HVACR - Basic Refrigeration

List the steps you must take as an AC technician to MANUALLY pump down a system.

1) Front seat the discharge service valve. 2) NEARLY but not completely front seat the suction service valve. 3) Run the system so that the compressor can pump the refrigerant to the high side. 4) Watch your low side gauge closely. Make sure to stop the system when the low side gauge reads 2 psig. 5) Now you can fully front seat the suction side service valve.

What are the two most harmful contaminants that can enter into a refrigeration system?

1) Moisture - can mix with oil to create acid and sludge which damage copper piping and compressor parts and windings. It can also freeze up at the metering device causing a restriction. 2) Air - migrates to the condenser taking up coil surface area, meaning less heat can be rejected.

List 3 compressor case designs

1) Open 2) Semi-hermetic 3) Hermetic

List 5 types of compressors

1) Reciprocating(piston) 2) Rotary 3) Scroll 4) Screw 5) Centrifugal

List the steps the AC system goes through, in order, on start up with automatic pumpdown.

1) Tstat calls for cooling. 2) Makes R-Y then R-G 3) Y energizes the liquid line solenoid valve coil, which opens the valve allowing refrigerant through to the low side. 4) G energizes the fan relay coil. 5) Once enough refrigerant pressure is in the low side, the LPC will close allowing power to travel to the HPC and on to the contactor coil.

List the steps the AC system goes through, in order, before shutting off with automatic pumpdown.

1) Tstat satisfies R-Y and de-energizes the liquid line solenoid valve coil which closes the valve so that refrigerant cannot be pumped into the low side. 2) Compressor continues to pump refrigerant to the high side into the liquid receiver. 3) Once the suction side pressure goes below the LPC cut out set point, the LPC opens which de-energizes the contactor coil.

At sea level, what would atmospheric pressure equal (in psia)?

14.7 psia

Given an ambient air temp of 75 degrees, flowing across an older 10 SEER system condenser coil designed to run with a 30 degree condenser split. What should the high side pressure be if the system is using R-22 refrigerant?

211 psig If our condenser split is designed for 30 degrees and the ambient temp outside is 75...then 75 + 30 = 105 degrees. The refrigerant needs to be 105 degrees in the condenser in order to be 30 degrees warmer than the outside air. Next we look up R-22 on a pressure/temp chart and find the pressure that corresponds to 105 degrees equals 211 psig.

What is the recommended TD (temperature difference/condenser split) for an older 10 SEER unit?

30 degrees. The temp of the refrigerant in the condenser should be 30 degrees warmer than ambient air (air around the condensing unit).

Given an ambient temperature of 90 degrees, flowing across a high efficiency 16 SEER condenser coil designed to operate with a 15 degree condenser split. What should the high side pressure be if the system is using R-410a?

340 psig If our condenser split is designed for 15 degrees and the ambient temp outside is 90...then 90 + 15 = 105 degrees. The refrigerant needs to be 105 degrees in the condenser in order to be 15 degrees warmer than the outside ambient air. Next we look up R-410a on a pressure/temp chart and find the pressure that corresponds to 105 degrees, which equals 340 psig.

What is the recommended TD (temperature difference/condenser split) of a high efficiency 20 SEER unit?

8 degrees. The temp of the refrigerant in the condenser should only be 8 degrees warmer than ambient air (air around the condensing unit). Since high efficiency units use such a low TD to save energy, they need much more condenser coil surface area to be able to reject the necessary amount of heat. This is why newer units are so much larger than older units.

If you wanted to convert 1 pound of water to 1 pound of steam, how many BTUs would be required?

970 BTUs

What is a positive displacement compressor?

A compressor that pulls vapor into an empty space, then replaces that empty space with something physical (like a piston). Reciprocating, Rotary, Scroll, and Screw type compressors use positive displacement.

Referring to refrigerant, what is a saturated state?

A saturated state is when the refrigerant exists as both a liquid and a vapor.

Assume we're using an R-400 series refrigerant. After we pull a vacuum, why do we add liquid refrigerant to the high side with the system off?

After pulling a vacuum, running the system could damage the compressor. We must add R-400 series refrigerants as liquid to ensure the proper chemical formula is added. Since there's currently extremely low pressure in the system, high pressure liquid will quickly rush into the low pressure environment of the piping. We add to the high side because we can add liquid quickly without worrying about damaging the compressor.

Why do we turn R-404a refrigerant cylinders upside down when we want to charge refrigerant?

Any refrigerants in the R-400 series are blended zeotropic refrigerants. In R-404a specifically, there are 3 different refrigerants blended together that all have different boiling and condensing points. As a liquid mixture, they are in the proper design ratio and will behave according to R-404a specifications. In vapor form however, there will be disproportionate amounts of each refrigerant since they don't all vaporize at the same pressure/temperature. We need to turn any R-400 series cylinder that doesn't have a dip tube(check cylinder label or look for arrows) upside down to make sure we only add it as a liquid, thus ensuring we are adding the proper amount of each refrigerant in the blend and maintaining the refrigerant's intended functionality and predictability.

What is an Azeotropic Refrigerant?

Azeotropic refrigerants are blended/made up of two or more refrigerants that all have the SAME boiling and condensing points. The R-500 series of refrigerants are azeotropic refrigerants.

What is a BTU?

BTU is short for British Thermal Unit. It's the amount of heat required to raise 1 pound of water 1 degree (F).

What is Temperature Glide?

Blended Zeotropic refrigerants are made up of two or more refrigerants that have different boiling and condensing points. The differences in the boiling and condensing points of the refrigerants in the blend is called "glide" or "temperature glide"

What is a non-positive displacement compressor?

Centrifugal compressors use non-positive displacement to increase the pressure of vapor. Kinetic energy is turned into pressure energy. A spinning impeller throws the vapor outward against the cylinder squeezing it against the wall.

Which component removes heat from the refrigeration system?

Condenser

Where do we find subcooling in refrigeration systems?

Condenser Subcooling - should take place from roughly the middle of the condenser to the outlet of the condenser. System/Total Subcooling - should take place from roughly the middle of the condenser all the way up to the inlet of the metering device.

What is Delta T?

Delta T is commonly used in residential AC to measure how well the evaporator is cooling the air. To measure Delta T, you find the temp of the return air about to flow across the evaporator, and subtract the temp of the air leaving the evaporator. Ex: The return air temp about to hit the evaporator = 80 degrees, and the air temp leaving the evaporator = 60 degrees. 80 minus 60 = Delta T of 20 degrees.

Which component absorbs heat into the refrigeration system?

Evaporator

Where do we find superheat in refrigeration systems?

Evaporator Superheat - should take place from roughly the middle of the evaporator to the outlet of the evaporator. System/Total Superheat - should take place from roughly the middle of the evaporator all the way up to the inlet of the compressor.

What is Evaporator Temperature Difference(TD)?

Evaporator TD is the difference in temp between the refrigerant in the evaporator coil versus the return air flowing across the coil. Ex: A residential system might be designed with a 35 degree evap TD(the refrigerant will be 35 degrees cooler than the return air flowing across it).

What two forces are trying to close the TXV valve?

Evaporator pressure and Spring pressure

What is Sensible Heat?

Heat that can be felt/sensed or measured.

Assume we're using an R-400 series refrigerant, the system is undercharged and needs a bit more added. When there's already refrigerant in the system, why do we add liquid refrigerant to the low side with the system running?

If the system is undercharged and needs a bit more, we need to run the system to get high and low side pressures to differentiate. Depending on the system components in place, the high and low side pressures will try to equalize when the system is off. Since we know high pressure will only move to low pressure, we need to add our high pressure refrigerant to the low pressure side of the system. Since we must add R-400 refrigerants as liquid, we must add them very slowly and carefully, so the liquid will have a chance to vaporize before getting to the compressor.

Why is the low side gauge on a refrigeration manifold called a compound gauge? What does it measure?

It's called a compound gauge because it measures more than 1 thing. It measures pressure and vacuum.

What is Latent Heat?

It's the heat required to change the state of a substance. (Also called "hidden heat"). Ex: The amount of heat absorbed by the liquid refrigerant inside the evaporator coil that causes the liquid to boil into a vapor. No sensible temperature change can be measured by a thermometer while the refrigerant is changing states.

If there's too much refrigerant in the evaporator coil(flooded evaporator) will superheat be high or low?

Low. Too much refrigerant in the evaporator means it will take longer for all of the liquid refrigerant to fully vaporize. The further down the evaporator coil the liquid gets without fully vaporizing, the less time there is for the refrigerant to have measurable/sensible heat (superheat) added.

When energized, will the liquid line solenoid valve coil open or close its valve?

Open.

What is a partial vacuum?

Pressure less than atmospheric pressure.

What are the only two substances that should be circulating through a normally operating refrigeration system?

Refrigerant and the proper oil for that refrigerant.

Why don't most compressor manufacturers want us to run their compressors when the system is in a vacuum?

Running a compressor while in a vacuum may cause arcing inside the compressor windings and damage the compressor.

What is the state of refrigerant in the middle of the evaporator coil, in a normally operating residential system?

Saturated. Right around the middle of the evaporator coil is where the refrigerant will begin to change state, thus there should be both liquid and vapor present.

What type of metering device can maintain the proper amount of superheat under small load variations?

TXV/TEV - Thermostatic Expansion Valve TXVs CANNOT maintain proper superheat under very high or very low loads. Ex: Very dirty filters and condenser coils, iced evaporator coils, low air flow, hot pull down, and a low charge or leaks will cause too much load variation for a TXV to maintain proper superheat.

What is the state of refrigerant after leaving the TXV/TEV on its way to the evaporator?

Technically, it's saturated. Roughly 75% liquid and 25% vapor(flash gas). After the TXV - Because of the dramatic pressure/temperature drop, as well as the fact that the liquid droplets are spaced further away from one another, it's easy for some of the low temp liquid droplets to absorb enough heat to vaporize before even making it to the evaporator.

What is Superheat

The amount of heat added to vapor refrigerant above its saturation point. Once the liquid refrigerant has fully evaporated/boiled into a vapor, any further heat added to the vapor refrigerant will raise it's measurable temperature. That increase in measurable temperature is called Superheat.

What is Subcooling

The amount of heat removed from liquid refrigerant below it's saturation point. Once vapor refrigerant has fully condensed, any further heat removed from the liquid refrigerant will lower it's measurable temperature. That decrease in measurable temperature is called Subcooling.

What is meant by Condenser Split or Condenser Temperature Difference(TD)?

The difference in temperature between the refrigerant in the condenser versus the ambient air flowing across the condenser. Ex: An older 10 SEER system using R-22 will likely have a temperature difference/condenser split of 30 degrees(The refrigerant in the condenser will be 30 degrees warmer than ambient air).

Why does Evaporator Temperature Difference(TD) matter?

The greater the degree of evaporator temperature difference, the greater the amount of moisture that gets condensed out of the air, thus providing for less humidity in the space. Comfort cooling needs less humidity, while preventing food spoilage needs more humidity. Ex: Residential AC = 35 degree TD = 50% humidity Reach-in cooler = 20 degree TD = 65% humidity Walk-in freezer = 10 degree TD = 85% humidity

What is ambient temperature?

The temperature surrounding us/the equipment in question. Ex: When calculating condenser split, ambient temperature is simply the outside air temperature that is pulled across the condenser coils by the fan.

What does Enthalpy refer to in HVACR?

The total heat content of a system. Sensible Heat + Latent Heat = Enthalpy

If the evaporator temp of the unit you're servicing is 50 degrees when operating normally, is this a high temp, medium temp, or low temp system?

This is a high temp system (high temp systems have evap temps between 47-60 degrees). A high temp system would be a comfort cooling AC system.

Why do we "pull a vacuum" - evacuate a system?

To dehydrate the system. Lowering the pressure to 500 microns ensures all moisture is boiled out of the system.

If the condenser is very dirty, will your pressures go up or down?

Up. Dirt on the condenser makes it difficult to reject heat to the ambient air. Too much heat will remain in the system which will directly increase the pressure.

What is a refrigerant's Bubble Point?

When dealing with zeotropic blends of refrigerant(such as R-404a, which has three different refrigerants in it), the different refrigerants in the blend will have different boiling points. The point at which the first refrigerant begins to boil is called the "Bubble Point"

What is a refrigerant's Dew Point?

When dealing with zeotropic blends of refrigerant(such as R-404a, which has three different refrigerants in it), the different refrigerants in the blend will have different condensing points. The point at which the first refrigerant begins to condense is called the "Dew Point"

When is the ONLY time the discharge pressure will be too low while the suction pressure is too high?

When the compressor is inefficient.

What is a Zeotropic Refrigerant?

Zeotropic refrigerants are blended/made up of two or more individual refrigerants that all have DIFFERENT boiling and condensing points. The R-400 series of refrigerants are zeotropic refrigerants.