How to Select a Recirculating Chiller

Considering a recirculating chiller? Follow a simple three-step process to narrow the search from a field of hundreds to the chiller that will work best for your process cooling operation.

Recirculating chillers are a popular choice for temperature-critical applications because they offer stable temperature control, a constant flow rate and constant pressure control.

Many process heating systems require cooling as well as heating. For example, many conveyor ovens feature a cooling zone. Injection molding requires bulk heat removal. And furnaces often have cold-water jackets. There are many ways of cooling such systems, including tap water, tower water, building chilled water and recirculating chillers.

Recirculating chillers are a popular choice for temperature-critical applications. They offer stable temperature control -- generally to within 0.2^oF (0.1^oC) -- and offer constant flow rate and constant pressure control. They can cool below ambient temperature and, when equipped with a heater, also can maintain temperatures above ambient.

A wide selection of products, manufacturers and options is available, and it can be a challenge selecting the right system for your application. However, there are three main considerations. Firstly you need to define the basic chiller requirements such as capacity, coolant temperature, coolant and flow rate. Second, you need to determine which, if any, additional options you need such as heaters, automatic start capability, filters, alarms and communications features. Finally, you should consider other details such as noise level, size, warranty and after-sales service and support.

Define Basic Requirements

Cooling capacity defines the chiller size. The amount of waste heat that must be removed from a system is called the heat load. Usually expressed in terms of watts (W) or BTU per hour, this is the minimum capacity you must look for in a recirculating chiller. If you do not know your exact heat load in W or BTU, see the sidebar for formulae to calculate this.

Nominal chiller capacity usually is quoted at 68^oF (20^oC) setpoint temperature and 68^oF ambient air temperature. However, every chiller has a characteristic performance curve (figure 1). This shows how the chiller's cooling capacity varies with setpoint temperature. You should check the cooling capacity curves at the setpoint temperature that you will be using in order to select the appropriate chiller.

Figure 1. Select your chiller based on the cooling capacity you need. For example, if you need 2.5 kW cooling capacity at setpoint of 77oF (25oC), Model C would suffice. However, if you require 2.5 kW capacity at a setpoint of 59oF (15oC), you need to move up to Model B.

Chiller capacity declines as ambient temperature increases. The performance of a chiller in a 90^oF (35^oC) environment may drop by 20 to 30 percent from the performance quoted for a 68^oF environment. It is therefore important to ensure that you pick a chiller that has sufficient capacity at the ambient temperature at which you will be using it.

Coolant Flow Rate and Pump Options. Most chiller manufacturers offer a choice of pumps with the chiller. Positive-displacement pumps are popular because they offer a constant flow rate regardless of the pressure drop across the system. One disadvantage is that they need replacing fairly regularly. Centrifugal pumps are much longer lasting, but because the flow rate depends on the pressure drop across the system, they may not be suitable for applications with a high pressure drop. A turbine pump has the reliability of a centrifugal pump but provides greater pressure drop. Figure 2 shows typical flow-rate/fluid-pressure curves for different types of pumps. To select the correct pump, you need to first consider your desired flow rate and pressure drop across the system and then, if more than one type of pump meets your requirements, choose based on factors such as maintenance and cost.

Figure 2. A comparison of pump performance for positive-displacement, centrifugal and turbine pumps in the same chiller model shows that different styles of pumps offer different performance. To select the correct pump, you need to first consider your desired flow rate and pressure drop across the system.

Selecting the Right Coolant. Most standard-model chillers can handle ordinary distilled or tap water as a coolant. Additives such as algaecide may need to be added to prevent algae growth. For setpoints below 50^oF (10^oC), a mixture of ethylene glycol and water is recommended to lower the fluid's freezing point. (As an added advantage, adding ethylene glycol inhibits corrosion and algae.) The use of a mix of ethylene glycol and water will affect the cooling capacity of the chiller as it does not transfer heat as well as pure water. Most chiller manufacturers also offer options for deionized water and systems for high purity applications, should your application require this.

Determine the Features You Need

Most chiller manufacturers offer a variety of safety, control and convenience features to ensure optimum performance. Safety features generally add little to the cost of the chiller and safeguard your system. They may include:

High/low temperature alarms or shutoff, which help prevent damage from coolant that is too hot or too cold. A visual display, audible alarm or a computer (if an RS232 interface is used) signals the fault.

Low flow alarm and shutoff, which protect both the chiller and your system. These features are critical because if the coolant freezes, it will expand and may cause the evaporator (the component where the coolant is chilled) to crack and possibly require extensive repairs. This and other causes such as a kinked hose can cause low flow that can affect system performance.

Low coolant level alarm or shutoff, which protects your system from underperforming due to coolant loss, possibly as a result of evaporation or leaks.

Audible alarms, which signal a fault condition to a remote operator.

Fault shutoff and relay contacts, which can automatically shut down the chiller or the system if a fault occurs.

Other optional features include:

Coolant filters, which protect the pump and other elements in the cooling circuit from harmful particles.

Air filters, which prevent dust and dirt from accumulating on the condenser (leading to a loss of cooling capacity).

Hot-gas bypass, which prevents unnecessary wear on the compressor. Unlike home refrigerators, in which the compressor is cycled on and off, with hot-gas bypass, the flow of hot refrigerant is diverted to bypass the condenser and flow directly to the evaporator.

Water-cooled condenser, which allows you to reject the heat from the condenser into a facilities water stream instead of into the air in the room.

Fractional degree setpoints, which allows accurate temperature setpoints in smaller increments such as 0.2^oF (0.1^oC).

Heaters, which enable above-ambient temperatures to be maintained and also enable setpoint temperature to be reached rapidly.

Remote start and shutdown, which lets you to remotely power up or down the chiller.

Communications features allow you to connect your chiller to a computer. A communications interface such as RS232 permits you to use a computer to start and stop your chiller remotely. With this kind of interface, you can use a computer to monitor or set parameters for the temperature setpoint, actual coolant temperature, coolant pressure and fault conditions. You also can be informed of conditions such as declining coolant pressure that forewarn of potential problems in your chiller's operation.

Beyond Equipment Specs

Several other considerations may be important for your application when choosing a chiller. If people are going to be working near the chiller, you may want to consider the noise level it produces, as this varies between manufacturers.

Ease of operation is another function. You should ensure that the chiller is intuitive and easy to program. If it is being used in equipment that may end up overseas, are the symbols displayed understandable in any language? You should check that operating temperature, setpoint, pressure and coolant level can be seen at a glance. From a practical standpoint, consider whether extras such as casters for easy moving or a rack-mount system are necessary.

Last, have you considered after-sales service? How long is the vendor's warranty? Can you get around-the-clock technical support? Do they offer on-site service or must you return the product to their factory? A little research into that now can save many headaches later.