Low Delta-T Syndrome. These are the dreaded words one hears when discussing the performance of a chilled water system. A chiller operating with low delta-T causes both increased operating costs and the potential of not meeting its load requirements. [A note here, this same discussion is valid for condensing hot water boilers as requirements not to exceed a design return water temperature to maintain efficiency is required.]
Chillers are designed for a specific flow rate at a design delta-T (differential temperature between the supply and return water temperatures). A lower than design delta-T can affect the ability of the chiller to load and meet the chilled water supply temperature setpoint which, in turn, could cause comfort issues in the space. Additionally once the flow through the system exceeds the chiller design an additional chiller will be brought on line to handle the building’s need for water and not it’s need for cooling. Unfortunately, running chillers to maintain the flow in the building can cause the building’s capacity to produce sufficient chilled water for the load to be decreased Besides affecting the load capacity of the chiller running additional chillers and pumps to maintain this flow will increase the energy costs.
Why does this happen?
First, overflow can occur at the control valve simply due to poor valve authority. When a control valve with a valve authority of less than 40% (where 100% is ideal) the modulating valve will produce more flow than required. Once the valve starts modulating closed as the control loop is satisfied the flow will reduce too much. Therefore a two-position flow will begin to occur (see Fig 1).
A control valve with a poor valve authority results in a net overflow at part load conditions. Once a load is brought to setpoint after occupancy, unless there is a large change load, it will typically be running in a part load condition.
Pressure independent control valves can solve this problem by eliminating the effects of poor valve authority. A pressure independent control valve is a control valve that supplies a specific flow for each value of the control signal regardless of the pressure variations across the control valve. In other words, a pressure independent control valve is a valve that operates as if its valve authority is 100%, even though it obviously isn’t when calculated.
The result is shown in Figure 2.
The instability of the flow through the control valve has been eliminated.
Next, let’s address the coil performance. Many issues can address the performance of the coil including, but not limited to physical damage, dirt, and coil sizing vs. load.
The key is to identify the point upon which additional flow to the coil does not yield additional heat transfer. By allowing flow beyond this point the coil water differential temperature will begin to decrease. The result (in a chilled water system) will be to allow a higher than design return water temperature to flow back to the system. The system differential temperature will be decreased and, once again, energy costs will rise due to inefficient loading of the chiller system while pumping this additional water through the building.
So, why doesn’t the control system recognize this overflow? Simply put it can, provided sufficient points are connected and monitored. These would include a BTU Meter and trending this data to view the performance of the coil versus flow.
For more information and solutions on how to fix low delta-T syndrome please visit www.energyvalve.com.
Bob Walker, LEED Green Associate
Regional Application Consultant-Midwest