The Refrigeration Cycle and How It Works

Gary McCreadie is an HVAC tech, the creator of hvacknowitall.com and the HVAC Know It All Podcast

To set this up correctly, I actually ought to convey up the primary time I put some lackadaisical thought into refrigeration or air con.  I used to be a younger teen strolling by the facet of my house on a scorching summer season day, we had an influence draining, inefficient beast of a window unit that intrusively frolicked the facet of the home, virtually obstructing the stroll manner. 

It was outputting some main warmth and water was dripping from the again onto the bottom.  I walked into the home and it was cooler and fewer humid than it was outdoors.  Sure, I had an epiphany: Was the warmth I felt coming from the gargantuan brown field truly being drawn from my house and was the dripping water being someway siphoned from the indoor air and being transferred outside?

I knew this needed to be true, however this was pre-internet or the web as we now comprehend it right this moment and the information I wanted wasn’t available.  It wasn’t till years later in commerce college that I discovered that my idea was certainly appropriate.  

Earlier than we get began, in case you’re searching for an expert on-line surroundings to debate the commerce, try the subscription based mostly HVAC Know It All app.  

The Refrigeration Cycle is a science, realizing it and realizing it nicely is a present and a curse.  Present, as we service techs know, it’s our bread and butter, the way in which we make a dwelling and supply for our households.  A curse, sure, as we service techs know, when it will get scorching outdoors everybody and their brother in regulation needs or wants a favour.  With a view to perceive the refrigeration cycle we have to perceive how refrigerant acts inside a system.  We additionally want to know how refrigerant reacts when its surrounding temperature adjustments because it pertains to its stress.  

After studying this text try this video explaining the refrigeration cycle with a extra visible expertise

Let’s take R410a, a preferred refrigerant for the time being and take a look at its saturated temperature at 72⁰F.  Discover the stress on the Danfoss RefTools Refrigerant Slider is 207.7 PSI.  If we enhance the saturated temperature of the identical refrigerant to 80⁰F, the stress will increase to 235.7⁰F.  The primary lesson right here is that refrigerant stress will increase with a temperature rise and conversely refrigerant stress decreases with a temperature drop. 

I discussed saturated temperature within the paragraph above so let’s take a look at that and take into account it lesson quantity two.  Saturation is actually the refrigerant’s boiling level, the place it’s a liquid and a vapour on the identical time.  As you possibly can see from the above photos on the Refrigerant Slider, R410a at atmospheric stress boils at -60.60⁰F.  Let’s evaluate this to a boiling level of water.  Water at sea degree will boil at 212⁰F, (100⁰C) as soon as the water reaches its boiling level, it’s a liquid (throughout the pot) and vapour (hovering above the liquid contained in the pot) on the identical time.

The water is at its saturated or boiling temperature.  Utilizing water for instance normally helps to know this idea.  However, water additionally follows the principles of stress/temperature relationship as nicely.  Water will boil at room temperature in a vacuum  at 29.92″hg (inches of mercury).  By inserting a glass of water in an enclosed surroundings after which slowly reducing the stress inside that surroundings by creating a vacuum, the water will boil, however at room temperature, that is proper, it is not scorching.

That is an train that many HVAC/R instructors use to showcase that stress and temperature are associated.  Similar to with refrigerant, as we drop the stress we drop the saturated or boiling temperature of the water, this normally begins the wheels in movement for a lot of college students.  

Try this experiment at house boiling water at room temperature in a vacuum

We won’t transfer on till we communicate briefly about superheat and sub-cooling, these are very important elements to understanding the refrigeration cycle and what’s occurring inside a system.  Superheat and sub-cooling are sometimes used to diagnose techniques and to dial within the refrigerant cost. 

Superheat In A Refrigeration System

Merely put, superheat is something above the saturation level.  Superheat ensures the substance is 100% vapour.  If we take R410a once more as our instance and we take a look at its saturation temperature at 118 PSI, it corresponds to 40⁰F.  Now, this can be a widespread stress/temperature relationship because it pertains to an evaporator for consolation cooling.  We generally seek advice from this because the saturated suction temperature or SST.  If we had been take an precise temperature of the suction line and it was measured at 50⁰F our superheat could be calculated at 10⁰F.  

Precise line temp 50⁰F – SST 40⁰F = Superheat 10⁰F

As we are going to contact on later, superheat within the suction line is required to make sure solely vapour enters the compressor throughout a operating cycle.  

This quick video explains methods to verify evaporator superheat

Sub-cooling In A Refrigeration System

Sub-cooling is the other of superheat, it’s something beneath the saturation level or boiling temperature, sub-cooling ensures the substance is 100% liquid.  A typical saturated condensing temperature in consolation cooling is within the vary of 100⁰F-110⁰F, let’s persist with 100⁰F for this instance.  Utilizing R410a once more, a saturated condensing temperature or SCT of 100⁰F corresponds to 317 PSI.  By taking the precise temperature of the liquid line and obtain a studying of 90⁰F, our sub-cooling for this instance is 10⁰F.

SCT 100⁰F – Precise Line Temp 90⁰F = 10⁰F

Sub-cooling is required within the liquid line to make sure our metering system receives a full column of liquid.  

The 4 Main Elements 

The refrigeration cycle can’t happen with out the next very important parts.  Perceive that many bells and whistles will be thrown into the combo so far as further objects inside a circuit, however for now we are going to concentrate on the 4 main items of the puzzle.  

1) Compressor

Compressors are available many shapes, sizes and kinds however all of them share the identical roll, to facilitate the motion of refrigerant by way of a system.  When the compressor is powered, its job is to take low stress, low temperature refrigerant vapour from the suction line and compress it right into a excessive stress, excessive temperature refrigerant vapour into the discharge line. Compressors are designed to maneuver vapour by way of a system, not liquid. 

Any liquid refrigerant that enters the compressor could cause injury and compressor failure. Most compressors comprise oil, the oil have to be appropriate with the kind of refrigerant the system is using.  The oil is strikes by way of the system with the refrigerant and retains the compressor and system elements lubricated.  Liquid refrigerant inside a compressor can wash away compressor oil, inflicting inner elements to fail. 

Widespread Sorts Of Compressors Are:

• Scroll 
• Reciprocating
• Rotary
• Screw

The time period semi-hermetic merely signifies the compressor just isn’t totally sealed and will be taken aside for service.  A totally airtight compressor or tin can, because it’s generally referred to, is a totally sealed unit and can’t be subject serviced.     

This can be a nice compressor verify up checklist from Danfoss Cool

2) Condenser

The condenser is a warmth rejection system.  The refrigerant system picks up warmth within the evaporator and in addition from the warmth of compression (from the compressor itself) and the condenser rejects or rids of the warmth.  A condenser will be air cooled, by utilizing a fan to maneuver air throughout the coil and fins, or water cooled by utilizing a pump to maneuver water although a coaxial coil, brazed plate warmth exchanger or condenser bundle. 

As a result of the condenser receives superheated refrigerant vapour from the discharge line, its first job is to chill the gasoline by de-superheating it.  As soon as the refrigerant reaches its saturated or boiling temperature, the condensers subsequent job is to sub-cool the refrigerant.  This ensures that because the refrigerant leaves the condenser down liquid line, there’s a full column of liquid making its manner in direction of the metering system.  

Widespread Sorts Of Condensers Are:

• Conventional copper coil w/aluminum fins
• Micro Channel
• Condenser Bundle
• Coaxial Coil
• Brazed Plate Warmth Exchanger 

2) Metering System

A metering system will be adaptive, reminiscent of a thermal enlargement valve or mounted, reminiscent of a capillary tube or mounted orifice.  Its job is to meter the sub-cooled liquid into the evaporator.  The metering system separates the system from the excessive stress facet and the low stress facet. 

Because the sub-cooled liquid passes by way of the metering system, it creates a flash gasoline.  This implies a portion of the liquid is straight away turned or flashed right into a vapour because of the drop in stress.  A rule of thumb is 75% liquid and 25% vapour coming into the evaporator. To Dig a bit deeper into metering units try this text.  

Widespread Sorts Of Metering Gadgets

• Thermal enlargement Valve
• Automated enlargement Valve
• Capillary tube
• Fastened orifice
• Digital Enlargement Valve stepper motor

The way forward for metering refrigerant 

Earlier than we discuss concerning the fourth main element, we must always briefly focus on smart and latent warmth and the variations between them.  

Smart Warmth

Smart warmth is the warmth required to vary the temperature of a substance.  For instance, if we take 80⁰F air and take away 10⁰F in order that air is now 70⁰F, we now have carried out 10⁰F of smart warmth removing.  The temperature considered on a normal thermostat show is an instance of a change in smart warmth.  

Latent Warmth

Latent warmth is the warmth required to vary the state of a substance.  By altering state we imply altering water to a vapour or water vapour again to a liquid, for instance. 

4) Evaporator 

Ah, for me that is the place the magic occurs.  The evaporator’s job is to soak up warmth and take away moisture from the air passing over it, if we’re talking by way of a normal evaporator coil.  A regular coil removes latent warmth and smart warmth from the air passing by.  Latent warmth removing is liable for eradicating moisture or humidity from the air, because the humid air comes into contact with the chilly coil, water vapour clings to it, the vapour adjustments state from a vapour to a liquid, generally known as condensation.  The condensation is collected within the evaporator drain pan and exits through a drain line. As soon as the latent warmth is eliminated (moisture from the air), smart warmth removing is ready to happen at a larger tempo. 

Smart warmth removing, as beforehand mentioned is liable for dropping the temperature of the air.  In chiller techniques, the evaporator absorbs warmth however not moisture, air doesn’t move over such a evaporator.  Chillers use an evaporator bundle, refrigerant and water or glycol enter the bundle and warmth is exchanged.  Chiller bundles can solely carry out smart warmth removing. For this instance we are going to concentrate on a normal air over coil.  

As refrigerant enters the evaporator by way of the metering system, the stress and temperature are diminished.  From our rule of thumb above we used a mix of 75% liquid and 25% vapour coming into the evaporator.  The remaining liquid is boiled off because it absorbs warmth from the air passing over the coil. 

In idea, the entire liquid is boiled off within the evaporator, any added warmth after boiling is further, that further warmth is know as tremendous warmth.  Superheat ensures that solely refrigerant vapour leaves the evaporator and enters suction line.  The compressor brings again the the superheated vapour and re-begins the method.  

Widespread kinds of evaporators are:

• Finned Evaporator (A coil and N coil)
• Evaporator bundle
• Plate evaporator 
• Naked Tube 

Try a brief podcast episode explaining the refrigeration cycle 

The Cycle

Now that we now have hammered down the foremost parts let’s put the cycle collectively as one.  

On name for cooling or refrigeration the compressor begins.  The compressor, performing as a pump will pressurize vapour refrigerant right into a excessive stress excessive temperature gasoline into the discharge line. 

The vapour refrigerant within the discharge line is superheated.  The discharge line feeds refrigerant into the condenser, as refrigerant strikes by way of the condenser the gasoline is cooled to take away the tremendous warmth.  The refrigerant strikes right into a saturated state (liquid and vapour on the identical time) as soon as it’s de-superheated.  At this level, additional warmth is eliminated, the refrigerant turns into a sub-cooled liquid as its temperature is now beneath its boiling or saturation level. 

The sub-cooled liquid enters the liquid line, at this level nonetheless at a excessive stress however diminished temperature.  The sub-cooled liquid strikes by way of the metering system dropping its stress and temperature because it flashes into the evaporator.  As air passes over the evaporator coil, the refrigerant absorbs warmth. 

The absorbed warmth causes the refrigerant attain its saturation or boiling level.  The refrigerant continues to soak up warmth previous its saturation level, the extra warmth superheats the vapour.  The superheated vapour enters the suction line and is introduced again to the compressor for the cycle to repeat itself.  

Key Factors

• The compressor acts as a vapour pump to maneuver refrigerant across the system.  A compressor just isn’t designed to pump liquid.

• The condenser rejects warmth picked up from the system (evaporator and compressor) and is liable for guaranteeing that the refrigerant leaving is a sub-cooled liquid.  

• The metering system regulates liquid refrigerant into the evaporator decreasing the temperature and stress.  It’s flashed into the evaporator as 75% liquid and 25% vapour as a rule of thumb. 

• The evaporator absorbs warmth, from air in a house for instance, boiling off the remaining liquid refrigerant.  The refrigerant picks up further warmth, the added warmth is named superheat.  The superheated vapour strikes into the suction line and again to the compressor.  

Further Elements 

As we now have coated the 4 main parts, there are just a few further parts which can be necessary inside a system we are going to cowl as bonus materials.  

Stress Switches

At minimal, techniques ought to be outfitted with excessive and low stress switches to guard the compressor.  There are a lot of security switches that we might focus on however excessive and low stress minimize out is tremendous necessary to any system.

Excessive Stress Change

The excessive stress swap will be discovered on the discharge line or the liquid line of any given system, it may also be mounted straight on the top of a semi-hermetic compressor.  It displays the system stress and can shut down the compressor on a excessive stress occasion.  Excessive stress occasions will be attributable to a grimy condenser coil, faulty condenser fan and over cost for instance.

The setting of the excessive stress swap will be adjustable or mounted relying on the model of swap that’s utilized.  The setting of the swap depends upon the refrigerant getting used.  Not all refrigerants produce the identical excessive facet stress and have to have completely different settings.  I’ve discovered that the cut-out setting normally sits in vary of 140⁰F to 155⁰F saturated condensing temperature or SCT.  In the event you take a look at the pressures that fall into that temperature vary for any given refrigerant, it is normally reference level for top stress cut-out.   

Low Stress Change

The low stress cut-out swap is mounted on the suction line, and even generally on the compressor physique as within the case with some semi-hermetic compressors.  Simply because the excessive stress swap, low stress switches will be mounted or adjustable.  Additionally once more, the swap setting depends upon the refrigerant and software.  Low stress switches in consolation cooling are normally set round or above a stress that corresponds to freezing.  As an example, R410a at 32⁰F saturation corresponds to a stress of 101.6 PSI. 

To guard the system from freezing up, the swap setting will must be set in and round 101.6 PSI.  When talking with regard to refrigeration, the low stress swap is normally set as much as shut down the system when it reaches set level or after a pump down.  The setting will likely be based mostly on the positioning particular software and refrigerant getting used and should require some thought.  

Low pressures switches typically may even shield the system in a low cost state of affairs.  If for instance the system had a leak and misplaced its cost the low stress swap would open up and never enable the compressor to run.  

Liquid Line Filter Drier

A liquid line filter drier is an important accent to most techniques.  It’s put in within the liquid line as near the metering system as doable.  Its job is to filter out any particles inside a system and take away hint quantities of moisture that is likely to be current utilizing a desiccant materials.  

Liquid Line Sight Glass

A sight glass is an inline system that can be put in within the liquid line, downstream (after) the filter drier.  It offers a sign {that a} full column of liquid is current coming into the metering system and in addition has a moisture indicator that adjustments color within the presence of moisture inside a system.  A liquid line sight glass may give indications that there’s a downside in a system.  

Receiver 

A receiver is a storage system that shops refrigerant within the off-cycle after a pump down or shops refrigerant till it is wanted.  An instance of that is winter operation, in chilly ambient temperatures extra refrigerant is required to pressurize  the system.  The refrigerant could be saved within the receiver in hotter ambient temperatures and because the ambient begins to drop, saved refrigerant is utilized to take care of system stress.  

Liquid Line Solenoid Valve

A liquid line solenoid valve is one other inline element that’s put in within the liquid line.  It’s used most often to pump down a system.  When the valve is compelled closed, the compressor continues to run, the refrigerant is pumped into the condenser/receiver.  Because the evaporator and suction line pressures drop, the low stress swap will open chopping operation to the compressor.  When the valve is opened throughout a name, the evaporator and suction line are pressurized by an inrush of refrigerant.  The low stress swap closes and the compressor re-initiates operation.  

This picture from the Danfoss RefTools App Troublehooter reveals a liquid line filter drier, sightglass and solenoid valve in sequence with movement transferring from proper to left

This can be a nice animation from Danfoss to point out how the pump down cycle works

The refrigeration cycle is a science that entails many elements and variables.  Realizing the cycle nicely, will guarantee success throughout service, set up and upkeep.  

Gary McCreadie

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