Understanding Portable Chillers
Process cooling essentially involves moving heat from a place where it is undesirable to a place where it is desirable. For small and medium processes, this operation typically can be accomplished with a portable chiller. Portable chilling systems are ideal when a central chilling system is too costly or too large for a facility, or is simply not the right choice for a specific application. As with any piece of equipment, there are advantages and disadvantages that need to be considered. By understanding the basic functions and components of a portable chilling system, you can choose the right equipment for your facility's needs.
Pros and ConsPortable systems range in size from 0.25 ton to approximately 40 tons and can be easily disconnected from one process and reconnected to another. Mounted wheels allow the chillers to be moved from one point to the next as needed. Air-cooled chillers are easier to move than water-cooled types, which require an additional set of cooling lines from a cooling tower or other cooling device (figure 1). However, the heat generated by the condenser in air-cooled chillers can warm the surrounding air and create an uncomfortable work environment.
Because portable chillers can be located close to the process, they can minimize a facility's piping and installation costs. However, a portable chiller generates approximately 15,000 BTUs of rejection heat for every 12,000 BTUs of process water that it creates. If the heat generated by an air-cooled chiller is excessive, it can be ducted to a more desirable location such as outdoors or to an unheated part of the facility, but this limits the chiller's portability. In addition, if the condenser is ducted outdoors, the amount of air that the condenser requires must be supplied by the plant's air system. If this air is also air-conditioned, the discharged air will increase the load on the air-conditioning system.
Another option for a small chiller is to locate the condenser outdoors and pipe the required hot gas and liquid refrigerant lines to and from the condenser. Although this method allows the chiller to be located at the process load, it also eliminates the chiller's portability. Additionally, if the system is located in a region that experiences cold winter climates, the condenser must be able to withstand these conditions.
Central chillers are used more often as a chilled water source for cooling a number of machines, or for cooling an entire plant when only one temperature is required. Running one central chiller at full capacity is more efficient and often less expensive than operating several portable chillers. However, central chillers become less efficient when they are not used at their full load capacity. For cooling applications that are below 40°F (4°C), and for applications in which multiple processes must be cooled to different temperatures, portable systems are preferred.
Portable Chiller OperationA portable chiller is designed using a basic refrigeration cycle. A compressor, usually a scroll, pumps a high-pressure, high-temperature refrigerant gas into the condenser. As this refrigerant is cooled, it changes from a gas to a liquid. From this point, the cooling is accomplished either by a fan or blower that blows air across a condenser coil, or by an external water source, such as a cooling tower, that circulates water through the system.
After the refrigerant liquid leaves the condenser, it is pumped through the liquid line into the expansion valve. The refrigerant then is released into the evaporator and boils back into a gas. This change of state from a liquid back to a gas requires a large amount of heat, which is absorbed from the water or cooling fluid that is circulating on the water side of the evaporator. After the refrigerant has picked up as much heat as possible and has completely boiled into a gas, it returns to the compressor to begin the process all over again. The basic components used in this operation are shown in figure 2.
A chiller's cooling capacity is measured in British thermal units (BTUs) and tons of refrigeration, where 12,000 BTUs equals 1 ton of refrigeration. Most chiller tonnage ratings are set at a 50°F (10°C) leaving water temperature and 95°F (35°C) ambient conditions, or, in the case of water-cooled units, 85°F (29°C) cooling tower water supplied to the condenser. If a lower or higher leaving water temperature is required, the chiller must be de-rated by 2 percent for each degree below or above 50°F, as shown in table 1.
Sizing MethodsIf the process that needs to be cooled is connected to an existing city or well water line, a simple method of chiller sizing is a weighed water test. This test measures the flow rate to the process in gallons per minute using either a flow meter or a timed fill of a fixed vessel. This information should be combined with the current process temperature rise of the water to determine the exact cooling requirement in BTUs of heat or cooling tonnage (table 2).
Another method of chiller sizing is to obtain the specific heat of the product that needs to be cooled and subtract the final desired temperature from the initial temperature. Specific heat tables such as the one shown in table 3 can be obtained from product bulletins, chemical reference books and heat exchanger suppliers' reference books.
Whatever sizing method is used, a safety margin of 10 percent to 25 percent should be added to the calculations for heat losses and other unknown factors such as ambient conditions, piping losses and the cooling of various components in the cooling system.
Flow CharacteristicsThe process flow of water or cooling fluid is based on a rate of 2.4 gal/min/ton, which assumes a 10°F (5.5°C) rise in process fluid — i.e., chilled fluid leaving the chiller at 50°F and returning at 60°F. Higher flow rates drop the temperature differential (ΔT). For example, a facility that requires 50°F water to the process and wants to limit the rise to 5°F (2.7°C) would require a 4.8 gal/min/ton process flow. Because the mechanical components of a chiller are normally rated at 2.4 to 3.6 gal/min/ton, this facility would need a secondary recirculating pump on the chiller. Additionally, portable chillers are normally self-contained units that include an internal reservoir. Depending on the turnover rate and the flow rate through the chiller, the tank capacity might need to be increased.
Temperature ConsiderationsA portable chiller typically has a temperature range of approximately 30 to 70 °F (-1 to 21 °C). Processes that require temperatures outside of this range will need a more specialized system. For example, portable chillers can be designed with special components to run at extremely low temperatures such as -30 °F (-34 °C). However, these systems must use high percentages of propylene or ethylene glycol, which affect the heat-carrying capacity of the fluid. Sometimes two to three times the normal flow is required when using 40 percent to 50 percent glycol solutions.
Operating at temperatures above 70 °F might require the use of special valving and bypass flow devices. Another option would be to use temperature control circulation units that produce a high-temperature flow to the process and, in turn, regulate a smaller amount of the standard chilled water temperature into the closed loop system. This solution would allow the chiller to run at normal temperatures while the temperature control unit could be set at any temperature from just above the chiller temperature to greater than 200 °F (93 °C).
When choosing a portable chiller, you should consider everything from the type of material being cooled to the efficiency of the compressor. Once you have all of the information you need about your specific process and requirements, finding the right system is just a phone call away.
Wayne Lange is water systems manager and Nichole Saccomonto is public relations/writer for Sterling, New Berlin, Wis., a manufacturer of chillers, cooling towers and other industrial equipment. For more information from Sterling Inc., call (630) 595-1060; visit www.sterlco.com; or e-mail firstname.lastname@example.org.