To provide some redundancy in the HVAC design, most designers will require two or more chillers. Multiple chillers also offer the opportunity to improve on overall system part load performance and
reduce energy consumption. Parallel chiller plants are straightforward to design and are easily modified for variable primary flow.
Figure below shows a parallel water-cooled chiller plant. Chilled water is circulated by the chilled water or primary pump through both chillers to the load and back to the chillers. The chilled water loop can
be either constant flow or variable flow. Variable flow systems increase the complexity but offer significant pump work savings. They also resolve the issue about chiller sequencing that occurs with parallel chillers, constant flow.
Variable flow systems are covered in Primary/Secondary Systems and Variable Primary Flow Design. A condenser loop is required for water-cooled chillers. This includes a condenser pump, piping and a cooling tower or closed circuit cooler. The condenser loop operates whenever the chillers operate.
For constant flow systems, the chilled water temperature range varies directly with the load. Depending on load diversity, the chiller design temperature range will be less than the temperature range seen at each load. In this case, the chiller temperature range is 8°F while the cooling coil range is 10°F . The overall result is increased chilled water pump and pipe capital cost plus higher annual pumping cost.
Parallel chillers experience the same percent load. For example, consider a chiller plant with a 100-ton and a 1000-ton chiller operating at 50% capacity. With both chillers operating, both chillers will operate at 50% capacity. The 100-ton chiller will be at 50 tons and the 1000-ton chiller will be at 500 tons. This occurs as long as the flows don’t change (i.e,. variable primary flow) and both chillers see the same return water temperature.
In most cases, the sum of the chiller capacities meets the design for the building or process. Additional capacity can be added, if required, by oversizing the chillers. It is common for parallel chillers to be the same size and type although this not a requirement. Water-cooled, air-cooled or evaporatively-cooled chillers can be used. Air and evaporatively-cooled chillers do not require a condenser loop including piping, cooling tower and pump.
Pumps can be constant or variable flow. The chilled water pump is sized for the design flowrate. Figure below shows one main chilled water pump providing flow to both chillers. An alternative method is to have two smaller pumps serving dedicated chillers. Figure below also shows dedicated condenser pumps and cooling towers for each chiller. The pumps and piping are sized for the design condenser flow for each chiller. Whenever the chiller operates, the condenser pump operates.
Water-cooled chillers will require cooling towers. Figure above shows dedicated cooling towers for each chiller. A common cooling tower is also possible but not common for parallel chillers.
Parallel Chiller Sequence of Operation
Parallel chiller plants create a unique situation when used in a constant flow system. Consider the system operating at 50%. From a chiller performance aspect, turning off one chiller and operating the other at full capacity is desirable. However, this will not happen. At 50% capacity, the return water will be 49°F. The chiller that is turned off will let the water pass through it unchanged. The operating chiller will only see a 50% load (49°F return water), and will cool the water down to the set point of 44°F. The two chilled water streams will then mix to 46.5°F supply temperature.
If the system is operated in this manner, the warmer chilled water will cause the control valves to open (increase flow) to meet the space requirements. An iterative process will occur and the system may stabilize. The issue is whether the cooling coils can meet the local loads with the higher chilled water temperature. Depending on the actual design conditions, the building sensible load could be met but high chilled water temperature will make it difficult to meet the latent load. Since this scenario is likely to occur during intermediate weather, dehumidification may not be an issue. In areas where humidity is an issue, this arrangement can result in high humidity within the space.
One solution is to operate both chillers all the time. This works and is a simple solution, however, it is not energy efficient and causes unnecessary equipment wear.
Another possibility is to lower the operating chiller’s set point to offset the mixed water temperature. This also works but has some difficulties. Lowering the chilled water setpoint requires the chiller to work harder, lowering its efficiency. In extreme conditions, it can cause chiller stability issues.
Adding isolation valves to stop flow through a chiller when it is not operating is not recommended for a constant flow system. It is unlikely that the pump will be able to provide design flow if all the chilled water is directed through just one chiller. The pump will ride its curve and a loss of flow will occur. Without design flow, it is unlikely that all the individual loads will receive their required flows. In the event the pump could actually provide the flow through one chiller, the maximum allowable flow rate for the chiller may be exceeded resulting in serious damage to the chiller.
The safe answer is to operate both chillers all the time chilled water is required, however, this is as expensive as operating a single chiller plant. Staging on the pumps and cooling towers is similar to that outlined for single chillers.
Parallel Chiller Plant Example
Consider the same model building used in the Single Chiller example. The design load performance is identical to the single chiller plant. There are small changes in real applications when two chillers are used instead of one. For instance, pump and chiller selections are not likely to offer identical performance, other than being half the size.
What is more interesting is the annual energy usage is the same for both single and parallel chillers. This occurs because both chillers were operated to provide 44°F supply chilled water at any plant load. With both chillers operating, all the pumps and towers had to operate as well. There was no opportunity to use only one chiller at light loads, shut down one tower and condenser pump and shift the single chiller further up its performance curve.
This could be accomplished by switching to variable primary flow, which would allow a chiller to be isolated at light loads, as well as to reduce the chilled water pump size and to lower its operating cost.