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Chiller selection criteria
Chiller is selected based on the following three parameters:
Chilled water temp out of the chiller evaporator
The lower the cold water temperature at the outlet of the chiller, the higher the capacity of the selected chiller and the lower the capacity of consumer equipment such as fan coils, ventilators, and so on. The best cold water temperature coming out of the chiller evaporator due to technical and economic issues is equal to 45 degrees Fahrenheit, which is usually considered in the design of refrigeration systems.
The amount of refrigeration capacity of the chiller
After obtaining the Sensible Heat load and the Latent heat load at the peak load (usually between 13:00 and 16:00 PM), the resulting value is calculated based on the refrigeration tone and the above number is considered as the basis for selecting the chiller.
Chiller refrigeration load based on refrigeration ton = [(QSH + QLH) X 1.05] ÷ 12000
- Chiller refrigeration load based on refrigeration tone
- QSH: Apparent load based on BTU / HR unit
- QLH: Latent load based on BTU / HR unit
- Number 1.05: Confidence factor
Note: Each BTU / HR 12000 equals one ton of 1 USRT refrigeration.
3- Outdoor humid temperature (TWB)
The humid temperature of the outdoor environment according to thermodynamic relations and psychrometric diagrams is the maximum temperature that can evaporate a maximum of one environment.
1-3 If the humid temperature of the outside environment is less than 85 degrees Fahrenheit
In this case, cooling towers are used to capture the heat generated in the condenser, which according to the humid temperature, the temperature entering the condenser or leaving the cooling tower will be equal to:
TSupply.Condenser = TWB + 10 ° F
The outlet temperature of the condenser (inlet to the cooling tower) varies according to the type of chiller. In condensing chillers, the difference between the inlet and outlet temperatures of the condenser is usually equal to 10 degrees Fahrenheit, and in absorption chillers, usually 10 to 15 degrees Fahrenheit is considered. Therefore, the output temperature of the chiller condenser will be equal to:
TExit.Cond = TSupplly.Cond + 15 ° F to 10 ° F
As a result, according to the cold water temperature parameters of the chiller outlet, the refrigeration capacity and the humid temperature of the outside environment that were mentioned, we select the chiller model by referring to the catalog of the manufacturer.
2-3 If the humid temperature of the outside environment is more than 85 degrees Fahrenheit
In this case, due to the fact that the outdoor environment has a high relative humidity and the humid temperature is above 85 degrees Fahrenheit, so due to the lack of proper evaporation in the cooling tower, the efficiency of cooling towers is very low and usually in the above projects of systems Cool air is used. To determine the chiller, the refrigerant distillation temperature is the selection criterion which is calculated as follows:
TCondensing = TDB + 25 ° F to 35 °TDB: Bubble temperature is dry.
Example: The refrigeration load of a building at peak times is 200,000 BTU / HR with a 5% confidence interval, it is desirable to choose the appropriate chiller according to the total refrigeration load:
1,200,000 / 12,000 = 100USRT = Chiller capacity based on tonnage
Also, the cold water temperature coming out of the chiller is equal to 45 degrees Fahrenheit, according to the statements.
The humid temperature (TWB) of Tehran is equal to 75 degrees Fahrenheit.
Therefore, the inlet and outlet temperatures of the chiller condenser will be as follows: (Assuming that the chiller is compression)
TSupply.Condenser = TWB + 10 ° F = 75 + 10 = 85 ° F TExit.Condenser = 85 + 10 = 95 ° F
Therefore, according to the above three parameters, we refer to the catalog of the manufacturer and the chiller.
For example: it is desirable to choose a chiller for a building with a refrigeration capacity of 600,000 BTU / HR (including 5% reliability) located in Nowshahr
According to 271 publications, the humid temperature of Nowshahr is 87 degrees Fahrenheit.
Chiller refrigeration capacity is equal to: Chiller refrigeration capacity based on refrigeration ton = 12,000 / 600,000 = 50USRT
The cold water temperature at the chiller is 45 degrees Fahrenheit.
Due to the fact that the humid temperature of Nowshahr is equal to 87 degrees Fahrenheit, the air condenser should be used instead of the cooling tower. Therefore, the distillation temperature (TCondensing) of the chiller will be equal to:
TCondensing = TWB + (25 ° F to 30 ° F) = 87 + 28 = 115 ° F
According to the above three parameters obtained, refer to the manufacturer’s catalog and select the chiller model.
