Many applications require water for cooling. Rotary evaporators, microscopes and spectrophotometers found in the laboratory can require cooling water. Industrial-scale applications such as semiconductor, additive manufacturing and extraction processes also typically require water for cooling. In many cases, tap water, cooling tower water or process cooled water may not be available, or it may not be suitable for use — for reasons such as cleanliness, incorrect temperature, inadequate temperature stability, inappropriate flow or pressure — or it may not be able handle the cooling load. As a result, facilities across the entire spectrum are presented with considerable process cooling challenges that are typically met by specifying a dedicated recirculating chiller.

Recirculating chillers supply a source of temperature-controlled fluid — typically water or an ethylene glycol/water mix — that removes heat from a process. This heat then is transported back to the chiller and transferred to the refrigerant gas. In addition, because chillers are electricity powered, they generate their own heat that originates from the system’s fan motor, compressor, pump and electronics. The total amount of heat released by a recirculating chiller into the room depends on whether the chiller has been specified with an air-cooled or water-cooled condenser.

Air-Cooled vs. Water-Cooled Systems for Industrial Fluid Cooling

Air-cooled chillers offer the key advantage of being standalone systems: They do not require the simultaneous use of other facility water sources. However, air-cooled chillers absorb the heat load from the application (process heat) and then release it into the surrounding environment via the system’s condenser. In addition, all electricity used by the chiller is converted to heat that also is added to the room where the system is located. As a consequence, air-cooled chillers release quite a large total amount of heat, making it necessary to have a sufficiently powerful HVAC system capable of removing the additional heat load from the application and air-cooled chiller.

Water-cooled chillers, by contrast, work by removing process heat from the condenser using a source of facility water. Most of the heat generated by the water-circulation pump and the compressor also is added to this water. The remaining heat generated by the chiller is released into the room, but it is significantly less than that of an air-cooled chiller. Assuming the availability of facility water at the required flow and pressure differential to cool them, chillers with water-cooled condensers are more desirable for large-capacity applications or where multiples of smaller chillers are installed.

Calculating the Heat Burden from a Recirculating Chiller

Employing a recirculating chiller in place of the typical facility water can benefit facilities in many ways. Some of these advantages include better temperature stability and accuracy, protection of valuable process equipment, a reduction in associated maintenance costs, and enabling cost-effective use of facility resources. However, an excessive rise in room temperature due to the heat load released by the chiller will degrade the system’s cooling capacity. To prevent this, accurately calculating the total amount of heat a recirculating chiller releases to a room is critically important for determining the most appropriate type of chiller to use, as well as for deciding where the system should be installed. Doing so will ensure optimal chiller performance and worker comfort while minimizing additional costs associated with running or upgrading HVAC systems.

Conventional methods used to calculate the total heat burden placed by a recirculating chiller often generate erroneous results. An alternative method that delivers precise results involves calculating the chiller’s power consumption and the process heat load. With regard to the energy usage of the chiller, this can be calculated by referring to the system’s serial-number tag, which details its electrical specifications, using the simple formula below to translate the chiller’s power consumption into watts.

V x Ø x A = W

where
V is voltage
Ø is square root phase
A is amps
W is watts

Essentially, this calculation requires knowledge of the voltage range, power phase and total amount of electrical current (amp draw) on which the chiller operates. On single-phase chillers, the voltage range and power phase normally are referred to on the system’s serial-number tag. On three-phase chillers, however, the amp draw may need to be calculated from the serial-number tag data. On three-phase units or where specified, the amp draw is the sum of the compressor running-load amps (Comp RLA), pump motor full-load amps (Motor Pump FLA) and fan full-load amps (Fan FLA).

The following real-life application examples demonstrate how this simple method of calculating the total amount of heat released by a recirculating chiller can be used to determine the most suitable type of system for each process.

Example 1: Air-Cooled Chiller

A powerful air-cooled chiller with one large 3-hp centrifugal pump and one fan was used. It offers a cooling capacity of 10 kW. The system’s electrical specifications were as follows:

  • 200 to 230 V, 60 Hz, 3-phase
  • MCA is 22.3 MOPD is 35.0
  • Compressor RLA is 10.4
  • Compressor LRA is 78.0
  • Pump motor FLA is 8.6
  • Pump HP is 3.0
  • Fan FLA is 0.7

Per the formulas below, the amp draw was first calculated, and this number was subsequently used to calculate the power consumption of the air-cooled chiller.

10.4COMP + 8.6PUMP + 0.7FAN = 19.7 A total

230 V x √3 Ø x 19.7 A = 7,848 W

To complete the room heat-load calculation, 10,000 watts (10 kW) of process heat generated from the application were added, summing up to a total heat load of 17,848 watts (17.8 kW or 60,915 BTU). The calculation is shown in figure 1.

This number then was converted to BTU using the following equation:

17.8 kWRoom x 3,412 BTU/kW = 60,734 BTURoom

This represents the worst-case heat load to the room from the air-cooled chiller under the specific process load.

Example 2: Water-Cooled Chiller

The same recirculating chiller was used, but it was equipped with a water-cooled condenser. The system’s electrical specifications were as follows:

  • 200 to 230 V, 60 Hz, 3-phase
  • MCA is 22.3 MOPD is 35.0
  • Compressor RLA is 10.4
  • Compressor LRA is 78.0
  • Pump motor FLA is 8.6
  • Pump HP is 3.0
  • Fan FLA is 0.4

As with the previous example, the amp draw was first calculated, and this number then was used to calculate the power consumption of the air-cooled chiller.

10.4COMP + 8.6PUMP + 0.4FAN = 19.4 A

230 V x √3 Ø x 19.4 A = 7,728 W

Adding the 10,000 W (10 kW) of process heat, the total heat load was calculated at 17,728 W (17.7 kW).

While this is not much less than the total heat load generated by the air-cooled chiller, the key differentiating parameter is that not all of that heat is being released into the room. Additional calculations are required to determine how much is actually released to the chiller’s surrounding environment.

With regard to the process heat, it all goes into the facility water supply. In addition, approximately 94 percent of the chiller’s compressor power is converted into heat by raising the refrigerant gas temperature during compression (heat-of-compression). This heat also is removed by the facility water-cooled condenser, resulting in a small amount of heat released into the room as shown.

Compressor heat:

230 V x √3 Ø x 10.4 A = 4,143 W (4.1 kW)

4.1 kW x 0.94 = 3.9 kW to the facility water

4.1 kW – 3.9 kW = 0.2 kW to the room

The system’s pump motor also generates heat, the total amount of which varies with pump horsepower, type, flow and pressure. For an approximation of the heat released into the room by the pump motor, the pump horsepower is converted to kilowatts and subtracted from the pump’s power usage. While the pump horsepower is not always on the serial-number tag, the manufacturer should be able to supply it, or it will be on the pump motor itself. The horsepower to kilowatts conversion was calculated as follows:

kWP = HPP x [0.746 kW / HP]

where

kWP is the total pump power in kilowatt as
HPP is pump horsepower

Pump heat:

30 V x √3 Ø x 8.6 A = 3,426 W (3.4 kW)

3 HP x 0.746 kW/HP = 2.24 kW

3.4 kW – 2.24 kW = 1.16 kW to the room

The remaining 2.24 kW goes into  the facility water.

Finally, water-cooled chillers use slightly less energy than air-cooled chillers because they do not require a large fan to move air across the condenser. Instead, the systems use a small fan to exhaust the heat from the case.

Fan heat:

230 V x √3 Ø x 0.4 A = 159 W (0.2 kW)

0.0 kW to the facility water

0.2 kW to the room

As a result, the total heat load released by the water-cooled chiller into the room was 1.6 kW, and that released into the facility water was 16.1 kW.  The total heat load from the water-cooled chiller to the room, and the total heat load from the water-cooled chiller to the facility water, are shown in figure 2.

The total room heat load was then converted to BTU using the following equation:

1.6 kWROOM x 3,412 BTU/kW = 5,459 BTUROOM

Table 1 summarizes the findings of the two application examples.

It is apparent that a water-cooled chiller releases a much lower amount of heat into the room compared to an air-cooled chiller. However, there are other key parameters to take into consideration when deciding which type of system to employ.

Key Considerations When Selecting a Recirculating Chiller

For facilities that have or will have either a chilled-water system or sufficient HVAC to run either air- or water-cooled chiller, then the amount of energy used to remove the heat from the facility is about the same.

The total heat burden placed by recirculating chillers is proportionate to the total number of systems installed and used. As such, with regard to the number of systems needed by a facility, future projections should take this into consideration. Even when there is sufficient HVAC capacity, as the number of air-cooled chillers in a room increases, there needs to be sufficient airflow through the room that also is directed to removing the hot air immediately surrounding the chillers. Ceiling vents alone may not accomplish this. Insufficient removal of the hot air around air-cooled chillers also can reduce their performance and make it uncomfortable for any workers in the area.

If it is determined that the future needs for HVAC or facility water cannot be met by the current systems, then a cost analysis for adding capacity to those systems should to be completed prior to committing to a condenser type. It may lead to a different purchasing decision.

As a general rule of thumb, when multiple chillers need to be installed, or when there is a high probability that more chillers will need to be added in the future, using water-cooled units might be a better choice. It typically is easier to add more water lines than it is to add HVAC ducting. By contrast, if just one or a few small chillers are to be employed in a facility, room or area — with little to no requirement for additional chillers to be installed in the future — and there is a HVAC system in place with a sufficient cooling capacity and airflow, then an air-cooled chiller is more likely to be the system of choice.

In conclusion, many industries rely on recirculating chillers to establish ideal temperature conditions that will ensure the efficiency of their critical applications. However, to address long-term cooling goals, it is vital to alleviate the heat burden placed by recirculating chillers themselves. This can be done by calculating the total amount of heat load released by the system into the room where it is located and combining this knowledge with other key considerations, including the number of chillers required, their location and the presence of workers in the room. Taking all of these into account can facilitate an informed choice between an air-cooled and a water-cooled chiller.