Flow Tolerance in a System with Variable Speed Pump Control
/By Chad Edmondson
In our last blog we discussed how to leverage system diversity to maximize energy efficiency. Specifically, we learned that we can size heating or cooling capacity based on peak block load rather than total connected load if the system is designed and operated correctly. Today, we will talk about flow tolerance and how it sets the limits on how much we can vary flow before comfort is negatively impacted.
Simply put, flow tolerance refers the amount flow reduction the system can tolerate and still maintain 97% of design heat transfer at design flow. The 97% minimum heat transfer is what ASHRAE suggests in a properly balanced system. Typical chilled and hot water design conditions require 10% flow tolerance to meet ASHRAE’s definition for a “balanced system.” In other words, you can only vary flow through a coil so much before you impair its ability to achieve sufficient heat transfer – and thus impact comfort. This is what we call a “miss.” A “miss” of greater than 10% is not acceptable.
Heating systems have a broader operating range than cooling systems when it comes to maintaining sufficient heat transfer. Let’s look at the following graph adapted from the ASHRAE Handbook to get a better feel for the comparative flow tolerances of heating and cooling:
Recommendations for Coil Flow Tolerance to Maintain 97% Design Heat Transfer (Carlson 1981, 2016 ASHRAE Handbook -- Systems and Equipment
This chart, originally developed by Gil Carlson of Bell & Gossett, has been adapted by ASHRAE and can be used to determine the coil flow tolerance for any given supply temperature at a given ∆T. The upper portion of the chart is for heating systems and the lower portion is for cooling.
Let’s first look at heating.
If we have a heating system with a supply temperature 140°F and a 30°F ∆T, we can look at this chart and determine that under these conditions we have a flow tolerance of 10% -- meaning that as long as we maintain 90% of design flow, we will stay within ASHRAE’s requirement for 97% heat transfer.
The same can be said of a cooling system where the supply temperature is 45°F and the ∆T is 12°F. If we read across from 45°F on the lower portion of the supply side axis of our graph and read across to where it intersects with the 12° F ∆T, we would read down vertically to find that again we have a recommended flow tolerance of 10%, meaning that we need to maintain 90% of design flow to maintain 97% of ASHRAE’s required heat transfer.
Remember -- chilled water systems are less forgiving in terms of flow variance than heating systems. This is because with a cooling system you are also dealing not only with a sensible load, but also a latent load.
It’s also important to note that ASHRAE says the system shall be designed to maintain the required flow tolerance. This includes part load. You must choose the right combination of variable speed pump control and balancing strategies to meet this requirement. We will explore these strategies in greater detail in upcoming blogs.
