Chiller plant cooling
How to reduce the heat load inside a chiller plant
- Understand chiller plant heat sources and how to calculate heat load.
- Learn about the how and why to manage cooling, ventilation, equipment and people.
Almost every electrical component in a chiller plant generates heat, but we’re going to focus on the major offenders. When dealing with electricity, nothing is 100% efficient. The energy losses associated with this inefficiency in most cases manifest itself as waste heat.
The single largest heat–generating component in chiller plants is often the chiller compressor motor. There are two types of chiller compressor motors, hermetic and nonhermetic. In a hermetically sealed chiller, the motor and compressor are both located within a common housing and are cooled by the refrigerant circuit within the chiller.
A nonhermetic — or open drive chiller — has an externally coupled motor which rejects heat to the equipment room in which the chiller is located. When calculating heat load, this is an important consideration to take into account. When selecting a chiller, the motor heat output is often expressed in Btu/hour or kilowatts and available from the manufacturer.
In addition to chiller motors, any additional smaller motors in the room must be taken into account when considering heat losses. In most cases these will be pump motors. ASHRAE 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings dictates minimum efficiency standards for electrical motors used in commercial settings. A map from the U.S. Department of Energy shows the adoption status for each state.
The DOE publishes a guide for motor selection and application. For motors of more than 10 horsepower, minimum standards exceed 90% efficiency with larger motors (200 horsepower and above) around 95% efficient.
Many chillers and pumps use variable frequency drives for greater efficiency at part-load conditions. While these drives allow for an overall more efficient system, they do introduce inefficiency in the conversion of the power from 60 hertz to a lower frequency. Most stand-alone VFDs (pump and tower motors) are air–cooled and reject waste heat to the space.
Chiller VFDs are often water cooled on a 480 volts or lesser voltage machine and do not reject waste heat to the space. When dealing with medium voltage machines (4,160 volts and higher) the VFD waste heat is regularly discharged to the space. Similar to motors, every VFD manufacturer publishes its own efficiency data on their products. The DOE publishes values representative of a “typical” VFD.
In addition to mechanical equipment, the following should be considered when calculating heat load:
- Transformers: While typically located outside, these can add significant heat load if located in the chiller plant.
- Harmonic control devices: If the electrical system can’t handle the harmonics associated with VFDs, harmonic control devices introduce efficiency losses and associated waste heat.
- Envelope loads: Dependent on space temperature requirements and geographic location.
- Small miscellaneous loads: Space lighting, uninsulated condenser water piping, electrical gear, uninterruptible power supply backups, control systems, etc.
See Table 1 for an example of a fully variable speed chiller plant load chart. As you can see, the impact of an open-drive chiller and an air-cooled chiller VFD have a significant impact on the room’s sensible heat load.Table 1: Heat loads: 2,000-ton chiller plant
This table shows the theoretical heat load in a 2,000-ton chiller plant. Courtesy: Jonathan Eveleth, RMF Engineering
There are two main ways to manage heat in a chiller plant, ventilation and mechanical cooling. The decision should be made between the two before designing the system to manage heat.
When ventilating a plant, both the climate and desired plant temperatures must be taken into account. The common calculation to determine ventilation airflow required is
hs= sensible heat (Btu/hour)
q = air flow (cubic feet per minute)
dt = temperature difference (°F)
ASHRAE 15: Safety Standards for Refrigeration Systems, Chapter 8.11 states that a machinery room shall not have less than 0.5 cfm/square feet or 20 cfm per person while occupied and also sets a maximum temperature rise of 18°F.
Considerations to take into account when using a ventilation approach are airflow paths and intake air space. As the heat is generated by specific sources in the chiller plant, it’s critical to ensure the air pathways don’t leave “dead zones” with little air movement. Even with the proper amount of ventilation air, if the air doesn’t move through the space appropriately the desired space temperatures will not be achieved throughout the plant.
When exhausting large amounts of air, there must be a louvered opening in the plant large enough to both allow the maximum required cubic feet per minute through and also not have water carry over during a rain event. When reviewing louvers, manufacturers publish a maximum air velocity (feet/minute) of the airflow to avoid carry-over.
The construction of louvers also results in a dramatic reduction of available free area compared to the size of the hole it fills. It’s common for louvers to have 50% to 60% free area, requiring the desired louver size to maintain a maximum air velocity to be close to twice the size of the hole. Finally, when using outside air to ventilate a plant, filtration is often desired to maintain clean equipment. Once airflow is known and a louver is selected, the following formula calculates the louver area required for ventilation.
In the case presented above, the airflow required for a 10°F temperature rise is approximately 42,500 cfm. To expand this example, if a louver has 50% free area and the desired velocity to avoid water infiltration is 800 feet per minute, the required louver size is approximately 100 square feet.
If mechanical cooling is desired, an air handling unit, fan coil units, air rotation unit or combination of equipment can be used to cool or partially cool the space. The desired setpoints should first be determined, along with the appropriate amount of outside air required to meet applicable codes.
There are multiple reasons that heat needs to be managed in a chiller plant and each individual owner’s circumstances will dictate how heat load is managed. The three major reasons heat load is managed are: Code Requirements, equipment protection and personal protection.
Code requirements are discussed above and set by ASHRAE 15.
Electrical equipment is all designed with a maximum operating temperature. The standard for commercial heating, ventilation and air conditioning equipment is 104°F. Depending on the geographical location, maintaining 104°F with ventilation alone is difficult. If peak outdoor air temperatures are 99°F, increased ventilation to achieve a 5°F temperature differential would be required to maintain 104°F. If 104°F is not attainable, equipment can be sized at a higher operating temperature or oversized and de–rated to allow safe operation at the plant conditions present. ASHRAE publishes climate data for national and international locations.
For example, if the chiller plant described above was located in Baltimore, and used ventilation for cooling, a 10°F temperature rise would keep temperatures at approximately 104°F on the 0.04% cooling design day (94.2°F).
The final reason to consider temperature control in chiller plants are for occupant comfort. When servicing, repairing or maintaining equipment in a plant the ambient temperature has a dramatic impact on the workers ability to efficiently and safely perform tasks. If a plant temperature is in the 90°F to 100°F range, worker safety measures such as frequent (multiple per hour) breaks, ice vests or portable coolers are necessary to maintain worker safety.
The Centers for Disease Control and Prevention and the National Institute of Occupational Safety and Health publish guidelines for the classification, measurement and control of heat stress when working in hot environments.
Using these guidelines and resources, a chiller plant can be designed to meet the owner’s operational needs while also providing adequate ventilation or cooling necessary to protect equipment and personnel.