Figure 5-2 shows an array of six nominal 5-ton air-to-water heat pumps that supply chilled water for cooling a small industrial facility equipped with chilled beam terminal units (to be discussed in section 7). These heat pumps also supply warm water for heating.
It’s important to understand that the piping shown in Figures 5-1a and 5-1b allows any of the heat pumps to operate, in the same mode, as required by the load, but does not allow some of the heat pumps to operate in cooling mode while others simultaneously operate in heating mode. Mixed mode operation is useful in some buildings, especially during swing season conditions. On a cool day in fall or spring, perimeter areas of a building may require heating, while core areas of the building require cooling.
Simultaneous heating and cooling from a multiple heat pump array is possible, but requires more piping, valves and controls. Figure 5-3 shows one example of piping that could be used with an array of three air-to-water heat pumps to allow simultaneous heating and cooling.
This system has a “hot” buffer tank and a “cold” buffer tank. The system controls monitor the temperature of each tank and call for a heat pump to turn on when the temperature of either tank deviates slightly from its target temperature or outside of a set temperature range. The water temperature in the hot buffer tank would be regulated based on outdoor reset control. The water for the cold buffer tank would be maintained between upper and lower temperature setpoints, such 45ºF and 60ºF whenever a cooling load is present.
When a heat pump is called to operate, the zone valve pairs associated with its mode of operation open. The status of the heat pump’s reversing valve is also set. A variable-speed pressure-regulated circulator operates to create flow through the appropriate buffer tank. The speed of the circulator is based on proportional differential pressure control. The speed automatically increases or decreases depending on how many heat pumps are operating.
The valving at each heat pump is also arranged so that the zone valves or heat pump can be isolated from the balance of the system if necessary for service.
Each buffer tank provides hydraulic separation between the heat pump circulators and the load circulators.
The heating zones are supplied by low temperature radiant panels. Flow to each manifold station is controlled by a zone valve. Each manifold station piping assembly is also equipped with a balancing valve and purging valve. These valves are arranged so that each manifold station and its associated zone valve can be completely isolated from the balance of the system if necessary for service.
The cooling zones are supplied by fan-coils. Each fan-coil piping assembly is equipped with a balancing valve and purging valve. These valves are arranged so that each fan-coil and its associated zone valve could be completely isolated from the balance of the system if necessary for service.
Because this system can simultaneously supply heating and cooling, one mode of operation must take priority when staging the heat pumps. Several possibilities exist. For example, during heating season the ability to maintain adequate water temperature in the “hot” buffer would likely be the priority. Once that temperature is established, at least one of the heat pumps would be allowed to operate in cooling mode if a cooling load is present. During the cooling season, it’s likely that all heat pumps would be prioritized to satisfy the cooling load.