Once you have determined that chilling is appropriate for your application and will improve your process throughput or quality, then you should decide which chiller is best for the application. Some of the factors to consider are:
- What are my total cooling requirements?
- What flow should I use?
- What temperature is right for my application?
- Do I use portable chillers or central chillers?
- Do I use air-cooled or water-cooled chillers?
Because cooling towers are able to produce temperatures down to approximately 85°F (29°C), depending on the geographical location, a chiller is used when lower temperatures are required. Chillers will have a rated capacity usually based on 50°F (10°C) leaving water temperature. In addition to the refrigeration system, there will be a hydronic system, including an appropriately sized pump circulating water or a glycol mix to the process.
Cooling RequirementsIf you have a plant-wide requirement for cooling, the first step is to make a list of all equipment that requires cooling and be sure to plan for any additional future equipment. The heat content of the process is found by the formula:
Q = M x C x ΔT
where M is the mass or weight of the item being cooled.
C is the specific heat of the item being cooled.
ΔT is the difference between the temperature in and out of the process.
Q is the heat in BTUs.
If this information is not known, it is possible to do a weighed water test, where you use city water through the process with an open line to drain. Get the process running at full production. Once it is, measure the temperature in and out of the process (using the same sensing device), and then time how long it takes to fill up a 5-gal bucket. This will provide a flow rate in gal/min. The tons then are calculated by multiplying the flow rate by the ΔT and dividing by 24. A chiller ton is 12,000 BTU/hr.
Flow RequirementsDetermining the correct flow for your process can save thousands of dollars. The objective is to remove the heat as quickly as the process will give it up - but not spend more money running a pump than is necessary.
Having turbulent flow in the process is a good starting point to maximize the amount of heat extracted from the process. Knowing what you need for turbulent flow is important for selecting the correct pump. Something that often is overlooked is that turbulent flow will occur in the part of the system with the smallest orifice or passage area. It is important to make sure that the hoses, quick disconnects and manifolds that feed the process are properly sized as well: Turbulent flow here could mean laminar flow in the process.
Conversely, having more pumping capacity than required wastes energy. For example a 10 hp pump or chiller, running 24/7 at $0.10/kWh will cost $6,500. Be sure your flow is turbulent, but beyond that, you are wasting money.
TemperatureThe best guidelines for selecting the chiller water temperature for your process are:
- Run the chiller water at the temperature that optimizes heat removal.
- Select a temperature that does not impact product quality.
- Select a temperature that optimizes chiller efficiency.
Another important consideration is condensation. For some processes, it is not a concern; for others, it can cause serious product quality problems. Chillers are usually rated at 50°F (10°C) for industrial applications. As the chiller is run colder, it becomes less efficient, losing about 2 percent for every 1°F (0.56°C) below 50°F (10°C). For example, a 100-ton chiller running at 40°F (4°C) will provide 80 tons. Conversely, as the temperature increases, the capacity will increase to as much as an additional 20 to 30 percent.
Most chiller manufacturers recommend that glycol be used below 45°F (7°C) to prevent freezing. While this will prevent freezing, it has a significant impact on achieving turbulent flow. A good guideline is that running a 25 percent glycol concentration will require flow three times that of water to achieve turbulence. This may lead to a larger pump and increased energy cost.
Portable vs. Central ChillersPortable chillers - usually up to 25 tons - are designed to provide cooling to a single piece of processing equipment where the cooling load is fairly constant. By contrast, central chillers generally are larger in capacity and more expensive to install due to plant piping requirements. The main benefit of a central chiller is the ability to handle varying process loads at a lower electrical consumption.
Usually, if the cooling load is 30 tons or more and the varying processes use the same temperature, then a central chiller often turn out to be the most economical decision. If some of the processes require warmer water than is provided by the central system, a temperature control unit can be used to elevate the water temperature to the proper level for the process.
Water- or Air-Cooled?Conventional thinking has been that water-cooled chillers are more efficient than air-cooled chillers. If you only look at compressor costs, this may be true. By employing modern technology with centrifugal compressors and variable speed control, however, air-cooled chillers often may be the better choice today (figure 1).
It is important to look at the total operating costs involved with a chiller or chillers, not just the compressor costs (table 1). Cooling tower operating costs should be added to the operating costs of a water-cooled chiller. A cooling tower system’s costs include the tower fan, water and sewer costs, chemical costs and pumping costs. In process applications, tower systems generally have process pumps and recirculation pumps, which can add a significant cost. The table compares the operating cost of an air-cooled, variable-speed centrifugal chiller vs. a water-cooled screw chiller. The table is based upon a 140 ton load, Chicago weather data, $0.07/kWh electrical costs, $5/1,000 gallon water and sewer costs, and 6,000 hr/yr operation.
Because the air-cooled chiller uses floating head pressure control, the compressor energy costs are less than the water-cooled chiller’s compressor costs - a $4,757 difference. Adding the tower system’s operating cost of $17,429 would result in a $22,186 per year operating cost savings. A more efficient variable speed water-cooled centrifugal chiller reduces the cost difference to $16,725, which is still a significant annual cost penalty.
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