Chiller Design Basics
A number of design variables can come into play when selecting and designing a process cooling system. The following formula commonly is applied.
Q (BTUs) = W x Specific Heat x gT
Where W is the weight of material to be cooled.
Specific Heat is a heat constant taken from tables.
gT is the initial temperature minus final desired temperature.
Note that 1 ton of refrigeration is 12,000 BTU/hr.
Example 1. A processor needs to cool a 55-gal drum containing 700 lb of corn syrup from 125 to 70oF (52 to 21oC). When applying the formula, the variables plug in as follows.
Q = 700 x 0.51 (specific heat) x 55oF (gT) = 19,635 BTUs
The amount of time required to cool the process also must be considered. In the above example, if the corn syrup was allowed one hour to cool, you would need 19,635 BTUs per hour of chiller capacity for more than one hour. Divide the heat load by the amount of time (in hours) to discover the hourly heat removal rate. Typically, 10 to 15% safety factor is added for losses that may occur from piping or uninstalled surfaces.
First, the weight of the oil needs to be calculated as follows.
6,000 gal x 8.34 x 0.92 (specific gravity) = 46,037 lb of oil
Q = 46,000 lb x 0.30 (specific heat) x 35oF = 483,000 BTUs
Divide by 3 hr = 161,000 BTU/hr
Divide by 12,000 = 13.43 tons
After the chiller is sized, add a 10% heat loss factor to arrive at approximately 14.8 tons of chiller. Cooling tonnage also equates to flow across the process. The following formula will provide an estimate for the quantity of water that must be supplied to the process.
Tons = gal/min x gT
Condenser SelectionOnce chiller size is determined, decide whether the chiller will utilize an air- or water-cooled condenser. An air-cooled portable chiller may be the most convenient to install close to the process. However, special attention must be paid to the amount of warm air discharging from the chiller into the atmosphere. Warm air discharge will be 125% of the actual cooling capacity of the chiller. With a small chiller in a large facility, this hot air may be a negligible factor. However, if a large chiller is discharged into a small facility, ambient temperature may rise to unacceptable levels. Also, if the plant is air-conditioned, the additional heat load will directly affect the air-conditioning load.
There are two alternatives to discharging air directly into the plant. The first is to duct the air through louvers out of the facility. Again, consider the amount of air pulled through the building and sent outside. An equal amount of makeup air must be introduced so as to not pull a negative pressure on the building. Duct sizing and makeup air requirements can be estimated by your contractor or chiller provider.
The second alternative is to utilize a remote condenser or split system. Refrigerant is piped in and out of the building so all the heat removed is dissipated outdoors. Installation is more involved. It may require the service of a professional refrigeration contractor.
An alternative to an air-cooled chiller is a water-cooled unit, which uses a refrigerant-to-water heat exchanger to dissipate the ab-sorbed process heat. The heat exchanger is supplied with water from an external source such as a cooling tower, city water, well water, spray ponds or other possible water source. Once the water-cooled condenser is connected with piping, it becomes less portable. However, if the cooling tower water is piped at various sources through the building, a portable chiller may be moved around to reconnect to those sources.
When sizing a chiller, it is critical to understand the actual operation temperature of the fluid the chiller will supply to the process. Most standard chiller capacities are rated at approximately 45 to 50oF (7 to 10oC). If the fluid temperature must be reduced due to the processing requirements, the capacity of the chiller must be derated by the manufacturer.