Assuming in a two-way valve building, differential pressure across a distant control valve will give the most savings.
You may want to change 3-way valves to 2-way valves to allow this strategy.
With 3-way valves, use differential pressure across the pumps.
Chilled water is presently circulated through the buildings by two xxx hp pumps and condenser water is delivered to the tower by two xxx hp pumps. These pumps are sized to deliver the required water flow on the warmest day of the year, but continue to operate at full flow throughout the cooler months too.
Most air handler cooling coils are fitted with two-way control valves. When the valves throttle back, the pumps are operating at a higher pressure head than necessary.[ Excess head is dissipated through a bypass valve controlled on differential pump pressure]
We recommend installing variable speed drives on both chilled water pumps and both condenser water pumps. We also recommend replacing the pump motors with inverter duty motors to increase reliability and protect the windings from overheating or burnout.
Pump speed should be adjusted based on <differential pressure across the pumps> <differential pressure across one of the more distant control valves>.
Chilled water flows should be adjusted based on differential pressure across the pumps. Condenser water flows should be adjusted based on temperature rise across the chiller. Minimum manufacturer’s flows must be maintained.
Commissioning of this measure is crucial to ensure that savings are maximized while maintaining adequate flows to prevent surge and other operational problems in the chillers.
Throttle pump discharge valves.
Issues and Concerns
SA Armstrong, promoting their variable-speed integrated pumping system, claims for a pump designed at 1000usgpm where 500 usgpm is needed:
- pressure bypass at pump = 28% savings
- control of differential pressure across pump = 44% savings
- control of dp across load and control valve = 67%
- control of dp across control valve only = 76%
Use the Armstrong spreadsheet IPS_Payback.xls to calculate savings.
Either build it up from parts (drives, controls, controller and programming) or get packaged price from SA Armstrong.
In retrofit we will not be replacing the pumps themselves, only adding controls, drives, and inverter-duty motors.