forcing it down to seal the valve.    The small iron plunger and spring don’t have the force required to force the valve closed but, by utilizing system pressure, we have a much larger amount of force available.

 

In truth, the large majority of solenoid valves a technician sees are pilot operated valves.

 

— Jeremy Smith CM

 

 

 

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

 

 

 

Let’s take a walk through the startup and commissioning procedure of a conventional or “single” refrigeration condensing unit.  We’re going to start with a unit that is fully piped in and has been pressurized for leak and strength testing. For brevity, we are going to assume a basic familiarity with industry standards, company and customer policies and requirements and the job site and any policies in place there.
Before we even swing a wrench at the machine, let’s familiarize ourselves with the job site and equipment.

Take a walk around, check with the job supervisors, check in with other trades, etc. Find all the equipment you’re to be starting and make notes.

Step one is to make final leak tests.   Typically, the installer records time, date and pressure data.  If you put the pressure charge on the equipment, you should have done so.  If you have had any temperature change, you MUST account for that.   While nitrogen is chemically inert, all gasses respond to changes in temperature by changing pressure.   The math is simple and I’ve addressed it in another article.

We’ll assume here the installers did their job properly and that there are no leaks to track down and fix.  Blow that charge off and break out a nitrogen cylinder.  Yeah, I know, you just blew off the nitrogen… That’s OK  – you aren’t leak checking anymore.

Disconnect the pressure controls; you’re going to SET them.   If they’re the little-encapsulated type, we’re at least going to check and record the operating pressures.  If they’re of the brazed in variety, you’ll need to try to isolate parts of the system to pressurize them to test.   I recommend referring to the manufacturer’s literature for proper control settings and, if they don’t offer a guideline, referring to the Heatcraft installation manual for guidance.

Use your nitrogen regulator and manifold gauges to adjust each control to precisely the setting desired.  The procedure that I typically use is to adjust the control to a setting that is near the top of the scale, set the applied pressure with the nitrogen to the desired pressure then adjust the control down until it closes.   Some controls have a very audible ‘click’ when they open and close, others will require you to use an ohmmeter to determine when the contacts open or close.

Use a sharpie to record that setting right on the cover of the control or, if you prefer, inside the electrical control panel.  Since many larger customers have specific commissioning paperwork they require, you might as well get your notebook out and record it there, too.

Once you’re done setting the controls, it’s time to evacuate the equipment.   Make sure you’ve opened any and all service valves in the system and that any control valves are open or that you’re connected on both sides of the valve.

Even though you’ve got a lot of work to do while the pump is running, I still prefer to use the faster no manifold, large hoses, core pulling method.   That way, I’m spending more time in a deep vacuum and, if something goes wrong and I have to make a repair and evacuate the system a second time, I’m not spending a lot of time watching a vacuum pump run.  I’m not even digging my micron gauge out just yet, just hook up the pump and let it run.

While the pump is running, you’ve got some details to attend to.

 

  1. Record model and serial of the condensing unit and the indoor equipment.
  2. Check phase rotation if possible. If not, you will check this during the initial startup. Remember not to energize equipment while under a deep vacuum.
  3. Check and tighten all electrical connections. I prefer to use a torque indicating device for this just to eliminate any chances I can for a problem down the road.
  4. If there are flanged or threaded connections on the refrigeration system, I’ll check and torque or tighten them at this time as well. Again, I prefer to use a torque indicating device when and where possible.
  5. Check that the metering device, condensing unit and oil type match the refrigerant being used. Make sure TEV bulbs are installed properly.
  6. If the unit has a headmaster and a fin/tube condenser, strip the panels off of the unit and measure the condenser for calculating a flooding charge. Go ahead and figure it and write that down, too.  Microchannel coils just have a lookup chart.
  7. Verify that any other trades involved have completed their work. It’s no fun to have a piece of equipment ready to run and not have power to it or to find out a day later that a condensate drain wasn’t properly installed or heated if necessary.
  8. Check any doors on fixtures to make sure they close and seal properly. Make any adjustments needed.

Get that micron gauge out now and let’s check the progress of our evacuation.   Again, much has been written on this subject, so I don’t want to belabor the point of the how and the why here.  Pull a proper deep vacuum on the equipment according to the customer’s standards, the manufacturer’s standards or industry standards and record times and evacuation levels here if the commissioning paperwork requires it.

Evacuation complete, here is where starting a refrigeration unit diverges from starting up a residential one.   Residential equipment typically comes precharged for a specific amount of line set length.  All you have to do is open the lines, start the equipment and check charge.   Split refrigeration equipment doesn’t come precharged because the manufacturer can’t know how their machine is going to be installed.  We will have to field charge it.
Final checks before charging:

  1. Power to the unit on? Leave disconnect open for now.
  2. Power to the evaporator unit? Go ahead and turn that on.
  3. Power to any control valves like a liquid line solenoid?

Put the cylinder on a scale and start adding refrigerant to the equipment.  Techniques vary somewhat here, but I start by adding liquid refrigerant straight into the receiver valve and liquid line while monitoring suction pressure.  Suction pressure is rising, so we’ve got flow through the system.  If we don’t see a suction rise, we need to stop and investigate.    Maybe a valve is closed or not energized properly. For us, everything is going nicely, go ahead and close that disconnect to the condensing unit to energize the equipment.  If you weren’t able to test phase rotation earlier, now is the time.  Verify that the compressor and fan motors are rotating in the correct direction and make any corrections necessary before proceeding any further.
While adding gas, the compressor is going to short cycle a fair bit while you’re getting enough refrigerant into the system to keep it running. Some guys like to bypass the low-pressure safety and I’ve done that.  I’m also not opposed to opening up some liquid to the suction line.   Not full flow, but get some in there.

 

For right now, we’re going to charge this unit to a moderately cloudy/bubbly sight glass.   We’ll come back and finalize the charge later and I’ve always found it easier to start low and add up to final charge than to add too much and have to remove some or be uncertain of our charge.  It’ll work, sorta, even low on charge.  Once the machine is running on its own, add just enough to get that sight glass in that cloudy state and stop.  Record the amount of refrigerant added in your notebook.  Monitor pressures and suction temperature for right now.  If your superheat really starts to drop into the flooding range, it’s time to go check the evaporator to see why, but that’s pretty rare.

Continually monitor the temperature in the box while monitoring the unit operation until box temp gets to within 5° or so of the desired temperature.

Now, we get to do some wrench twisting.

First things first, you MUST HAVE a solid column of liquid to continue, so add the rest of the charge.   Clear the glass and add your flooding charge. Record that total amount of refrigerant added both on the unit and in our notebook
Now, let’s go check and set the superheat.   Having a box that is close to temp and a solid column of liquid is important because without both conditions being present, a TEV cannot properly regulate superheat at the coil.   Connect your gauges and temperature probes and monitor for a couple minutes.    Again, record the information.   Pressures, temperatures, superheat….    Write it down.    Adjust the superheat to the manufacturer’s or customer’s specifications.   Be aware as you’re doing this that the unit may cycle off and throw your readings off.    You can adjust thermostats or bypass controllers to keep the unit running while doing this but be careful to not allow the unit to get too cold as this will affect the operation of the valve at normal conditions.
Final details and checkout.
Now our unit is running and we’ve got everything set up right where we need it, we need to turn our attention to details.

  1. Set the thermostat or temperature control and verify the setting with an accurate thermometer.
  2. Set the defrost timer to manufacturer’s specifications or customer’s specifications. Test operation of the timer as well and ensure that it not only keeps time but switches properly. Then set the timer to correct time of day.  If requested, provide the customer with the defrost schedule.   Verify that any defrost heaters draw proper amperage and record.
  3. Many cases and freezers have mullion heaters to prevent frost and condensation on doors and frames. Check these for proper amperage and record.

Now we can sit down, fill out the customer’s paperwork and submit that to them.

 

Before leaving the job site, it should go without saying that we need to clean up any debris left behind.  I also like to present the customer with their case manuals, give them a quick run through of the equipment and answer any questions they have.   Be sure to leave a business card because even though we’ve been diligent in starting and commissioning this equipment, they may have problems or just questions down the road.

— Jeremy Smith CM

 

 

 

One of my most popular YouTube videos goes over how to adjust TXV superheat. It’s a very simple little video that I did at my desk and the other day I got this comment –

“Good Video but I hate to say this BUT, with the title SCHOOL, why would you show the public an instructional video depicting the use of an adjustable wrench on a valve stem? Service valves, valve stems on TXV’s and Acetylene tanks should NEVER be touched with anything but a service wrench… Its hard to unlearn bad habits”

It was a well-deserved rebuke, in my haste I used an adjustable wrench to show the adjustment of the TXV stem rather than a service wrench.

The refrigeration service is as much a staple of the HVAC/R industry as a gauge manifold. It’s really just a square drive ratcheting box wrench, usually with several sized built in with 1/4″, 3/16″, 5/16″ and 3/8″ being the most common

There are many purposes for the refrigeration wrench including –

  • Opening and closing acetylene tanks
  • Adjusting TXV superheat
  • Opening & Closing multiposition service valves to the backseat, front seat or neutral seat
  • Opening typical residential HVAC Service valves using a 3/16 (liquid line) &  5/16 (suction line) combo hex key (Like shown below)
  • Adjusting other square refrigeration valve stems

The primary lesson is that whenever you are making an adjustment on a device, tank etc… you want to use a tool that will do the least amount of harm by damaging the stem edges as well as use a tool that will apply the correct amount of force without providing enough torque to break anything.

A refrigeration wrench fits the bill in many applications and in general getting away from adjustable wrenches is a good idea anyway.

— Bryan

 

I was watching this video on the HVACR Videos channel by my friend Chris Stephens and I was impressed by how completely he diagnoses the system both visually and with his tools. Take a look.

I say this all the time, but a good, efficient technician is one who sees everything and can also quickly diagnose the main thing

First off… and this isn’t shown in the video. Chris knows what type of appliance this is, what sequence of operation it runs in and how the controls work. He likely knows all of this because he has a ton of experience on these freezers. If he didn’t then the first course of action would be to do a bit of reading up on the manufacturer’s site or app so that you have the basics before proceeding.

So let’s explore everything Chris found and how he found it

  • Failed door seal
  • Dirty Condenser Coil
  • Frozen Evaporator Coil
  • Defrost Heater inoperable
  • Failed Defrost Limit

The main thing wrong with this machine is a failed defrost limit but Chris was able to diagnose the problem quickly because he used these skills of perception –

  • Cycling a defrost mode (prior knowledge of the equipment or reading the manual)
  • Noticing how quickly the suction line froze (visual and touch)
  • Visual observation of the frozen coil (visual)
  • Looking at the wiring diagram to find the circuit (Diagram reading)
  • Testing for voltage to the defrost heater (electrical diagnosis)
  • Measuring amperage on the heater circuit to see if it is working (electrical diagnosis)
  • Looking at the diagram again and finding a limit inline (diagram Reading)
  • Finding the high limit failed open (electrical diagnosis)

In addition to these efficient diagnosis skills, Chris was thinking about –

  • Proper authorization for repair (getting approval before defrost)
  • Protecting the equipment and workspace (covering the control with a bag)
  • Noting and quoting other repairs needed for optimum performance (condenser coil cleaning, door seal)

These additional diagnosis items for optimum performance took no additional time and will result in a more profitable repair ticket and a piece of equipment that will run better, with lower power consumption and less likelihood of an expensive callback.

You will notice that his first instinct wasn’t to hook up gauges or use fancy tools. In this case, he really didn’t need them and it would have only wasted time. When he returns he will make the repairs and observe temperatures only to gauge the progress of the machine to ensure it’s going to get to and maintain temperature.

In refrigeration, especially small refrigeration like this you need to think about –

  • Coil cleanliness (Condenser & evaporator)
  • Fan and compressor operation (is everything running when it should and going off when it should)
  • Defrost type and function (is it coming in and out of defrost)
  • Product and boxes aren’t in the way of proper condenser or evap airflow
  • Door seals are in good shape
  • Door fits properly and hinges aren’t bent or damaged
  • Box achieves and maintains the proper temperature for the product contained within

Of course, there is a time to connect gauges or probes and refrigeration is no exception, but the best techs use their senses first, knowledge of the equipment or reading second and their tools only once they know exactly what they are looking for.

Great job Chris! and I would suggest subscribing to his channel HERE

— Bryan

We have discussed DTD (Design Temperature Difference) quite a bit for air conditioning applications, but what about refrigeration? Let’s start by defining our terms again

Suction Saturation Temperature

Saturation temperature is the temperature the refrigerant will be at a given pressure if it is in the process of changing state. This change of state would be from liquid to vapor (boiling) in the case of the low side (evaporator / suction line). When we look at saturation temperatures instead of pressures we can use similar rules and we will see similar saturation temperatures across all refrigerants when the application is the same. Experienced HVAC and refrigeration techs pay far closer attention to the saturation temperatures than they do pressures.

Evaporator TD and DTD

Evaporator TD (temperature difference) is the measured difference between the suction saturation temperature (evaporator boiling temperature) and the box temperature. DTD (design temperature difference) is the designed or expected TD.

Delta T

Many A/C techs will confuse TD with Delta T. Delta T is the difference between the evaporator AIR temperature entering the coil to the air temperature leaving the coil. The Delta T will vary based on the humidity in the box where TD will not.

Target Box Temperature 

The temperature the refrigeration box should maintain when the system is operating properly

Superheat

The increase in temperature between the suction saturation temperature and the suction line temperature leaving the evaporator. Superheat is the temperature (sensible heat) gained between the point that all of the liquid boiled off in the evaporator coil and the suction line at the outlet of the coil. in refrigeration, like HVAC 10°F(5.5°K) of superheat  is average with a range from 3°F to 12°F(1.65°K – 6.6°K) depending on the equipment type (10°F(5.5°K) for med temp, 5°F(2.75°K) for low temp, 3°F(1.65°K) for ice machines ).

Hot Pull Down

Refrigeration equipment is unlike HVAC equipment in that the evaporator will spend most of its life running in a very stable environment with minimal fluctuation in the box temperature.

On occasion a refrigeration system will see a huge change in load in cases where it was off and needs to “pull down” the temperature, or when doors are left open or when a large quantity of warm product is placed in the box. When a piece of refrigeration equipment is in hot pull down it cannot be expected to abide by the typical DTD or superheat rules and must be allowed to get near the design box temperature before fine adjustments are made to the charge, TXV superheat settings or to the EPR (Evaporator Pressure Regulator) if there is one.

Design Temperature Difference (DTD)

In air conditioning applications a 35°F DTD is a good guideline for systems that run 400 CFM(679.6 m3/h) of air per ton of cooling (12,000 btu/hr). In refrigeration the DTD is much lower than in air conditioning.

There are several reasons for this but one big reason is the desire to maintain relatively high relative humidity levels in refrigeration to keep from drying out and damaging product. Keep in mind that NOTHING is a substitute from manufacturer’s data but here are some good DTD guidelines for traditional / older refrigeration equipment. Keep in min dthat the trend is toward lower evaporator TD on newer equipment.

Walk-ins  10°F(5.5°K) DTD +/- 3°F(1.65°K)
Reach-ins  20°F(11°K) DTD +/- 5°F(2.75°K)
A/C 35°F(13.75°K)) DTD +/- 5°F(2.75°K)

You then subtract the DTD from your box temperature / return temperature to calculate your target suction saturation. You can then use this target saturation / DTD and compare it to your actual measured saturation and DT once the box is within 5°F – 10°F(2.75°K – 5.5°K) of it’s target temperature to help you set your charge, TXV and EPR as well as diagnose potential airflow issues when compared with suction superheat and subcooling / clear site glass.

For Example –

If you have a medium temp walk-in cooler with a 35°F(1.66°C) box temperature you would expect to see a suction saturation of  25°F +/- 3°F

When doing a quick inspection of a piece of refrigeration equipment without gauges you can use this data to do the following calculation –

35°F – 10°F DT + 10°F superheat = 35°F suction line temperature +/- 3°F 

In this particular case logic tells us that the suction line could be no WARMER than 35°F(1.66°C) because that is the temperature of the air the refrigerant is transferring its heat to. However by the time you factor in the the accuracy of your box thermometer and line thermometer and the assumed saturation temperature you would still expect a 35°F(1.66°C) suction line temperature +/- 3°F(1.65°K)

For a -10°(-23.33°C) box, low temp reach-in you would calculate it this way

-10°F- 20°F DT + 5°F superheat = -25°F suction line temperature +/- 5°F 

Clearly, this is NOT the way to commision a new piece of equipment or to benchmark a system you haven’t worked on before, but it can give you a quick glimpse at the operation of a piece of refrigeration equipment without attaching gauges, especially on critically charged or sealed systems.

The best practice is to know the equipment you are working on, read up on it and properly log benchmark data the first time you work on a piece of equipment or during commissioning.

It should also be noted as Jeremy Smith pointed out, in recent years TD’s have been decreasing as manufacturers seek higher efficiency through higher suction and lower compression ratios.

This means that TD’s as low as 5 can be designed into some units but keep in mind… the suction line can still be no warmer than the box so as DTD drops so does superheat and the critical nature of expansion valve operation.

— Bryan

 

This article is written by Christopher Stephens of JVS Refrigeration in California with just a few additions by me (Bryan) in italics. Thanks, Chris!


Reach in refrigerators are an interesting side of our industry, often looked at as frustrating and troublesome. Often working in kitchens or convenience stores the refrigerators are never located in a convenient place to work on them, and that tends to lead to frustration on the technician’s part. Please understand my article pertains to medium temperature refrigerators. I also advise you to use manufacturers OEM parts when possible as the unit was designed to work with them. One of the more misunderstood and misdiagnosed parts is the temperature controller.

Keep in mind that some refrigeration temperature controls sense the evaporator coil temperature (not the desired box temperature) some use intake air sensors and some use supply air sensors. The medium being sensed (Coil, return air (intake) or supply air (discharge) will greatly impact how the controls function and what impacts them.

Personally, I break temperature controllers down into five different types, please understand that these are generic descriptions and you should always lean on the manufacturer if possible to understand their control strategies. 

  1. Standard Pressure Control – these work on the principle that at any given pressure saturated refrigerant is a constant temperature. This style of control is not used very much anymore as a means of temperature control because it is not very precise and to an untrained technician, it can be hard to set the temperature correctly.To use this control strategy, you need to understand what evaporator T.D. (Temperature differential) your reach-in was designed with, you will need a temperature pressure chart, you will need an accurate set of refrigeration gauges, and an accurate thermometer. With all these tools you can take your desired box temperature and find it on your pressure chart read across the pressure chart and find the corresponding pressure for your desired box temperature and that will be your cut-in pressure to set your control at. Now we need to find the cut-out setting typically we want the system to have about a 5-8 degree differential between the cut out and cut in to reduce system short cycling this will likely be about 8 degrees colder than the cut-in temperature, so take your desired box temperature subtract your differential of 5-8 then subtract your designed evaporator T.D. (specific to the equipment but likely 20 -30 degrees for reach ins) and find that number on the temperature pressure chart than read across the pressure chart and find the corresponding pressure and that will be your cut out setting.  Understand that pressure controls are never exact, so you will need to adjust accordingly in the field.
  2. Constant Cut in Control (electromechanical) – These are one of the most common temperature control’s that you will find in reach in refrigerators because they are the most economical for the manufacturers as they have an off cycle defrost built into them. They work by inserting the sensing bulb into the evaporator coil and they have a set temperature that they turn back on (cut in) at no matter how cold you turn the dial. They work very similar to the pressure control as they are designed with the evaporator T.D. (Temperature Differential) in mind, but instead of using pressure they sense the evaporator temp on the surface of the coil, they do have a knob to adjust the cut-out temperature, but you have no control over the cut in temperature that is why they are called constant cut in. By design they also have a built-in defrost as the cut in temperature is usually 37 to 41 degrees (for a cooler/refrigerator) depending on the manufacturer. They rely heavily on proper superheat and proper refrigerant charge. If the charge is incorrect or the superheat is not correct the coil could get too cold and the control could prematurely shut off. This could lead a technician to diagnose a bad control if they did not understand how they work.  If you come upon a reach in that is short cycling and shutting off too soon, make sure to check the charge and measure the evaporator superheat before you diagnose a bad control.
  1. Constant cut in (Digital) – These work the same as the electromechanical control, but they typically have two probes one to be located in the coil and one to be located in the return air stream. They tend to have more features that are available, such as an added defrost cycle based off time (every four hours, every six hours, etc.…..)  while still using box temperature as a fail-safe. For example, say the control has a defrost every four hours if the coil temperature comes above a pre-determined temp say 40 degrees the control will terminate the defrost. The controls can also shut off the evaporator fan motors during the off cycle to save energy and reduce warm is intrusion into the unit. These types of controls are on many HC (Hydrocarbon) units being built today.
  2. Universal electromechanical – these typically have one sensing bulb that you mount in the return air stream and they turn on and off via the temp setting.
  3. Universal digital – These are usually aftermarket controls and can have several different control strategies and can usually be customized to do anything, from heating to cooling to defrost depending on the manufacturer.

 

Something to understand is that reach in refrigerators is usually designed to perform in a certain environment and if something changes such as the ambient temp in that environment, or if doors are left open. The box will not perform correctly, I suggest you take a step back before you start throwing parts at a reach in and evaluate the environment you may find your problem there!

— Chris

As always I suggest “Commercial Refrigeration for Air Conditioning Technicians” by Dick Wirz as the bible for refrigeration training 

This article is written by Jeremy Smith CM, experience refrigeration tech and all around great dude. Thanks, Jeremy


A very common means of control seen on refrigeration equipment is the pump down control. Why do we use this rather than just cycling the compressor off and on like a residential HVAC unit?

Since most refrigeration equipment tends to be located outdoors, it comes down to ambient temperatures and the basic properties of refrigerant we all understand about temperature and pressure and how they can conspire to kill a compressor.

During periods of low ambient temperatures, if we were to just cycle the compressor off, it can easily get colder at the compressor than it is inside the space.   If the compressor cycles off for long enough as it would during a defrost cycle, refrigerant vapor will start to condense within the crankcase.  If we are lucky, the extent of this problem will be a unit that doesn’t start because the pressure of the refrigerant is lower than the cut in setting of the pressure control.  What typically happens, though, is that enough refrigerant will condense to start to settle under the lubricating oil causing a lack of lubrication on restart leading to bearing wear and premature failure.  If enough refrigerant condenses within the compressor housing, the resulting damage could cause valves, pistons and other internal parts to break if liquid gets into the cylinders.

How can we prevent this?

One thing that is applied across almost all sectors of our industry is crankcase heaters.   These small heaters, either immersion style heaters or wrap around style heaters add a small amount of heat to help keep the compressor oil warm and help to prevent vapor from condensing there. The effectiveness of these are limited by the wattage of the heater, the ambient temperature and the size of the compressor.   Too low an ambient or too large a compressor and they start to lose some effectiveness.

So, how else can we prevent condensation within the compressor?  Let’s look to the pressure/temperature relationship of refrigerant for the answer.   If we lower the pressure in the crankcase to a point where the saturation temperature of the refrigerant is below the ambient temperature the compressor is in, the refrigerant cannot condense.   This is why we use a “pump down” type system.

In operation, a pump down control consists of little more than a liquid line solenoid valve, a thermostat control, and a low-pressure control.   When the thermostat or defrost control opens, the solenoid de-energizes, stopping the refrigerant flow and allowing the system to pump the suction pressure down before the low-pressure control turns the compressor off.

How low should we set that cut-out?   The Heatcraft installation manual has us setting the cut out as low as 1” Hg vacuum, depending on the minimum expected ambient.  I like to set the cut in just below the lowest expected ambient temperature so that you don’t wind up in a situation like I mentioned earlier.   If the ambient gets too low and the cut in is too high, your unit won’t cycle on until it warms up enough resulting in a preventable service call.

Combining a pump down control with a crankcase heater and ensuring that all controls work properly at all times can save your compressor from damage in cold weather.

 

Jeremy Smith, CM

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