Step 1: Evaporator

The vapor compression cycle is the primary cycle used in commercial refrigeration systems.

The Vapor Compression Cycle for the Mechanical PE Exam

The vapor compression cycle starts at (Step 1) the evaporator, with cold, low-pressure, liquid refrigerant. It absorbs heat and evaporates to a low-pressure gas. Then the gas is (Step 2) Compressed to a high-pressure, high-temperature gas and (Step 3) condensed to a high pressure gas. Finally, the gas is condensed at the (Step 4) expansion device to a cold, low-pressure liquid refrigerant.

Step 1: Evaporator. The first step in the vapor compression cycle is the evaporator, which can also be called a liquid cooler. The evaporator is simply a heat exchanger. Heat is exchanged from the warm medium (air or water) to the cold, liquid refrigerant. The heat gained by the liquid refrigerant causes it to change phases to a refrigerant gas. The refrigerant liquid gains the heat necessary to overcome the latent heat of evaporation, in order to change to a gas. There are two types of evaporators, (1) an air cooled evaporator and (2) a water cooled evaporator. Figure below shows the (1) air cooled evaporator which is most commonly referred to as a direct expansion system. In this evaporator, warm air from an air conditioned space is cooled and redistributed to the space. Also shown in the figure below is the water cooled system, where chilled water return is cooled and supplied to the chilled water distribution system.

Evaporator for the Mechanical PE Exam

The most common system is the direct expansion system. This system is prevalent throughout smaller systems, like those serving residential systems. In this system, the hot air from the space is used to directly evaporate the refrigerant to a hot gas. Note that the hot air from the space is roughly ~75 °F and the refrigerant liquid is typically 40 °F. The 75 °F room air is cooled down to ~55 °F and then distributed back to the space. In a water-cooled system, which is more common for larger commercial systems, chilled water typically at 55 °F is cooled by the evaporator down to ~45 °F. The colder chilled water is then supplied to another heat exchanger, where air is cooled and then distributed to the space.

Besides the two different types of evaporator systems, there are also different types of heat exchangers used in refrigeration. The most common heat exchangers include: (1) Shell and Tube, (2) Tube in Tube and (3) Brazed Plate.

(1) Shell and Tube: This heat exchanger is the most common and consists of copper pipes arranged in a coil that is constructed in a cylindrical shell. One fluid is provided in the shell and contacts the outer surface of the inner tubes. Another fluid is contained inside of the tubes. Heat exchange occurs in the shell at the outer surface of the tubes. Often times aluminum fins are provided on the copper pipes. These fins provide more surface area for heat exchange to occur.

(2) Tube in Tube: A tube is constructed in a tube, sealed separately to keep the fluids in one tube from contaminating the other. Heat exchange is conducted at the outer surface of the inner tube and the inner surface of the outer tube.

(3) Brazed Plate: This type of heat exchanger consists of multiple thin plates separated by a small distance. Each plate either carries the hot or cold fluid. Heat exchange occurs between the surface areas of each plate.

As previously mentioned the evaporator acts as a heat exchanger with a cold side and a hot side. The cold side consists of a mixture of refrigerant gas and liquid. At this point, the partial liquid-gas refrigerant mixture moves through the evaporator, picking up heat from the hot side. But instead of heating the gas, the heat is used to boil the remaining liquid. It is important for the evaporator to boil all of the liquid, prior to the refrigerant entering the compressor in the following step. Once all the liquid has boiled, the liquid-gas mixture turns into a refrigerant gas (vapor), called a saturated vapor. Any additional heat will now increase the temperature of the refrigerant vapor, into a region called super heat. Any release in heat will cause some of the gas to condense back to a liquid.

It is important for the engineer to understand that the amount of cooling provided through the evaporation of the refrigerant liquid is much more than simply increasing the temperature of the refrigerant liquid. For example, R-134a takes 92.82 Btu of heat to change 1 lb of refrigerant from liquid to gas. While it takes 0.204 Btu of heat to increase 1 lb of refrigerant gas by 1°F.