Part Load Efficiency Values (PLEV) Part 5: BTUH Load Output vs. Percent of Flow Tolerance

By Chad Edmondson (JMP) and Norman Hall (RLD) and

We recently introduced the part load efficiency value (PLEV) aspect of the Bell & Gossett online pump selection tool. PLEV for pumps is based on flow rate while the Integrated Part Load Value (IPLV) for chillers is based on load. This final segment on PLEV will look at the difference.

Let’s begin by examining the flow tolerance for hydronic heating systems.

The figure below shows a typical heating coil output based on percentage of the coil manufacturer’s recommended coil velocity. Clearly in heating systems, we can get a large percent of the heat transfer at a lower flow rate. The graph assumes a constant supply temperature. If your design is using temperature reset, there is less tolerance to reduced flow.  The actual velocity through the coil and even the type of terminal unit will affect the flow tolerance in hydronic heating systems. Since most heating systems have temperature reset along with modulating two way valves, percentage of flow rate will approximate the percentage of load.

Flow Tolerance for Hydronic Chilled Water Systems

Chilled water systems require enough flow rate to meet both sensible and latent loads. The chart below is from ASHRAE 2015 Applications Chapter 38 and shows flow tolerance for a particular coil at 45°F supply and 10°F ΔT. Note that although there is 90% of the total heat transfer at 80% of the design flow rate, there is only 70% of the latent heat transfer.

Percent of flow rate is a good approximation of load when looking at chilled water coils.

 CHILLED WATER TERMINAL HEAT TRANSFER VERSUS FLOW

CHILLED WATER TERMINAL HEAT TRANSFER VERSUS FLOW

B&G integrated PLEV into the ESP-PLUS pump selection program as a useful tool for the engineer to evaluate pump efficiency over a range of operating points. Obviously the pump selection program does not differentiate between heating and chilled water and does not require the user to enter the type of terminal units and temperatures. PLEV is simple to see and use with minimal input and we recommend its use when comparing variable speed variable flow pump selections in HVAC applications.