Chilled-Water Low Delta-T Screen and Pump Penalty

ScopeFive hundred is 8.34 lb per gallon times 60 minutes per hour times water's specific heat of one, so a circuit's delta-T is fixed once the load and the flow are known -- a plant designed for a 12 degree delta-T and running 9 is moving a third more water than it needs to, and the water is doing the same job either way. The consequences compound. By the pump affinity laws power follows the CUBE of flow, so 25% excess flow is nearly double the pumping energy -- an upper bound, since real systems with static head and fixed control setpoints save less (Taylor, ASHRAE Transactions 2002) -- and that line is usually the largest number in the whole conversation. Beyond the pumping cost, a low delta-T means the chillers see a warmer return than they were selected for, so the plant has to run more machines at part load to serve the same tons and each one runs less efficiently: a plant with a chronic low delta-T is short of capacity long before it is short of chillers. A 240 ton plant at a measured 600 gpm against a 12 degree design is running 9.6 degrees, 120 gpm over the 480 it needs, with the pumps drawing 1.95 times design power -- roughly 20 hp of continuous waste on a 40 hp pump, spent to move water that is not picking up heat. The causes are all downstream: three-way valves left in place, coil control valves that never fully close, coils fouled or selected for a low delta-T, and a bypass that was supposed to be temporary. This does not find them, but it quantifies why finding them is worth doing. A diagnostic screen; the plant's own instrumentation, a calibrated flow measurement, and the chiller selection govern.

Formula or tableactual_delta_t_f = load_btuh / (500 x actual_gpm); design_flow_gpm = load_btuh / (500 x design_delta_t_f); excess_flow = actual_gpm - design_flow; pump_penalty = (actual_gpm / design_flow)^3.

Source and editionThe chilled-water sensible relation Q = 500 x gpm x delta-T (500 being 8.34 lb/gal x 60 min/hr x water's specific heat), with the pump-affinity cube law applied to the excess flow, by name. Public hydronics. The plant's own instrumentation, a calibrated flow measurement, and the chiller selection govern.

Free accessThe sensible relation and the affinity law are public hydronics applied to the plant's own measured load and flow.

What governsEstimate. AHJ and licensed professional govern.

API field namesload_btuh, actual_gpm, design_delta_t_f, actual_delta_t_f, design_flow_gpm, excess_flow_gpm, pump_penalty

• The 500 constant 8.34 lb/gal x 60 min/hr x specific heat 1.0, for waterpublic hydronics
• Cube law pump power follows the cube of flow, so 25% excess flow is nearly double the energypump affinity laws
• Capacity, not just energy a warm return makes the plant run more machines at part load for the same tonschilled-water plant practice