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The compressor is considered the heart of the refrigeration system. The best term that describes a compressor is vapor pump because it increases the suction pressure level to the discharge pressure level. In a low-temperature, R-404A refrigerant system the suction pressure may be 21 psig and the discharge pressure may be 236 psig. The compressor increases the pressure 215 psig (236 - 21 = 215). It is also possible for a system to have a different increase in pressure. A medium-temperature system may have a suction pressure of 56 psig with a discharge pressure of 236 psig. This system has an increase of 180 psig (236 - 56 = 180).
There is a common technical expression for pressure difference when it comes to sizing compressors, Compression ratio. It is the high-side absolute pressure divided by the low-side absolute pressure. Absolute pressures in contrast to gauge pressures are used when we are looking to determine the compression ratios in order to keep the calculated compression ratio from becoming a negative number. Absolute pressures keep compression ratios positive and meaningful. For example, when a compressor is operating with R-404A, a head pressure of 331.8 psig (125°F), and a suction pressure of 19.6 psig (216°F), the compression ratio would be:
A compression ratio of 10.1:1 would indicate that the absolute or true discharge pressure is 10.1 times as great as the absolute suction pressure. Assume now that this same refrigeration system is operating with R-134a as the refrigerant. With the same condensing temperature of 125°F (184.6 psig) and an evaporating temperature of 216°F (0.7 in. Hg vacuum), the compression ratio would be:
Notice that the compression ratio for the R-134a system is higher than that for the R-404A system even though both systems have the same condensing and evaporating temperatures of 125°F and 216°F, respectively. The compression ratio for the R-134a system is higher because of the higher condensing pressure and lower evaporating pressure associated with R-134a at these temperature ranges. Either an increase in head pressure or a decrease in suction pressure will cause higher compression ratios. Any time a system has high compression ratios the compressor&#;s discharge temperature will also be elevated. This is because of the higher heat of compression during the compression stroke associated with higher compression ratios. When compression ratios become higher, more energy is needed in order to raise the pressure of the suction gases compared to the pressure of the discharge side. Due to that more heat is generated as the heat of compression during the compression stroke.
The compression ratio is used to compare compressors. When the compression ratio becomes too high (more than 12:1 for a hermetic reciprocating compressor) the refrigerant gas temperature at the outlet of the compressor rises to the point that oil for lubrication may become overheated. Overheated oil will come in return into carbon and create acid in the system. It is possible to overcome through two-stage compression, this will reduce the compression ratio per stage. One compressor discharges into the suction side of the second compressor.
Two-stage or compound compression is not generally used until the compression ratio has exceeded 10:1
Types of Compressors
I won't elaborate in detail regarding to this section, however, I shall only introduce the application per each type with figures. There are five major types of compressors are used in the refrigeration and air-conditioning industry. These are the reciprocating, screw, rotary, scroll, and centrifugal.
The reciprocating compressor in common is used in small- and medium-sized commercial refrigeration systems. The screw compressors are used in large commercial and industrial systems.
The rotary as the case with the reciprocating compressor is also used in residential and light commercial air-conditioning. Centrifugal compressors are used extensively for air-conditioning in large buildings.
Selecting a Compressor
The first component selected for a refrigeration system is the compressor. The compressor is the heart of a refrigeration system, mechanical energy is produced by an electric motor is used to pump refrigerant through the system. The refrigerant picks up heat from one place (evaporator) and releases it in another place (condenser).
Generally, a compressor must remove vapor from the evaporator fast enough in order to enable the refrigerant to vaporize at the correct low pressure. Refrigerant vapor must be removed from the evaporator as fast as heat enters the evaporator in order to vaporize the refrigerant.
In practice, the compressor is selected based on manufacturers&#; tables of compressor capacities. The compressor&#;s capacity has to be sufficient to move enough refrigerant through the system in order to remove the total heat load within the operating cycle. The compressor's capacity varies based on the suction line temperature and the condensing temperature. It is worth mentioning that the compressor selection determines the refrigerant to be used to operate the system at maximum efficiency.
Compressor selection determines the refrigerant to be used to operate the system at maximum efficiency
The suction line temperature is calculated based on the design temperature of the conditioned space and the temperature difference (TD) between the design temperature and the refrigerant in the evaporator. Other system conditions affecting compressor sizing include the desired relative humidity in the conditioned space and the defrost method used in the system.
The capacity of a compressor is determined in part by its operating cycle, which is the number of hours that it operates per day. A smaller compressor with a longer operating cycle can be as effective as a larger compressor with a shorter operating cycle.
Example: A particular walk-in cabinet used to store lean beef at 35°F (2°C) requires 160,524 Btu/day. The system has a TD of 10°F (5°C) and, thus, a suction line temperature of 25°F (&#;4°C). Based on the cabinet and suction line temperatures, warm air defrost can be used for the system. For a system with warm air defrost, the operating cycle is 16 hours. Determine the required compressor capacity.
The required capacity is 10,033 Btu/hr (0.83 tons). Compressor manufacturers provide tables listing compressor capacities. The capacity of a compressor is determined by three factors:
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The condensing temperature is the ambient design temperature plus the TD. Condensers can be air cooled or water cooled. For commercial refrigeration systems, air-cooled condensers are commonly used. Air-cooled condensers in commercial systems generally have a temperature difference (TD) of 10°F to 30°F (5°C to 17°C).
Condensing temperature =
Ambient design temperature + TD
To select a compressor for a given load and conditions, first, locate tables for available compressors. Next, check the compressor capacities based on the refrigerant type, suction line temperature, and condensing temperature.
The condensing temperature on our previous example is Ambient temp + TD = 100°F + 20°F =120°F.
The following figure shows the selection of compressor MTZ032-4 from Danfoss :
The selection is made at 50 Hz, and the cooling capacity of the selected compressor is 12,925 Btu/hr. The upper side of the figure shows the first stage of selection of the compressor based on refrigerant and capacity. The lower side of the figure shows a chart that is called the operating envelope and is used to validate the performance of the compressor. The operating envelope consists of evaporating temperature (dew point temperature) on the x-axis and condensing temperature on the y-axis.
The selected compressor is oversized in terms of capacity because our design cooling capacity is 10,033 Btu/hr. That is why computed aided selection software could be a great tool to avoid time-consuming and get the right compressor that complies with the best EER.
Compressor Selection using Coolselector®2
Coolselector®2 is a nice tool from Danfoss that can be used to select many system components such as compressors, condensers, heat exchangers, valves, and line components.
The following explains how to use the Coolselector®2 ( Note: configuration can be changed from US to metric units):
[1] The specifications related to applications, refrigerant, power supply, compressor type, and speed adjustment.
[2] The cooling /heat capacity in kW or Btu/hr.
[3] Evaporator dew point temp.
[4] Useful superheat.
[5] Additional superheat.
[6] Condenser dew point temp.
[7] A compressor that matches the specifications.
The recommended compressor can achieve the requirement for this cycle and match the demand. You can cross-check the match in the last column in the selection specification . To show the relevant details from the previous example, the columns shown for the selection specification are different from the default ones.
Some superheat is required for the refrigerant at the compressor inlet to ensure the avoid of liquid droplets in the compressor. The useful superheat is the superheat inside the evaporator, which contributes to the cooling capacity. However, a very high useful superheat decreases the evaporator efficiency as well as the density at the evaporator outlet which results in higher compressor consumption. This value is set to 8 K by default in Coolselector®2.
Additional superheat happens after the evaporator is in the suction line. A longer length of the suction line would result in a higher additional superheat. This is set to zero by default, as it is highly affected by the length and size of the suction line, which is not provided in Coolselector®2. However, you should try to provide an accurate value or estimation for a good selection.
If you change the additional superheat to 5 K, the suggested compressor will allow a slightly higher volumetric flow rate to support the given cooling capacity.
The reason is that increasing the useful superheat would result in a decrease of density after the suction line at the compressor inlet. The mass flow rate required for the cooling capacity would be the same (you can check that in the performance details tab), but a lower density means a higher volumetric flow rate, which results in demand for a slightly larger compressor. Another important aspect regarding additional superheat is the discharge temperature, which can be affected significantly, and which would affect the selection of components in the discharge line, as well as compressors or condensing units. Hence providing additional superheat correctly is important for proper selection and suggestion.
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