Five Considerations for Chiller Plant Design
Chiller plant design can be challenging; one may not know where to start. Below are the five design considerations to get going.
- Type of Chiller
You have to decide between air-cooled vs. water cooled; centrifugal vs modular. Air-cooled are good when you have space outside but limited space indoors. Air-cooled tend to be less expensive but require outdoor space and can be loud. Water-cooled are more expensive and require the addition of cooling towers and condenser water.
Centrifugal tend to have a much large footprint but are more efficient and cost effective for large systems. Modular chillers allow for chilled water when there is a limited square footage available and are great for retrofits.
Standard equipment is rated for 125 psi. Pressures tend to become a concern in buildings taller than about 280 feet tall and the chiller plant is located at the top or the bottom of the system, or when the greatest working pressures exceed the 125 rated capacity. When this condition occurs, the designer must look into providing a pressure break through a heat exchanger or look to locate the chiller plant in a central location. In tall and super-tall buildings, pressures can exceed 300 psi, and the chilled water system may require multiple pressure breaks. In these situations, the floor on which the chiller plant is located on becomes critical.
Typical temperature deltas for chilled water systems range between 10 and 16 degrees. Supply temperatures range between 38 degs F and 44 degs F. Lower supply temperatures and higher delta Ts result in smaller coils at air handling units and terminal units but result in higher energy consumption at the chiller. Tall buildings requiring multiple pressure breaks require the use of lower design temperature because at each pressure break, two degrees of temperature are lost.
It is best to talk with the building operator to determine when the chiller plant will need to operate. Are there critical loads that will require cooling 24/7, 365 or is the chilled water system responsible for space cooling and will only operate during the summer? One also has to consider the load profile the chiller will see. The operation of the system will rarely be constant, and the equipment must be capable of turning down to meet low load conditions.
For large systems, the piping system is typically variable, primarily because there is a lower upfront cost and lower energy use. For modular chillers, primary-secondary pumping arrangements can be beneficial due to the need to maintain minimum flow through the chiller on low load days to avoid cycling the chiller or turning off due to low flow. Primary secondary remains a good option for modular chillers when the load profile is constant.