Regardless of your application requirements, understanding the major factors when sizing a cooling system is crucial to obtaining the right product fit. Pairing this information with the proper use of an online selector tool can help improve efficiency, reduce frustration and even correctly repurpose existing systems when necessary.
A working knowledge of chiller performance factors is a necessary solid foundation to incorporate proper consideration of them into the early design stages (figure 1). That knowledge also will help with troubleshooting or service issues.
1. How Much Cooling Capacity Do You Need?
The most important factor in choosing a cooling system is to determine the amount of heat rejection into the cooling loop. An undersized chiller has the potential to create issues such as:
- The chiller will not provide fluid at the desired temperature.
- The system being cooled suffers in performance or experiences a catastrophic failure.
- Overtemperature alarms are triggered, potentially shutting down the system and process. (If overtemperature automatic shutoff is not included, the chiller can be damaged.)
Typically, the manufacturer of the equipment requiring cooling will provide the necessary specifications to size a chiller. If the information is not available, or if the process is an in-house design, then the heat load can be determined experimentally or theoretically. Experimental results are preferred for their accuracy. If experimental measurements are not practical, theoretical methods can be applied. They are inherently conservative, however, and they typically result in an oversized system.
Experimental Method. The equation
Q = m x CP x ΔT
can be used where
Q is the heat load (BTU/hr).
m is the mass flow rate (lb/hr).
CP is the specific heat (BTU/lb-°F).
ΔT is the change in temperature (°F).
Using two thermocouples, a flowmeter in combination with a cooling fluid that has known properties, and a pump (to collect inlet/outlet temperatures and flow rate data) will allow you to determine values for these variables so Q can be derived.
Theoretical Method. This method uses the first law of thermodynamics (conservation of energy). If electricity is the only form of energy entering your system, it can be conservatively assumed that all the power going into a device is being given off in the form of heat. A significant portion is consumed by equipment operation, but this is a conservative approach, limiting any potential for undersizing.
For example, suppose a system requires 230 V, single phase, at 22 A. Use the equation to calculate power consumption:
P = V x I
where P is power in watts; I is current in amps; and V is voltage in volts. Using the example system:
P = 22 A x 230 V
P = 5,060 W of heat requiring dissipation
Note: For a system using three-phase power, the result must be multiplied by three.
2. What Cooling Fluid Temperature Does Your Process Require?
The setpoint temperature will affect the cooling capacity of a chiller. Decreasing the temperature will put more load on the refrigeration system, and vice versa for increasing. There is a direct relationship between the temperature at which you have set the chiller and its total cooling capacity. Therefore, it is important to review the chiller’s performance (figure 2). Typically, these are published in the user manual or on the manufacturer’s website; if not, contact the supplier. When running variable temperatures/heat loads, the curves will be a useful reference.
As another example, follow along the x-axis of the graph in figure 2. Note the chiller output temperature vs cooling capacity.
3. In What Environment Will the Chiller be Operating?
Ambient Temperature. An air-cooled chiller’s ability to dissipate heat is affected by the ambient temperature. This is because the refrigeration system uses the ambient air/refrigerant temperature gradient to induce heat transfer for the condensation process. A rising ambient air temperature decreases the temperature differential (ΔT) and, subsequently, reduces the total heat transfer. An example of the decrease in performance for a 86°F (30°C) ambient environment as opposed to 68°F (20°C) can be seen in figure 2. Typically, this is a 2 to 3 percent cooling capacity decrease for every 1.8°F (1°C) increase in the ambient temperature of the chiller’s operating environment.
If the chiller uses a liquid-cooled condenser, high ambient temperatures can still have negative effects on key components such as the compressor, pump and electronics. These components generate heat during operation, and elevated temperatures will shorten their lifetime. As a guideline, the typical maximum ambient temperature for nonexterior rated chillers is 104°F (40°C).
Spatial Constraints. Adequate air circulation space around the chiller — typically specified in the user manual — is important to maintain the proper ambient air temperature. Without proper airflow, recirculation of the resulting insufficient air volume rapidly heats that air. This decreases chiller performance and potentially can damage the chiller.
Electrical Supply. Based on region, electric power frequency is either 50 or 60 Hz. At 60 Hz operation, motor speed (rpm) is approximately 20 percent higher, delivering more power to the compressor, pump and fan. This factor is demonstrated by the performance curves in figure 2. The dashed lines represent 50 Hz and the solid represents 60 Hz.
The available frequency in the country of operation as well as the electrical input rating of the chiller (voltage, frequency and number of phases) must be known to correctly size a chiller.
4. What Type of Process Fluid Will Be Used?
There are two main concerns when considering fluids: performance and compatibility.
Performance. The performance of a fluid is based upon its properties at a given temperature. Relevant parameters are specific heat, viscosity and freezing/boiling points. There is a direct relationship between specific heat and cooling capacity. Mixing some percentage of ethylene or propylene glycol with water (typically in the 10 to 50 percent range, depending on value boundaries) is recommended when low or high setpoint temperatures are desired.
Compatibility. The potential for corrosion and the early degradation of seals are common failure modes for incorrectly sized systems. Materials of construction and fluids should be compared prior to use. The inclusion of a corrosion inhibitor in the cooling fluid is strongly recommended to maintain system integrity and prolong optimal operation.
5. What are the Flow and Pressure Requirements of the Process Loop?
Pump life is of primary concern when configuring an industrial cooling system, but the pressure loss across the system and necessary flow rate first must be satisfied by the pump choice.
Pressure. Using an undersized pump will prevent the fluid from flowing through the entire cooling loop. If the chiller has been equipped with internal pressure relief, the flow will be diverted around the process and back into the chiller. If there is not internal pressure relief, the pump will attempt to provide the necessary pressure and run at what’s referred to as dead-head pressure, or its limit. When the latter occurs, the pump’s life will be drastically reduced. When a pump enters a dead-head state, liquid ceases to flow and the liquid in the pump becomes hot, eventually vaporizing. The liquid vaporization disrupts the pumps ability to cool, causing excessive wear to bearings and impellers.
Note that when potential for blockages to occur in the process exists, the option for a pressure-relief valve is strongly recommended.
Determining the pressure loss across a system requires plumbing pressure gauges at the process’s inlet and outlet, then using a pump with sufficient pressure to performance, to get values at the desired flow rate.
Flow. Inadequate flow will yield inadequate heat transfer. The flow will not remove the heat necessary for safe operation of your process. As the fluid temperature increases beyond the setpoint, the surface/component temperatures also will continue to rise until a steady-state temperature — greater than the initial setpoint — is reached.
Most systems will detail the pressure and flow requirements. When specifying the necessary heat load removal as part of the design, it is important to account for all hoses, fittings, connections and elevation changes integral to the system. These ancillary features can significantly increase pressure requirements when not sized appropriately.
In summary, adding these five considerations to your technical toolbox will help in determining the optimal industrial cooling system for your application.