Part 2

     

Main Components

There are 4 main components in a mechanical refrigeration system. Any components beyond these basic 4 are called accessories. The compressor is a vapour compression pump which uses pistons or some other method to compress the refrigerant gas and send it on it's way to the condenser. The condenser is a heat exchanger which removes heat from the hot compressed gas and allows it to condense into a liquid. The liquid refrigerant is then routed to the metering device. This device restricts the flow by forcing the refrigerant to go through a small hole which causes a pressure drop. And what did we say happens to a liquid when the pressure drops? If you said it lowers the boiling point and makes it easier to evaporate, then you are correct. And what happens when a liquid evaporates? Didn't we agree that the liquid will absorb heat from the surrounding area? This is indeed the case and you now know how refrigeration works. This component where the evaporation takes place is called the evaporator. The refrigerant is then routed back to the compressor to complete the cycle. The refrigerant is used over and over again absorbing heat from one area and relocating it to another. Remember the definition of refrigeration? (the removal and relocation of heat)

Heat Transfer Rates

One thing that we would like to optimize in the refrigeration loop is the rate of heat transfer. Materials like copper and aluminium are used because they have very good thermal conductivity. In other words heat can travel through them easily. Increasing surface area is another way to improve heat transfer. Have you noticed that small engines have cooling fins formed into the casting around the piston area? This is an example of increasing the surface area in order to increase the heat transfer rate. The hot engine can more easily reject the unwanted heat through the large surface area of the fins exposed to the passing air. Refrigeration heat transfer devices such as air cooled condensers and evaporators are often made out of copper pipes with aluminium fins and further enhanced with fans to force air through the fins.

Metering Device

We will now take a closer look at the individual components of the system. We will start with the metering device. There are several types but all perform the same general function which is to cause a pressure drop. There should be a full column of high pressure liquid refrigerant (in the liquid line) supplying the inlet of the metering device. When it is forced to go through a small orifice it loses a lot of the pressure it had on the upstream side of the device. The liquid refrigerant is sort of misted into the evaporator. So not only is the pressure reduced, the surface area of the liquid is vastly increased. It is hard to try and light a log with a match but chop the log into toothpick sized slivers and the pile will go up in smoke easily. The surface area of zillions of liquid droplets is much greater than the surface area of the column of liquid in the pipe feeding the metering device. The device has this name because it meters the flow of refrigerant into the evaporator. The next graphic shows a capillary line metering device. This is a long small tube which has an inside diameter much smaller than a pencil lead. You can imagine the large pressure drop when the liquid from a 1/4 or 3/8 in or larger pipe is forced to go through such a small opening. The capillary line has no moving parts and can not respond to changing conditions like a changing thermal load on the evaporator. Some labels have been added showing the names of some of the pipes.

The Magic of Latent Heat

The Evaporator

The metering device has sprayed low pressure droplets of refrigerant into the evaporator. The evaporator could be the forced air type and could be constructed of many copper tubes which conduct heat well. To further enhance heat transfer the pipes could have aluminium fins pressed onto them. This vastly increases the surface area that is exposed to the air. And this type of evaporator could have a fan motor sucking air through the fins. The evaporator would be capable of reducing the temperature of air passing through the fins and this is a prime example of the refrigeration effect. If that evaporator was located in a walk in cooler, the air would be blown out into the box and would pick up heat from the product; let's say it is a room full of eggs. The flow of heat would be egg core/egg shell/circulating air/aluminium fins/copper evaporator pipe/liquid droplet of refrigerant. The droplet of refrigerant has the capability of absorbing a large quantity of heat because it is under conditions where it is just about ready to change state into a gas. We have lowered it's pressure, we have increased surface areas and now we are adding heat to it. Just like water, refrigerants also have ratings for Latent Heats of vapourization in BTU's per lb. When heat is picked up from theair stream, the air is by definition cooled and is blown back out into the box to take another pass over the eggs and pick up more heat. This process continues until the eggs are cooled to the desired temperature and then the refrigeration system shuts off and rests. But what about our droplet of refrigerant. By now it might have picked up so much heat that it just couldn't stand it anymore and it has evaporated into a gas. It has served it's purpose and is subjected to a suction coming from the outlet pipe of the evaporator. This pipe is conveniently called the suction line. Our little quantity of gas joins lots of other former droplets and they all continue on their merry way to their next destination.

The Compressor

The compressor performs 2 functions. It compresses the gas (which now contains heat from the eggs) and it moves the refrigerant around the loop so it can perform it's function over and over again. We want to compress it because that is the first step in forcing the gas to go back into a liquid form. This compression process unfortunately adds some more heat to the gas but at least this process is also conveniently named; The Heat of Compression. The graphic shows a reciprocating compressor which means that it has piston(s) that go up and down. On the down stroke refrigerant vapour is drawn into the cylinder. On the upstroke those vapours are compressed. There are thin valves that act like check valves and keep the vapours from going back where they came from. They open and close in response to the refrigerant pressures being exerted on them by the action of the piston. The hot compressed gas is discharged out the...you guessed it; discharge line. It continues towards the last main component.

The Condenser

The condenser is similar in appearance to the evaporator. It utilizes the principles to effect heat transfer as the evaporator does. However, this time the purpose is to reject heat so that the refrigerant gas can condense back into a liquid in preparation for a return trip to the evaporator. If the hot compressed gas was at 135 °F and the air being sucked through the condenser fins was at 90 °F heat will flow downhill like a ball wants to roll down an inclined plane and be rejected into the air stream. Heat will have been removed from one place and relocated to another as the definition of refrigeration describes. As long as the compressor is running it will impose a force on the refrigerant to continue circulating around the loop and continue removing heat from one location and rejecting it into another area.

Superheat and Slugging

There is another very common type of metering device called a TX Valve. It's full name is Thermostatic Expansion Valve and you might be thankful to know that its' short form is TXV. (It can also be called TEV) This valve has the additional capability of modulating the refrigerant flow. This is a nice feature because if the load on the evaporator changes the valve can respond to the change and increase or decrease the flow accordingly. The next graphic shows this type of metering device and you will note that another component has been added along with it.

The TXV has a sensing bulb attached to the outlet of the evaporator. This bulb senses the suction line temperature and sends a signal to the TXV allowing it to adjust the flow rate. This is important because if not all the refrigerant in the evaporator changes state into a gas, there would be liquid refrigerant content returning down the suction line to the compressor. That could be disastrous to the compressor. A liquid can not be compressed and if a compressor tries to compress a liquid something is going to break and it's not going to be the liquid. The compressor can suffer catastrophic mechanical damage. This unwanted situation is called liquid slugging The flow rate through a TXV is set so that not only is all the liquid hopefully changed to a gas, but there is an additional 10 °F safety margin to insure that all the liquid is changed to a gas. This is called Superheat. At a given temperature any liquid and vapour combination will always be at a specific pressure. There are charts of this relationship called PT Charts which stands for Pressure/Temperature Chart. If all the liquid droplets in an evaporator have changed state into a gas, and they still have 1/4 of the evaporator remaining to travel through, this gas will pick up more heat from the load being imposed on the evaporator and even though it is at the same pressure, it will become hotter than the PT Chart says it should be. This heat increase over and above the normal PT relationship is called superheat. It can only take place when there is no liquid in the immediate area and this phenomena is used to create an insurance policy of sorts. Usually TXV's are set to maintain 10 °F of superheat and by definition that means that the gas returning to the compressor is several degrees away from the risk of having any liquid content. A compressor is a vapour compression pump and must not attempt to compress liquid.

That extra component that got added in along with the TX Valve is called a receiver. When the TXV reduces the flow there has to be somewhere for the unneeded refrigerant to go and the receiver is it. Note that there is a dip tube in the outlet side to insure that liquid is what is fed into the liquid line. Liquid must be provided to the TXV not a mixture of liquid and gas. The basic premise is to change a liquid to a gas so you don't want to waste any of the evaporator's capacity by injecting useless vapour into it. The line that comes from the condenser and goes to the receiver is also given a name. It's called the condensate line.

Accessories

Even though there are only 4 basic components to a refrigeration system there are numerous accessories that can be added. The next graphic shows a liquid line filter and a sight glass. The filter catches unwanted particles such as welding slag, copper chips and other unwanted debris and keeps it from clogging up important devices such as TX Valves. It has another function as well. It contains a desiccant which can absorb a minute quantity of water. (a mere drop or two) Hopefully a proper evacuation removed all the air and moisture content during the installation of the equipment.

The sight glass is a viewing window which allows a mechanic to see if a full column of liquid refrigerant is present in the liquid line.

Earlier we discussed heat transfer rates and mentioned surface area as one of the factors. Let's put some fins on our condenser and evaporator. While we are at it lets also add a couple of fan motors to move air through those fins. They are conveniently called the condenser fan motor and evaporator fan motor.