Pressure
We have a saying in refrigeration: We manipulate pressure to change temperature.
There is a condition that changes the boiling and condensing temperatures of substances. It is pressure.
If we change the pressure on a gas or liquid, we change the boiling point of the substance.
Take the pressure cooker. It cooks food faster than an open pot. Why? Because when the water is heated to the boiling point, it is enclosed so there is no place for the steam to go. So pressure goes up. As the pressure goes up, the boiling point goes up. Pressure cookers have a relief valve to maintain a prescribed pressure inside. If the pressure is 5 psig, the boiling point of water will be 227F (108C). The food cooks faster because the temperature is higher.
If we manipulate pressures on a liquid or gas, we can bring the boiling point below the ambient temperature so heat can be absorbed from the conditioned space or substance and the liquid is boiled so the heat goes into the gas.
Then, if we increase the pressure on the gas we can raise the condensing point to above the ambient temperature of the area we want to remove the heat to. Then with that temperature difference, the gas will condense to a liquid and the heat will be released.
So now lets see how this is physically done.
If we change the pressure on a gas or liquid, we change the boiling point of the substance.
Take the pressure cooker. It cooks food faster than an open pot. Why? Because when the water is heated to the boiling point, it is enclosed so there is no place for the steam to go. So pressure goes up. As the pressure goes up, the boiling point goes up. Pressure cookers have a relief valve to maintain a prescribed pressure inside. If the pressure is 5 psig, the boiling point of water will be 227F (108C). The food cooks faster because the temperature is higher.
If we manipulate pressures on a liquid or gas, we can bring the boiling point below the ambient temperature so heat can be absorbed from the conditioned space or substance and the liquid is boiled so the heat goes into the gas.
Then, if we increase the pressure on the gas we can raise the condensing point to above the ambient temperature of the area we want to remove the heat to. Then with that temperature difference, the gas will condense to a liquid and the heat will be released.
So now lets see how this is physically done.
The refrigeration cycle
There are 4 parts that are necessary to make the cycle work. The compressor, evaporator, condenser, expansion device.
We will start with the Evaporator. Depending on the medium being cooled, there are many designs for this part. We will start with a coil that cools air.
We will start with the Evaporator. Depending on the medium being cooled, there are many designs for this part. We will start with a coil that cools air.
The coil above is an air conditioning coil. It consists of a large number of copper pipes with aluminum fins that increase the surface area exposed to the air passing thru. This coil is capable of reducing the temperature of the air passing thru it by 20 to 30 degreesF. If the air passing across this coil is 80F (26.6C), and the coil has refrigerant inside that is boiling at 40F (4.4C) the air coming out of the coil could be 50F (10C). The coil has exposed its colder refrigerant to the hotter air passing thru and latent heat is absorbed by the boiling refrigerant.
The boiled gas contains the heat from the air that was passing thru. Then the gas passes to the compressor.
The compressor usually consists of a motor-compressor assembly. The compressor increases the pressure of the gas.
The boiled gas contains the heat from the air that was passing thru. Then the gas passes to the compressor.
The compressor usually consists of a motor-compressor assembly. The compressor increases the pressure of the gas.
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On the left is a motor compressor assembly. It draws in cool air from the evaporator, compresses it to a higher pressure and temperature. The discharge of the compressor is called "hot gas". The gas contains the latent heat from the evaporator and the sensible heat of compression from the compressor operation.
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As the hot gas moves out of the compressor, it goes to the condenser. The condenser is very similar to the evaporator in design. Here, the hot gas, which is at a higher temperature than the surrounding air, loses its heat, both latent from the evaporator and sensible from the compressor to the outside air.
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The last component is the expansion device. This part acts as a separation between the low pressure side and the high pressure side.
If there was no restriction between the high side (head pressure) and the low side (suction pressure), this system would simply pump refrigerant around the piping. The restriction restricts the flow to the point that the low side pressure reduces the boiling point of the refrigerant to below the temperature of the air passing across the evaporator. At the same time, refrigerant backs up behind the expansion device and raises the boiling point of the refrigerant to above the air passing across the condenser. There are several types of expansion devices. It could be as simple as a small hole that restricts flow just due to its size to more complicated types that use the temperature of the gas as it leaves the evaporator to control flow. On the left is a an expansion device of this type. |
When the refrigeration system is at rest, the pressure in the system will be at the saturated temp-pressure for the type of refrigerant and the ambient/saturated-pressure-temperature.html temperature. Once the compressor starts, the refrigerant begins moving thru the system. Because of that expansion device discussed above, the pressure will rise in the condenser and, because the compressor is drawing gas from the evaporator the pressure will drop in the evaporator The video below shows how the refrigerant moves thru the system.