The volume of chilled water in the chilled water system acts as a damper and smoothes out the effects due to load change. Process loads tend to be the most abrupt and can cause the chiller to shut down on safety if the system is not designed correctly. As well, when a chiller is close-coupled to a large dominant air conditioning load (A chiller connected to a single large air handling unit for example) the chiller can “hunt” on the control valve and result in unstable operation.
There are other issues, which need to be considered as well. For instance, a small chiller using scroll compressors will change capacity in discrete steps. For example, a four compressor chiller will have a stepped capacity of 25, 50, 75 and 100% cooling capacity. If the load is between the steps, for example, 65% capacity, then the chiller will operate at the step above and then the step below the actual load. The chiller will require the chilled water fluid volume to “dampen” the effect of the chiller either over or undercooling the chilled water.
Another example is where the load drops below the minimum capacity the chiller can operate. In this case, the chiller will cycle a compressor on and off to meet the load. If there is not enough fluid in the system, the compressor will incur too many starts, which will result in undue wear and tear on the chiller.
1.0 Estimating System Volume
To evaluate whether there is sufficient volume in the system requires first estimating the amount of fluid in the system. To find the volume, the amount of fluid in the chiller evaporator, piping, and coils must be added together. Table 7 provides the fluid volume per foot for standard piping. Chiller evaporator volumes can be found in the chiller catalogues and/or computer printouts. Coil volumes are often provided by computer selection outputs or can be estimated by assuming 0.15 USgal/ft2*row.
Whether there is sufficient fluid volume can be found using the following formula:
Where:
Ts = time from start to start, minutes
Vw = fluid volume, USgal
H1 = minimum operating capacity of the chiller, Btu/hr
H2 = minimum applied load on the chiller, Btu/hr
TD = dead band of chiller controller, °F
The values used in this formula should come from the specific chiller and application. Where these values are not immediately known, Table 8 provides some guidance. Chiller short cycling is a serious issue that can shorten the life or damage equipment and result in poor performance. Whenever a chiller is close-coupled to a single large load, the designer should review this issue carefully.
