Last month, we discussed how a constant volume heating, ventilation and air conditioning (HVAC) system would continually pump the same flow rate of chilled water through the entire system regardless of the cooling load. This worked well when maximum cooling was needed on a hot day, but during times of low cooling load, much of the chilled water would go through the bypass. This method allowed the system to be controlled, but at a high cost. Also discussed was how centrifugal pumps connected to variable speed drives (VSD) could be used to reduce the pumping cost in a chilled water system by varying the flow rate through the system to match the system’s cooling loads.
A New System
Since HVAC systems are recirculating systems, there is no static head for the pump to overcome. As a result, the pump simply needs to make up the head loss associated with the flow through the interconnecting pipelines and the air handlers. The head loss in a closed system can be simplified by graphing the head loss versus the flow rate (see Figure 1). By superimposing head loss curve on the pump curve, one can see how the system can work together. The flow rate through the system occurs at the intersection of the pump and head loss curves.
By installing a VSD on the pump, the pump’s speed can be adjusted so the system flow rate can be matched to the flow rate required for the system heat load. Using this approach, the flow rate through the system can be controlled by adjusting the speed of the pump using the VSD.
In chilled water systems, a design temperature drop is established for the heat exchangers in the air handlers. The flow rate through the air handlers is adjusted so the air outlet temperature of the air handler equals the temperature set on the occupied space thermostat. If the temperature of the occupied space is too high, the chilled water flow through the air handler is increased. When the temperature of the space is below the set value, the chilled water flow rate through the air handler is decreased.
Now let’s see what happens when the thermal load of the space is only 50 percent of the designed heat load. Since the heat load of the space is only 50 percent of the design value, the chilled water flow rate through the air handler only needs to be 50 percent of what is needed.
This causes the pump to operate back on the pump curve, resulting in greater discharge pressure. As a result, the differential pressure across the temperature control valves is higher at lower flow rates.
In HVAC systems, with the large number of cooling circuits, air handlers and temperature control valves, the system must be designed to minimize pressure drop across the temperature control valves. This is why constant volume systems are still widely used in HVAC chilled water systems.
Constant Flow Systems
In a constant volume system (described in last month's column), the flow rate through the system is maintained at a constant flow rate regardless of the system’s thermal heat load. The space room temperature is maintained by bypassing some of the chilled water around the air handler. In our example, 50 percent of the design flow rate will pass through the heat exchanger with the remainder bypassing the heat exchanger. Once the system is balanced, the differential pressure across the temperature control valve can remain at a low value under a variety of flow rates. Since the chilled water flow rate through the system must meet the system’s highest thermal load regardless of the current thermal load needed to meet the operating conditions, this will control the system but at a higher pumping cost.
Variable Volume Systems
In a variable volume system, only the chilled water needed to meet the thermal load passes through the system. Without a variable speed pump, the temperature control valves in each circuit are not large enough to accompany the higher differential pressure required. Figure 2 shows what happens when operating the system at 50 percent of the thermal cooling load.
Using a pump with a VSD installed, the rotational speed of the pump is adjusted so the pump curve intersects the head loss curve at the 50 percent flow condition. Not only is the pump passing only 50 percent of the design flow rate, but the system requires less head at the flow rates. This reduction in pump head and flow rates results in a tremendous energy savings.
Looking at how a pump with a VSD installed saves energy, building owners were installing them on their chilled water systems. This was successful when installing VSDs on new systems, usually HVAC chilled water systems, but there were problems when installing them in existing systems.