Hey guys, so I thought that this would help me write my essay for my Final in my HVAC (Heating Ventilation AC) course that I'm taking. I noticed something interesting today. When I'm writing about something to people on here, I can easily write two paragraphs and not even notice it. But suddenly when it's an assignment for class the thought of writing a 2 page essay double spaced sounds horrible. SO I thought maybe if I write it on here, I could trick my brain into thinking I'm doing this for fun and it'll make this essay a breeze! :D Plus since I know I'm explaining it to people who have no idea about how refrigeration systems work rather then picturing my instructor reading it, it'll make me explain things in great great detail.. not only filling up pages quicker but making sure I write things thoroughly and hell it might even get me a great grade on it. : 3 It just feels better to talk to you guys you know? I love you guys. *crushing hug* Hey look, I already wrote a paragraph, too bad this one doesn't count le sigh.


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Finals Essay: Sequence of Operation For a Refrigeration System
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A refrigeration system is surprisingly simple in design, brilliantly simple actually, the true complexities don't really come through until you look at what's actually going on with the refrigerant as it travels through the system loop, the thermodynamic laws and control of pressures and boiling points that engineers have managed to harness to make these systems work with the best efficiency possible is very fascinating. But with these finely balanced systems comes with a need for HVAC technicians to understand how all these internal systems work together to make sure that outside forces don't cause these systems to go out of whack.


To simplify things, a standard AC system can be broken down into two main sub-groups, the physical refrigerant lines/mechanical systems, and the electrical system which powers and regulates the system's operation. Lets start from the beginning and describe how an AC system activates from the moment the thermostat calls: The thermostat, usually located in the interior of the house, or inside the cooling area of a refrigerator, is what regulates the desired temperature inside the area that is to be cooled (the product) and this temperature is usually set by the user of the system. A thermostat is a simple device usually consisting of a coil of metal connected to an electrical switch, thermal expansion causes the coil of metal in the thermostat to contract or expand based on the surrounding room temperature. Due to this change in size the coil of metal will either make or break an electrical contact, closing the contact when indoor temperature raises above the specified temperature and breaking the contact when the interior temperature has met the desired level.

When the thermostat calls for the system to activate, the circuit is closed and power is sent to the solenoid valve, this is located on the high pressure liquid line of the refrigerant line. The solenoid is an electromagnetic switch that either allows or blocks the refrigerant flow through it, this is a critical component of the "pump down" sequence which we well touch on in a while. As the solenoid is powered the valve opens and allows refrigerant to flow. As refrigerant begins to cycle through the line set of the system and build up pressure the low pressure switch is then tripped, a simple electronic switch that senses fluid pressure in the refrigerant line. Once the pressure switch is tripped it signals the contactor coil, another electromagnetic switch. The contactor coil closes and in doing so slides the main electrical contacts together and closed. This in turn sends power to the heart of the AC system, the compressor.

A standard refrigeration system is made up four main components all connected in a cycled loop by a line-set usually comprised of copper. The starting point of this loop can really be anywhere but for the sake of minimizing confusion we'll start with the compressor, as the compressor activates refrigerant begins to travel to the condenser coil, from there it travels through the copper liquid line to the metering device, from the metering device it travels into the evaporator coil. As it leaves the evaporator coil it enters the suction line and then goes right back into the compressor again to repeat the cycle. We will now talk about each part of the system in detail as well as describe the state of the refrigerant in each part of the system as it passes through it. The compressor is a motorized vapor pump, they come in a variety of designs but they all perform the same function, it is responsible for building up pressure in the refrigerant and forcing it through the entire system, this pressure not only moves the refrigerant through the system, but it also raises the boiling point of the refrigerant. There is an intrinsic link between a liquid's pressure and the temperature at which it will boil. If a liquid is pumped up to a pressure higher then atmospheric pressure, which is 14.7psi at sea level, it's boiling point will raise from 212 degrees onward, if a liquid is put in a vacuum it's boiling point will decrease below the 212 degrees of sea level atmosphere.

As the refrigerant leave the compressor it is a high pressure, high temperature super-heated vapor. The refrigerant is heated from the mechanical insides of the compressor, but most of the refrigerant's heat is from earlier in the system at the evaporator coil. The refrigerant is at the highest pressure it will ever be in its cycle through the system, this is why this segment of the copper line-set is referred to as the 'high side' high pressure line, it is also referred to as the discharge line in reference to being ejected from the output of the compressor.

The superheated vapor now enters the inlet of the condenser coil. It can help to literally think of this as the exhaust system of the refrigeration system, its design is also almost identical to the radiator of an automobile, main refrigerant lines coiling back and forth surrounded by then coils of heat-sink metal fins. An electric fan, switched on at the same time the condenser, forces air through the condenser fins, this draws all of the collected heat from the refrigerant as it passes through, taking the heat collected from inside the house or refrigerator cabinet and expelling it outside. As the refrigerant enters one end of the condenser it is a superheated vapor, roughly halfway through the heat is removed and the temperature decreases until the refrigerant is what is called 'saturated' a mixture of both liquid and vapor. As it exits the outlet of the condenser is has turned into a high-pressure subcooled liquid having expelled most if not all of it's heat, think of hot steam cooling and then condensing into a liquid, that's the same action that is happening in the condenser. At this point as the liquid refrigerant passes through the liquid line, it may be passed through a filter to remove any harmful contaminants that may be in the refrigerant, it may also pass through what is called a sight-glass, which is merely a service device: a port-hole that allows the technician to make sure no air bubbles are in the refrigerant because at this point in the cycle the refrigerant should be %100 liquid. The liquid refrigerant passes through the now open solenoid and into the metering device.

Metering devices come in two flavors, a capillary tube and a TXV or a (Thermal Expansion Valve), capillary tubes or pistons as they're also called are the cheaper and less complex of the two types. It is merely a copper fitting that is large in diameter on the inlet and smaller on the outlet. A TXV is a more advanced option, a mechanical thermostatic valve connected to a remote temperature sensing bulb located on the suction line further down the cycle. This allows the TXV to open or close it's metering iris more or less depending on the state of the refrigerant later in the cycle allowing it properly control the super-heat state of the liquid further down the line. The metering device's purposes is to cause a sudden decrease in pressure. This sudden change to a lower pressure lowers the boiling point of the liquid as we discussed earlier. This causes the refrigerant to "flash" and suddenly boil back into a saturated level refrigerant or a mixture of liquid and vapor just like water boiling in a pot. Now just to clarify, even though the refrigerant is boiling this does not mean that it is hot, in fact the refrigerant is about 40 degrees, refrigerants are made to boil at much lower temperatures then water.

The refrigerant, now a low pressure saturated liquid (a ratio of around %75 liquid %25 vapor) enters the inlet of the evaporator coil. This coil is located inside the building or inside the cooling area of a refrigerator. It's design is nearly identical to the condenser coil though it's form factor may be a different shape usually in the shape of an upwards triangle inside most furnaces in buildings, they are rightly dubbed A-Coils. This radiator coil, again with it's own electric blower fan, draws hot air from inside the area to be cooled. As the hot air passes through the fins of the evaporator, the heat is drawn from the air into the refrigerant in the coil. Naturally heat always travels from a hotter place to a colder place, so refrigerant systems are designed so that the refrigerant entering the the evaporator is always colder then the air in the building. Likewise, at the condenser the liquid within is always warmer then the hot air outside so that the heat is drawn out of the condenser into the outside air.

The process is the reverse of the condenser, the saturated liquid absorbs the heat of the product air, the refrigerant warms until it is a superheated vapor. The air passing through the evaporator is then cold as it comes out the other side, it is then pumped back up into the building or refrigerator cabinet to cool it.

The refrigeration system has been designed in a way that if everything is working correctly, the refrigerant is always in a saturated state when it enters the condenser and evaporator coils. This is because of something called latent heat, latent heat occurs during a state change of a liquid, in this case when water boils and becomes steam or when steam cools and condenses into water. If you ever boil water to its boiling point (112 degrees at sea level) you will notice that as the water starts to boil off it will NOT increase temperature no matter how much heat you apply until it finishes changing state. Refrigeration units exploit this phenomena by making sure that latent state change always occurs as the fluid is passing through the coils, absorbing or expelling heat, this allows the refrigerant to continue to absorb or expel heat the entire time it is traveling through the coils without actually changing temperature. Because as we discussed, the refrigerant has to be either warmer or colder then the air outside for heat transfer to take place.

To continue, as the superheated refrigerant, now full of heat energy collected from the building, leaves the outlet of the evaporator it makes its way through what is called the suction line which leads into the intake of the compressor. It is imperative that the refrigerant has been superheated into %100 vapor by this point as the mechanical compressor is a vapor pump and liquid over time would damage the compressor. This is where metering devices like the aforementioned TXV comes in very handy as it will sense the suction line for the right temperature superheat in the refrigerant, and adjust itself to vary the pressure change to insure that the right superheat is maintained. As the refrigerant enters the compressor it is pressurized again and makes the cycle all over again expelling the buildings heat at the compressor, cycling around to the evaporator to move more heat from the internal air again.


So, now the building or refrigeration area has reached its designated cooled temperature, it's time to cycle the system back down again until the heat raises again. The cold internal air contracts the metal coil inside the thermostat, breaking the connection and causing the thermostat to call for a system shutdown. The liquid solenoid valve in the line-set looses it's power state, the magnetic switch closes and cuts off the refrigerant flow going to the evaporator. This is where the pump down procedure begins. The outgoing refrigerant is blocked as the compressor continues to run and draw the refrigerant from the rest of the system, through the compressor and condenser, and up to the blocked solenoid. This process draws all of the refrigerant from the surrounding system and draws it to one spot in the liquid line at the solenoid valve. As the refrigerant is drawn out of the surrounding system the low pressure switch in the suction line is triggered and shuts down, this breaks the electrical connection at the contactor coil which in turn cuts off power to the compressor and coil fans finally shutting the system down.

The reason for this pump-down procedure is primarily to make sure that the refrigerant is away from the compressor, so that when it kicks back on to cool again, the refrigerant has to go through the metering device and evaporator. This makes sure that no harmful liquid refrigerant is drawn into the compressor at start-up. A refrigerant receiver tank is usually located in the line set between the compressor and condenser in refrigeration units to help hold refrigerant and prevent it from seeping back into the compressor during it's off cycle.


Hopefully this helped you better understand the start-up, shut-down and running operation of the standard AC and refrigeration system.