Choosing a Process Recirculation Chiller
Your process needs cooling, and you've been given the task of finding a chiller. You're not a process cooling expert, and the company you bought your process equipment from probably didn't tell you much about the cooling requirement. That's usually when the problems begin. Choose the wrong chiller, and your problems could avalanche out of control.
Say you need a 10-ton chiller. That's a good starting point, but it's only a starting point. Poll 10 random chiller companies to get a specification on a 10-ton chiller, and the net result will be that all 10 units are rated differently. Which unit is best for your application? By understanding the basic selection parameters, you can make a more informed decision on your chiller purchase and maximize your equipment investment.
Leaving Fluid TemperatureWhat is the leaving fluid temperature (chilled water temperature) required for your process? Compressor manufacturers rate capacities based on a number of conditions, including the saturated suction temperature (SST), which is determined partly by the leaving fluid temperature and partly by the evaporator (heat exchanger) being used for your process. The chiller manufacturer must know the required leaving fluid temperature to size both the compressor and the evaporator, and to determine the SST.
Note that the SST required to reach the desired fluid temperature might (and most likely will) vary for each manufacturer depending upon the approach used to size the evaporator. Just because a compressor is rated for certain capacities doesn't mean that each chiller manufacturer uses the same approach. For this reason, total chiller capacities will vary even though different companies might use the same compressor model made by the same manufacturer.
Further, compressors are rated as high-, medium- and low-temperature based on the SST. Most air-conditioning compressors are high-temperature compressors and are rated for air-conditioning standards (EWT 54, LWT 44, 39F SST, 65F return gas). Wineries, dough-making facilities and other food processing applications might require a 28°F (-2°C) glycol process fluid, which is medium temperature. Other applications such as chocolate companies and laboratories might require a fluid below 10°F (-12°C) and low-temperature compressors. A high-temperature chiller (compressor) might work in a medium-temperature application, but premature compressor failure can occur because the equipment is not designed to operate in such conditions.
Returning Fluid TemperatureWhat is your returning fluid temperature (warm water from the process)? An off-the-shelf air-conditioning chiller might not be able to accommodate a return temperature greater than 10°F above the process temperature. If the chiller is not rated for your returning fluid temperature, it will not provide adequate cooling and might even shut down due to high head pressure.
Flow Rate and Pressure DropWhat is the flow rate and pressure drop through your process? The flow rate is required to calculate the BTU/hr (figure 1). The flow rate and pressure drop also are needed to properly size the pump (if the chiller will be equipped with a pump) and evaporator (heat exchanger).
Fluid TypeWhat type of fluid is your system circulating? Water, propylene glycol, ethylene glycol, synthetic glycol and oil all have different properties that affect cooling. If you run glycol in an off-the-shelf chiller that was rated (sized) based on water (figure 2), the chiller's capacity might drop below the threshold at which it can adequately provide cooling to your process. If the manufacturer knows up front that you will be using a certain glycol or other fluid, the capacity can be properly calculated. In addition, the evaporator can be properly sized so that the chiller will provide full cooling capacity under those conditions.
Operation TypeIs your process a single-pass or batch operation? Batch-style chillers vary by cooling demands and require additional information such as batch size and temperature, number of batches per hour, fill and dump rates, and the properties of the fluid being cooled.
Process ConditionsIs your process constant or does it fluctuate? Make sure the chiller is designed to work for your specific process conditions. A low-cost, off-the-shelf chiller might not work well under constantly changing loads, with a high or low flow rate, or with a high temperature differential. Many off-the-shelf chillers are designed under air conditioning conditions that are 44°F LWT, 54°F EWT and 2.5 gal/min per cooling ton (figure 2). Proceed with caution if your conditions differ from this norm. Ask questions and ask for a written verification from the manufacturer's representative or distributor that the chiller will work properly for your conditions.
Chiller LocationWill the chiller be located indoors or outdoors? If you place your air-cooled chiller indoors, keep in mind that it will reject heat from the process back into the air inside your building. You must have a large enough area and adequate ventilation so that the ambient air around the condenser doesn't warm up beyond the designed temperature of the condenser. Most indoor chillers are rated between 85 and 90°F (29 and 32°C). If the air around the chiller exceeds this rating, the chiller will begin to loose capacity and, in extreme cases, might turn off because of high pressure.
If your chiller is located outdoors, you must make sure the chiller is rated for both the warm and cold ambient air temperatures in your region. Failing to do so can lead to diminished capacity, poor temperature stability and the chiller shutting off on high/low pressures. You must also make sure the chiller is built as a weatherproof unit.
Cooling MethodDo you need air or water cooling? Water-cooled chillers can be advantageous for indoor chiller installations where space and ventilation are lacking and where a water source is readily available, while air-cooled chillers are best for outdoor use.
Critical IssuesIs your process critical? This is the most subjective of all the criteria used in selecting a chiller. All chillers are mechanical, and all things mechanical will fail at some point. You should do a cost analysis to determine the level of redundancy that makes sense for your application. A cost-effective backup might be achieved simply by using a domestic water supply. A more reliable backup might include purchasing redundant chiller features such as dual pumps and refrigeration circuits, or purchasing a complete backup chiller. Some factors to include in your analysis are lost labor costs, product waste and lost revenue during downtime.
CostsWhat can you afford? Paying more for a chiller does not mean that you are getting a better chiller, nor does paying less for a chiller always mean that you are getting the best value. Many factors affect the cost of a chiller, including the quality of the parts installed, the number of alarms and fail-safe indicators, the complexity of the controls and control circuits, single vs. dual stages of refrigeration, types of evaporators, labor costs and efficiencies, the manufacturing company that you are buying from and the number of commissioned salespeople who have influenced your sale and deserve a commission.
It is best to ask the chiller manufacturer or manufacturer's representative if the equipment is engineered to perform to your specific criteria. Often, more than one option is available to meet your budget and unique set of circumstances. As a rule of thumb, a chiller made for air conditioning should not be used for a process cooling application.
The next time you buy a chiller, understand the basic selection criteria and perform adequate comparisons before making your purchase. After all, in addition to cold water, isn't it value that you are buying? Maximize yours.