Tag: pump down

This tip comes from market refrigeration and controls technician Kevin Compass. Thanks Kevin!


A little tip when changing liquid cores. If you start pumping them down begin bypassing discharge gas into the shell to warm it up and to push out the remaining liquid and bring the shell above dew point so it doesn’t sweat when you change the cores.

This helps drive out the liquid refrigerant in the shell and helps prevent moisture contamination from condensation in the shell.

Work quickly so the system is open for the shortest possible time.

Two spring clamps make holding the lid on the cores cake so you can put the bolts back the easy way.

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

 

 

 

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

 

 

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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