Regardless of your industry and process, making sure that you have sufficient cooling is critical to optimal performance. While it may seem that there is not much to consider when making a decision as to the type of chilling equipment to purchase, there are a number of variables that should be taken into consideration.
The first step is to calculate the total cooling requirements of the plant. This should take into consideration the heat generated by the process itself, but do not
overlook other areas in the plant that might require cooling, including hydraulic cooling, air compressors and possibly even the HVAC system. The desired temperature for optimal cooling then needs to be determined for each cooling point. Usually 85°F (29°C) is sufficient for hydraulics and air compressors.
Depending on the process, you may need anywhere from -30 to 500°F (-34 to 260°C).
Once all this information is compiled, it is time to start evaluating which type of equipment will be best for the application and will work best in your plant. The most important criteria to consider will be total cooling load, upfront cost, floor space, noise, total operating cost, processing performance, the chilling equipment environment, and maintenance and reliability.
The first decision is usually whether it makes more sense to have a dedicated chiller for each machine or whether a plant-wide system makes more sense. The second decision is whether an air-cooled or water-cooled chiller is the right choice. Let’s look at each of the different variables.
Total Cooling Load. Central chillers begin to make sense if the total chilling requirement is 20 tons or more and there are at least three machines that require cooling, all using the same temperature water. Otherwise, a portable chiller will be the correct choice.
First Time Cost. Portable chillers will be more expensive than a central chiller in cost per ton. However, purchasing portable chillers allows you to buy only what you need for now. If your plans change in the future, you have not spent money for cooling capacity that is no longer necessary or purchased cooling that will not meet your requirements.
Installation costs vary significantly depending on the specific plant layout, the type of equipment selected, how much work is done internally, and how the portable chillers are installed. A central system will cost more to install, and they usually are installed by independent installation companies. Portable chiller installations typically can be done by in-house maintenance personnel as all that is required is to install hoses and piping between the chiller and process, connect the power to the chiller, and fill with water or a water/glycol mixture.
Floor Space. In most plants, floor space is at a premium. Portable chillers are located close to the process on the production floor, taking up valuable space that could be used for other equipment or more primary equipment. Central chillers, by contrast, can be located in an equipment room or outdoors.
Noise Level. Depending on whether air-cooled or water-cooled portable chillers are used, the noise level can be quite high by the machine, affecting operators. Removing the cooling equipment from the production floor substantially reduces noise. Even among central chillers, the noise level can be dramatically different depending on the type of compressor used. If this is a significant concern, a centrifugal compressor should be considered.
Operating Cost. A portable chiller must be selected for the largest cooling requirement that the machine it is cooling might require. And, because portable chillers are only available in specific sizes, invariably the portable chiller will be oversized for the application, thus costing more to operate. Central chillers have different methods of providing less capacity at a reduced energy cost, making them more efficient at reduced loads. It is not unusual to see a central chiller with a screw or scroll compressor save 30 percent in energy vs. individual portables. Oil-free centrifugal chillers with variable-frequency drives can save up to an additional 30 to 50 percent beyond those savings.
Processing Performance. Proper flow and pressure are critical factors for optimal processing. How critical is the temperature control and stability for your process? Central systems are ideal for “brute force” cooling while portables are suited for fine temperature and flow control. Central systems are ideal when neither the water temperature provided nor the temperature stability is a large concern. Central chillers only can provide one temperature for all of the various processes, and without proper system balancing, it is quite common for one process to use too much water while another process is starved for water. Portable chillers manufacturers will design a chiller specifically for a customer’s requirements, so if temperature accuracy and stability are critical in your process, a portable chiller may be a better solution.
Maintenance Considerations. Downtime is always a concern. With individual portables, if one fails, then that machine is down, unless a spare portable is available. With a central system, there normally are standby pumps for quick switchover in the event of a failure. The chiller itself can be oversized with an extra standby compressor. Regardless of which type of chiller you have, preventive maintenance should be done at a minimum of every six months. Consider the actual cost of your downtime and consider that cost when making an upfront purchase. Look for equipment with a history of reliability from a company with quick service response time.
Process Fluid Considerations. The type of fluid your process requires can also be a consideration. Most portable chillers are built with nonferrous materials
|Most portable chillers are built with nonferrous materials that will not rust, while many central systems use carbon steel components that will require corrosion inhibitors to be added to the fluid.|
that will not rust, while many central systems use carbon steel components that will require corrosion inhibitors to be added to the fluid. In fact, most portable chillers are filled with either tap water or distilled/deionized water, and because the system is closed, it will only need to be filled once, with additional water added sporadically. If a central system will be located outdoors, it will require using a glycol mixture in freezing conditions. Besides the initial cost of the glycol, there is also the additional cost of maintaining a glycol system, and disposal.
Air-Cooled vs. Water-Cooled Chillers. Whether you purchase a portable chiller or a central chiller, you will need to determine if you want one with an air-cooled condenser or a water-cooled condenser. Water-cooled chillers require a secondary cooling source, usually a cooling tower, to provide 85°F (29°C) water to the chiller. If there are many other places in the plant where 85°F (29°C) water is acceptable, then putting in a cooling tower system can make sense. If there are not, it usually is not cost effective to put in a cooling tower just to provide cooling to the condenser of the chillers.
It is important to realize that if an air-cooled chiller is located in the plant, it will take in plant air and discharge significantly warmer air. While this can be beneficial in winter months, if your plant is air conditioned, it can be a definite disadvantage. In those cases, you will either want to locate the chiller outside of the air conditioned area or possibly outside of the building.
The final consideration to be aware of in purchasing an air-cooled chiller is to recognize that if you have an operation that generates quite a bit of dust, it will be necessary to clean the condenser of the chiller more frequently so as to not negatively affect performance.
This article does not cover all the individual considerations that may be unique to your application. It is strongly recommended that you consult with manufacturers of chilling equipment that have technical personnel to evaluate your personal application and assist in selecting the right equipment for your optimal cooling performance.