Recirculating chillers are “off-the-shelf” liquid-cooling systems that offer precise temperature control and cooling below ambient temperatures. A number of options typically are available to configure the system to a particular application, including protection and convenience options such as external flow valves and pressure-relief valves, anti-siphon systems, air filters, water filters and remote-start capabilities. Selecting the appropriate options simplifies chiller operation and maintenance as well as minimizes the risk of premature system failure.
External ValvesFlow Valves. An external flow valve is a manually adjustable globe valve that is used to control the coolant flow rate to the application. Using an external flow valve provides maximum flexibility for the chiller to be used at different flow rates and, by extension, process cooling application temperatures. As the valve is opened, more flow bypasses the process cooling application and is redirected to the chiller’s return line. The external flow valve is installed on the back of the chiller in parallel with the chiller coolant supply/return (figure 1). This option is useful in applications where the process pressure drop is unknown or a specific flow is required.
Pressure-Relief Valves. An external pressure-relief valve is a manually adjustable check valve that allows the recirculating chiller to continue to run and maintain temperature and pressure if the flow to the application is temporarily interrupted - for example, if the application is cycling on and off. The external pressure-relief valve also can be used to prevent the chiller supply from exceeding a predetermined pressure setting.
The external pressure-relief valve can be set to 50 to 100 psig. When flow is interrupted, the valve opens and bypasses flow to the chiller’s return line. This valve typically is installed on the back of the chiller in parallel with the chiller coolant supply/return (figure 2). When using a positive-displacement pump in conjunction with an external pressure-relief valve, the pump’s integral bypass valve setting usually is increased to 90 psi. This setting prevents the pump from bypassing flow and creating a low-flow condition on the return line, causing the chiller to turn off. Minimal flow rates of 0.5 gal/min are required to prevent this low-flow condition in some chillers.
The external pressure-relief valve and the external flow valve options should not be selected together as the interaction between them might be difficult to control.
Anti-Siphon SystemsAn anti-siphon system allows a chiller to be installed at an elevation below that of the application. The chiller tank is vented, so when the chiller is shut off, coolant in the hoses between the chiller and the application will flow back to the tank if the application is at a higher elevation than the chiller. In this situation, the fluid can overflow the tank if there is no anti-siphon system installed. The anti-siphon system option is most beneficial when space is an issue, and the only location available for the chiller is below the application or in a clean room application.
Some anti-siphon systems consist of internal check and solenoid valves that are installed on the supply and return lines, respectively. The check valve (figure 3) allows coolant flow to the application while preventing it from flowing back to the chiller. The solenoid valve opens to allow coolant flow when the power is on and closes to prevent coolant from flowing back to the chiller’s vented tank when power to the chiller is shut off.
FiltersAir Filters. Air filters can help prevent dust buildup that could lead to decreased chiller cooling capacity. They also reduce the need to clean the condensing coil and other internal refrigeration components. Typically mounted inside the chiller behind the front grill, the air filter is easily accessible and should be replaced on a set schedule, depending on the level of dust in the air (figure 4). For example, in a clean environment such as in an air-conditioned laboratory, the air filter might only need to be replaced every six months. However, in a dusty or dirty environment such as on a machine shop floor, the filters might need to be replaced as often as every month.
Coolant Filters. Just as important as an air filter is a water or coolant filter. A filter capable of removing particles up to 5 µm from the coolant supplied to the application should be used to protect equipment from blockage or damage due to particulate buildup.
Coolant filters usually are located on the supply line of the chiller, and all of the coolant flow is directed through the filter. If a chiller has both a coolant filter and a pressure-relief or flow valve, the filter will be installed after the valve. In this situation, 100 percent of the coolant flow to the application will pass through the filter. To maintain optimum performance, the filter should be inspected the day after the initial setup and on a weekly basis after that. Failure to replace coolant filter cartridges can lead to a decrease in flow and, consequently, a decrease in chiller cooling capacity or a system shutdown.
Coolant filters can be used with turbine pumps and are especially beneficial when used with positive-displacement pumps, which are sensitive to particles. Coolant filters are not recommended with centrifugal pumps because the pressure drop through the filter is too high. Figure 5 shows a standard water filter configuration.
Remote StartThe remote-start option, which is more for convenience than for safety, allows an external circuit to control the chiller's on/off function through dry contacts on the rear of the chiller. An auto-restart is included with most standard chiller controller packages; however, a remote start is beneficial for those who would like to simultaneously start their equipment and the recirculating chiller from a computer or relay for ease of operation. The remote start also can prevent accidental operation of the applications without the chiller, and vice versa, which could cause damage, unnecessary energy consumption and other problems.
In summary, valves, filters and remote-start functions are just a few of the options available on standard recirculating chiller systems. Selecting the right options can help facilitate system operation and maintenance while also minimizing downtime and costs.