When choosing a chiller or system of chillers, one consideration that’s often overlooked is the amount of heat generated by each unit (also known as heat rejection). This is important for a number of reasons including HVAC considerations and ensuring the cooling capacity of the chiller is not impacted.
Here, we discuss more about the reasons you’d want to calculate the amount of heat a chiller produces and how to carry out the necessary calculations.
Why Generated Heat Is Important
We know that any equipment that runs on electricity generates at least some heat. Chillers, in particular, can produce a significant amount of heat, mainly generated by the compressor, fan motor, pump, and electronics.
Quantifying the level of heat is important for several reasons, the main one being HVAC considerations. In most facilities, ambient temperature will be important in ensuring a comfortable environment for workers. In addition, higher ambient temperatures can affect the performance of the chiller itself by lowering its cooling capacity.
Calculating how much heat is generated by a chiller enables you to determine the burden on the HVAC system of the building. The more heat generated by a chiller, the harder air conditioning units will need to work to remove heat from a space. Conversely, with more heat generated, heating systems won’t have to work as hard.
Performing these calculations for various units can help you make decisions such as which chiller to purchase, which location a chiller should be placed in, and what size HVAC system you require. Although the calculations below result in estimated rather than precise values, these numbers can at least provide a basis on which to tweak your setup to minimize overall costs.
How to Calculate the Heat Generated by a Recirculating Chiller
The way you calculate the heat generated will depend on one important factor: the type of condenser the chiller uses. Air-cooled condensers will release heat into the air rather than transferring it to the facility water as do water-cooled condensers. As such, you can expect the heat generated by models with air-cooled condensers to be far greater than by units with water-cooled condensers.
The calculations below are sourced from this article published by Process Cooling magazine. The information you need to calculate the heat generated by a chiller can be found in the chiller specifications. If you’re looking at an existing chiller, these numbers should be on the label. If you’re considering purchasing a chiller, you may be able to find these specifications online or you can ask the distributor or manufacturer to provide you with the necessary information.
Note that we are estimating so these calculations won’t give you an exact figure but rather an approximation.
Air-Cooled Condenser
In an air-cooled condenser, all electrical power is converted to heat and is released into the surrounding environment. To calculate the electrical heat generated, use the following calculation:
We = V x (√Φ) x A
Where:
- We = electrical power
- V = voltage
- Φ = phase
- A = total amp draw
A few notes:
- For the voltage, taking the upper limit (for example, using 230V where the range provided is 200–230V) will give you a “worst case” approximation of the heat generated.
- The phase refers to whether the chiller has a single-phase or three-phase power system, denoted by “PH” in the specifications. So you will either use √1 or √3.
- The amp draw part of the equation is the total amp draw from the compressor, pump, and fan. These components may not all match in terms of phase, for example, the pump and fan may be single-phase even if the unit itself is three-phase, so again, we may end up with an overestimation of the amount of heat generated.
For an air-cooled chiller, you also need to add the process heat as all of this will be released into the room. To find the upper limit of heat generated, you can use the full amount of heat that the chiller can handle cooling, for example, 5,000 W.
Then your total heat generated can be calculated by adding the electrical heat and process heat (Wp).
Wtotal = We + Wp
While your answer will be in watts, to convert watts to BTU/hr, you can use the following conversion:
1 W = 3.412142 BTU/hr
Water-Cooled Condenser
With a water-cooled condenser, you don’t need to consider process heat as this will be transferred to the facility water.
Additionally, not all of the electrical heat will be transferred to the room, but this differs for the various components (fan, compressor, and pump). As such, instead of considering the total amp draw of the three components as above, we need to break down the electrical heat calculation into its separate components.
Fan
All of the fan heat will dissipate into the room, so this part will remain unchanged.
Wfan = V x (√Φ) x A
Compressor
Most (94 percent) of the compressor heat will be removed by the facility water. So the total compressor heat will be:
Wcomp = V x (√Φ) x A x 0.06
Pump
The amount of heat produced by the pump will vary depending on type, horsepower (HP) pressure, and flow. To estimate the amount of heat from the pump, we can subtract the HP from the power usage. To convert the HP to watts the formula is:
Wp = HPp x (746W/HP)
Where:
- Wp = pump power
- HPp = pump horsepower
For example, if the pump horsepower is 3, you would have:
Wp = 3HP x (746W/HP) = 2,238 W
Then the amount of heat released into the room from the pump will be:
Wpump = (V x (√Φ) x A) - Wp
Total
To determine the total heat released into the room, you calculate the total heat released from the fan, compressor, and pump.
