Making a Case for Chillers
- Cutting to size silicon crystals used in the manufacture of computer chips.
- Cooling dies used in an extruding process.
- Cooling machine-tool spindles and cutting oils used to manufacture close-tolerance parts.
Other processes that can benefit from close temperature control of the process coolant are:
- Compounding rubber.
- Controlling chemical reactions that occur in jacketed reactor vessels.
- Maintaining the proper stable temperature of food during processing and packaging operations.
The chiller control components used to achieve close temperature control depend on the capacity of the chiller and the number and type of refrigeration compressors utilized.
Regardless of the chiller components used, a requirement for all chilled-fluid systems is a fluid reservoir of sufficient size to act as a heat sink. A rule of thumb for a properly sized reservoir is a minimum of 25 gal of fluid volume per oF of temperature tolerance required per ton of heat load. For example, a 10-ton chiller operating with a 1oF temperature tolerance requires a reservoir with a minimum fluid volume of 250 gal. The thermal characteristics of the process combined with the type of controls used on the chiller are what actually determine the reservoir size. In general, though:
- If the process has a relatively constant heat rejection rate, then a smaller reservoir may be appropriate.
- If the chiller is equipped with hot-gas bypass control, then a smaller reservoir can be used.
- If the process has wide fluctuations in heat load, especially if the fluctuations can exceed the chiller capacity on an instantaneous basis, then the reservoir size should be increased. This volume is in addition to the volume contained in the process piping system and process equipment.
The purpose of the reservoir is to adsorb and spread out over time an input heat load spike to the chiller that is greater than the chiller's heat rejection capacity. This reduces the possibility of the cooling fluid's temperature changing at a rate faster than the response time of the chiller's controls and keeps the input heat load to the chiller within the chiller's design capacity.
An example of the need for a larger reservoir can be found in chemical applications such as when the contents of a jacketed chemical reactor vessel are at an elevated temperature and must be cooled rapidly and then maintained within a tightly controlled tolerance at a significantly lower temperature. The reservoir must be able to adsorb the input heat load that is in excess of the chiller's heat rejection capacity without the temperature of the cooling fluid rising beyond the process design requirements.
A process chiller with a closed-loop cooling system mounted and piped as a part of an air-cooled chiller package can cut water consumption to zero and at the same time provide important benefits for process cooling.
The condenser of an air-cooled chiller can be selected to operate satisfactorily in a wide range of ambient conditions. Because the process cooling system is closed to the environment, environmental consequences virtually are eliminated from consideration, as is the need for makeup water (if water is used as the cooling medium). Therefore, a chiller is a desirable method for cooling a process when little or no makeup water is available or if makeup water quality is poor.
The most obvious reason to limit water usage is the need to conserve potable water for use in the community. Reduced precipitation in the Great Lakes Basin has caused the water level in the Great Lakes to drop to levels not seen since the 1960s. Increasingly, companies are being faced with the need to conserve water or face water restrictions. An air-cooled chiller equipped with a closed-loop cooling system is one way to eliminate water consumption.
Fluid SelectionWater is not the only fluid that can be used in chillers. The materials of construction in the chiller evaporator (the heat exchanger used by the chiller to transfer heat from the fluid being cooled to the refrigerant) can be selected to be compatible with a range of fluids. For example, direct cooling of deionized (DI) water, hydraulic oil, glycol/water solutions, water-soluble oils and heat transfer fluids is possible.
When a process requires fluids other than water to be used as the cooling medium and other systems such as cooling towers are used, an intermediate heat exchanger must be used to separate the two fluids. The use of a heat exchanger complicates the process cooling system. Two sets of pumps are required, materials used in the piping systems for each fluid may be of different materials, and the electrical and control components increase in quantity and complexity. Further, the heat exchanger introduces an approach temperature into the design of the cooling system that the process engineer must take into consideration. For example, if the process requires 85oF (29oC) process-cooling temperatures, and the isolating heat exchanger has an approach temperature of 10oF (5.5oC), summertime cooling tower water temperatures may be too warm to provide proper cooling.
Fouling and scaling of the isolating heat exchanger caused by cooling tower water quality also is a concern.
The direct chiller with its packaged closed-loop cooling system can be used to supply constant temperature process cooling fluids to manufacturing equipment or process heat exchangers. Because the chiller uses mechanical refrigeration, it can maintain any desired temperature without regard to fluctuations caused by seasonal ambient temperature changes.
The choice between a cooling tower (with or without an isolating heat exchanger) and direct cooling using a chiller equipped with a closed-loop system should be made after consideration of all the relevant factors. Capital cost, water quality, maintenance and operation costs, and the ability to maintain water or coolant temperatures within the desired parameters must be considered.