Installing VSDs in Existing Systems
Many building owners were looking at installing VSDs on chilled water pumps in existing systems. This is where problems started occurring. Looking at the simplified curve in Figure 1, the head loss in the system is represented as a single loss curve, which in reality consists of multiple loops working together. Each circuit is supplied by the common pump, but each path has head losses associated with the interconnecting pipelines, air handlers and a control valve. The outlet temperature of the air handler is adjusted by controlling the flow rate through the circuit.
In HVAC systems, there are many control valves needed for the various circuits. The temperature control valves are sized with low differential pressures to keep the cost of the valve actuators down and minimize the airborne noise associated with a high pressure drop. If the differential pressure across a temperature control valve is too great, the circuit will not operate properly.
When converting an existing constant volume system to a variable volume, or when adding cooling loads to existing systems, many of the control valves show excessive differential pressure. However, there are ways to overcome the problems associated with high differential pressure to take advantage of the VSD savings. They include installing and setting balancing valves in circuits with high differential pressures, installing differential pressure controllers to minimize differential pressures across temperature control valves or installing pressure independent control valves.
Installing Balancing Valves
Manual balancing valves can be installed upstream of the temperature control valves. The balancing valves are manually set to pick up some of the circuit head loss, resulting in minimizing the head loss on the temperature control valve. This requires installing and setting the balancing valves for each circuit in the system. Any time the thermal load in a circuit or system changes significantly, the system must be rebalanced to meet the design changes.
Installing Flow Limiting Regulators
A variable volume system can be designed by installing flow limiting regulators upstream of the temperature control valves set to the circuit’s maximum flow rate. In our example, let’s assume a maximum flow rate through the cooling circuit is 50 gallons per minute (gpm) and a flow limiting regulator is installed upstream of the temperature control valve. The flow limiting regulator will not allow more than the set value regardless of the inlet pressure. The flow limiting regulator also results in head loss at the lower flow rate, much like a manual balancing valve (as described above) thus reducing the differential pressure across the temperature control valve.
This approach eliminated the need to balance the system when changing the thermal loads on other circuits in the system, but the valve must be replaced when increasing the head load on the affected circuit.
Installing Differential Pressure Controllers
As the number of circuits in an HVAC system increases, the differential pressure across the temperature control valve increases as well. This is especially true across the circuits physically closer to the pump.
In HVAC chilled water systems, the pumps provide all the hydraulic energy needed for each circuit within the system. The circuits physically located closest to the pump have less head loss in the interconnecting piping than the circuits located farther away from the pump. As a result, the circuits closest to the pump typically have a much higher differential pressure across the circuits than the heat loads located farther from the pump.
The use of a differential pressure controller senses the pressure at the supply and discharge headers across the circuits (see Figure 3). The differential pressure controller positions the regulator to absorb the excessive hydraulic energy across the supply and return header, thus allowing the temperature control valves to operate properly. This approach requires the installation of the differential pressure controller along with the connections for the pressure sensors. Since the differential pressure of a circuit or a group of circuits supplied by a common supply and discharge header is maintained at a set differential pressure regardless of flow rate, the associated temperature control valves will be easier to control.
The disadvantage to this approach is potential overloading if there are too many circuits supplied by a given differential pressure controller.
Installing Pressure Independent Controllers
Pressure independent controllers (PICs) can successfully regulate the flow rate through the temperature control valve with a wide range of differential pressure. This is accomplished by installing a differential pressure regulator (DPR) in the same valve body with the temperature control valve.
The DPR uses internal passages to set the differential pressure across the temperature control valves to a constant differential pressure regardless of the flow rate through the valve.