.Decades ago, the primary cooling means for machines or processes was tap water. Once considered bountiful and inexpensive, water now is recognized as a valuable natural re-source with costs incurred for both use and disposal. Using a refrigerated circulator or chiller instead of tap water in certain applications makes sense for many reasons. Such units allow you to:
- Maintain Internal System Cleanliness. In a closed-loop temperature control system,cooling fluid impurities can be controlled or eliminated. Tap water contains impurities such as mineral deposits that can inhibit heat transfer by plating internal system parts.
- Control Cost. Tap water is expensive to use and of which to dispose. If a piece of equipment uses 4 gal/min of water during a normal 40-hr work week, annual usage is approximately 500,000 gal. Of course, water usage and disposal rates vary, but based on current rates in the Chicago area, for example, you could expect to pay more than $1,000 for 500,000 gal. In Houston, the water and sewer charge would be more than $3,000 per year. Payback on a refrigerated circulator or chiller may be achieved in less than a year, depending on the unit's size and the local costs for tap water and disposal.
- Maintain Accurate Temperature Control. When the seasons change, tap water temperature also changes. Refrigerated circulators and chillers provide year round precise temperature control for process cooling applications.
- Address Environmental Concerns. Using tap water for applications that could use circulators or chillers is a waste of a natural resource.
- The basics of how the devices work.
- The types of equipment available.
- The application's precise temperature control requirements.
- The sizing, including calculated heat load, cooling flow volume and pressure requirements.
- Additional options.
A temperature controller is the circuitry that controls the required range of temperatures. In many cases, it also controls the sophisticated operations of more complex process cooling devices. A controller can be as simple as bimetallic on/off thermostats or as sophisticated as analog and microprocessor proportional-integral-derivitive (PID) controllers.
Devices DefinedThough refrigerated circulators, chillers and immersion coolers have integral refrigeration circuits, they provide effective fluid cooling only if properly sized and applied.
A refrigerated circulator has a reservoir, which is useful for processes requiring container or material immersion. They also are ported to allow for the pumping and return of the fluid for external circulation. Refrigerated circulators are precision temperature control devices providing temperature stability in the range of I0.01°C. They generally are limited, however, to approximately 1,000 W cooling capacity and pressure flow rates of less than 10 gal/min.
Refrigerated circulators also have heating elements and provide a fast, precise range of temperature control capabilities. Manu-facturers offer basic models that hold temperature constant at the control setpoint. More elaborate programmable models have operator-adjustable setpoint parameters. Programmable models also are able to provide multistep temperature setpoint ramping/decline at adjustable rates.
Chillers, like circulators, have reservoirs, but they are used as thermal storage devices instead of sample chambers. Chill-ers circulate water out to an application. Most adequately sized chillers are able to provide temperature stability of I0.5°C to a process. Unlike small-capacity circulators, chillers are capable of providing a range of cooling capacity from approximately 100 W cooling at 32°F (0°C) to hundreds of tons of cooling capacity. Chillers provide a range of bulk heat removal capabilities but are not designed and built to control temperatures as precisely as refrigerated circulators. Applications for chillers are numerous throughout industry. They include welding, laser cutting and cooling, paint manufacturing, plastics injection molding and air compressor jacket cooling. Any process or machine that requires cooling below 85°F (29°C) may benefit from using a chiller.
An immersion cooler is an instrument that provides low temperature capabilities for nonrefrigerated circulators, tanks or baths. Most immersion coolers have a probe for directly applying the unit's full cooling capacity at the required point. Immersion coolers often replace dry ice or nitrogen, and they provide maximum cooling at point-of-use. Although useful for specific applications such as freeze-point determination, crystallization, impact testing or quick-cooling requirements, immersion coolers generally are limited to approximately 1,000 W of cooling capacity.
How to Size Your Cooling DeviceProper selection begins with sizing the device for the application. Some manufacturers will provide you with this information, but if it is not provided, the amount of cooling required can be determined by analyzing the amount of heat that must be dissipated. Accurate sizing relies on following a series of steps:
- Determine the wattage.
- Adjust the wattage calculations.
- Consider the effect of process-specific considerations.
Calculating the amount of energy to be removed by a circulator or chiller requires measuring the change in water temperature before and after it cools the equipment. Next, multiply a constant, which incorporates water fluid density, specific heat and conversion of calories, to watts. Finally, divide by the time in seconds that it takes to fill a 1 l measure. The equation is:
where TK equals the difference between the output and input temperature and S is the number of seconds.
Be sure to use the same thermometer to measure both temperatures. If the difference is less than a few degrees, slow down the fluid and remeasure. This is a critical measurement, so any error will cause a large error in the final measured power. Measure in Celsius or Fahrenheit - the constant (K) is shown both ways.
K is the conversion constant that takes into consideration the water's specific heat, its density and the ratios to convert the units measured into watts per hour for either Celsius of Fahrenheit. Measured in Celsius, the calculation reads
Once you have completed your calculations, the results (in watts) should be adjusted for the following:
- If the ambient environment (average room temperature) is above 68°F (20°C), add 1% above the calculated wattage for each 0.5°C above 20°C.
- If the circulator or chiller is to operate at 50 Hz current rather than 60 Hz, add 20% to calculated wattage previously obtained.
- If your line voltage runs consistently below rated voltage or you are working at high altitudes, add another 10% to calculated wattage.
When converting from watts to BTUs, remember that 1 W equals 3.41 BTU/hr. If converting from BTUs to tons cooling, remember that 12,000 BTU/hr equals 1 ton.
Though the calculations provide a basis for determining the cooling power requirements necessary to size your apparatus, you may want to consider future or alternative application requirements that may require additional cooling capacity.
What Kind of Device Do I Need?Today products are engineered to fit almost every process cooling application imaginable. To help you understand and specify the correct level of flexibility and temperature control precision you require, ask yourself the following questions. The answers will point you to the right cooling device.
What is the temperature range required?
Be certain the device provides the full range of temperature operation your equipment or process requires. If it is below 86°F (30°C), think refrigeration. But if it is above 86°F (30°C), think heating.
What degree of stability do you require?
If you need temperatures held to I0.01°C, think circulator. Or, if your process requires I0.5KC or greater, think chiller.
How much cooling is required?
Use the manufacturer's recommendations or follow the steps in the article to size equipment for the cooling requirement. In general, for cooling below 700 W, think circulator, and for cooling above 700 W, think chiller.
What fluid pressure and flow is required?
Proper pump selection is critical to ensure proper pressure and flow. If you are using a fluid other than water, be sure to get the manufacturer's pump-type recommendation.
Are there additional application factors you need to consider?
If you are buying a chiller that will cool only water, do you need to add an ethylene glycol solution? This depends on the low temperature requirement, application and manufacturer suggestions. It is a good idea to discuss your equipment uses with the manufacturer.
Are you replacing a nonworking refrigeration device with a new one?
Federal laws strictly regulate the disposal of refrigeration products and gases. Some manufacturers provide services to assist you with disposal.
Does the process cooling device have all of the operating, safety and reporting features you require?
Most circulators and chillers have alarms or shut-offs for low liquid levels and high temperature situations, and additional alarms may be available. Also, many process cooling units have RS232 ports for connection to computers for remote operation or data recording.
What does the manual suggest?
Ask to review the manufacturer's manual before you purchase a chiller. Most circulators, chillers and immersion coolers require minimal periodic maintenance. It is a good idea to determine what type of maintenance will be required. An important maintenance item is periodic condenser cleaning. Be sure to follow the manufacturer's recommendations for air and fluid filter cleaning and replacement. Also, some pump motors occasionally require lubrication. Review the maintenance requirements for all the device components to maximize the life of the unit.