Combined use of differential pressure controllers and variable speed pumps delivers high energy savings. By Jean-Christophe Carette (M. Sc. Eng., PhD, ASHRAE member)

With 40% of the world’s energy being used in buildings and 60% of this being used in HVAC, our industry carries a heavy responsibility for delivering energy-efficient installations. Chillers used in cooling and boilers/heat pumps used in heating situations are the devices which use the highest amount of energy in installations. On the waterborne side, pumping energy consumption comes a close second, especially in cooling, where it represents between 15 and 20% of the annual energy consumption. It is, therefore, necessary to undertake any effort to maximise the energy savings allowed by modern chillers, boilers and pumps, while making it possible to deliver the required comfortable indoor climate in buildings.

The chilled water return temperature issue

Chiller efficiency is usually indicated by its coefficient of performance1 (COP). In order to keep the COP of a chiller as high as possible at partial load, the difference between the chilled water supply temperature (Ts) and the chilled water return temperature (Tr) should be kept as high as possible. In these conditions, the mean temperature difference between chilled water and the refrigerant is kept high, enhancing the heat transfer in the evaporator and, thus, the chiller COP. With a constant chilled water supply temperature Ts as generally considered in cooling, keeping a high delta T means that the chilled water return temperature should be kept as high as possible at partial load.

When proportional control is used for variable flow circuits, the temperature monotonically increases when the flow is reduced through the terminal unit (figure 1). Hence, keeping a stable and accurate control of the cooling output of a terminal unit will benefit the chiller COP. On the contrary, if proportional control deteriorates to an uncontrollable on-off level of behaviour, energy consumption and uncomfortable room temperature levels will become unavoidable.

Stable and accurate control

To ensure room temperature is controlled as accurately as possible, the global control characteristic of the circuit, relating the cooling output of the terminal unit to the control signal, must be as linear as possible. All other characteristic shapes lead to high gain in some parts of the control range, leading to uncontrolled room temperature oscillations. The global control characteristic of a circuit results from the combination of the characteristics of the different circuit components (figure 2).

For typical cooling conditions, the characteristic of a terminal unit is nonlinear. At small and medium loads, small variations of flow lead to rapid changes of the emission of the terminal unit, making control difficult. Choosing a control valve with an equalpercentage Kv characteristic that compensates for the non-linearity of the terminal unit is essential to solve this problem. Unfortunately, in variable flow systems, when the flow in the system is reduced, pressure drops are reduced through terminal units, pipes and accessories. This results in higher applied differential pressure across control valves, which distorts the relation between the flow and the control valve Kv.

With a linear actuator characteristic and an equal-percentage control valve, the degree of nonlinearity of the global circuit characteristic depends mainly on the importance of the Dp variation effects (figure 2). It is, therefore, essential to focus on avoiding differential pressure variations on control valves. This is best performed by locally stabilising the differential pressure with differential pressure controllers, either stand-alone or integrated into control valve bodies (pressure-independent control valves). In this later case, care must be taken that the basic control valve Kv characteristic is indeed of the equalpercentage type.

Compensating an improper control valve characteristic or Dp variation with actuator signal reprogramming is not a viable solution, as this leads to position control valves at low lifts and, thus, near or within its range limit, where it will deliver unstable on-off behaviour.

Variable speed pumps and differential pressure controllers

When a system is equipped with a variable speed pump (VSP), one could wonder if the use of differential pressure controllers is really needed as both devices seem to work according to the same basic principle: controlling the differential pressure at one point in the system. Variable speed pumps are meant to maximise pumping energy savings at varying load in a variable flow system. They do act where they are installed, on the total flow going through them, based on the differential pressure sensed at one location by their Dp sensor. Even if multiple sensors are used, at any one moment in time, a VSP can only adapt its speed according to the signal of one of the sensors.

Thus VSPs cannot guarantee by themselves a stable and accurate control to all circuits distributed all over the system. This is true whatever the control mode and Dp sensor location that is selected. Differential pressure controllers are required to protect local control valves from large differential pressure variations experienced at varying load in variable flow systems.

Minimising pumping energy use

When differential pressure controllers are used consistently over a variable flow system, enhanced pumping energy savings can be obtained by using a remote Dp sensor for the VSP. This is made possible, because differential pressure controllers are self-acting balancing devices adapting their opening according to the variations in the system. Without differential pressure control, the use of a remote Dp sensor in the middle or the end of the system always lead to having some circuits unable to deliver their design cooling output under average part-load conditions.

The process to obtain optimum energy savings together with good controllability of all circuits works in three steps:

1. Perform dynamic balancing with Dp controllers on each branch/zone or on each unit by using pressure independent control valves2.
2. Install the VSP Dp sensor on the index, Dp controlled, branch or circuit.
3. Adjust the set-point of the VSP to the largest required differential pressure amongst all Dp controlled branches/circuits.

This last step ensures that all Dp-stabilised areas will receive enough primary differential pressure at low load. In order to implement this process, it is warmly recommended to perform a complete differential pressure calculation of the system.

Conclusion

Ensuring a good controllability to hydronic circuits is crucial for avoiding instability leading to chilled water delta T degradation and, thus, higher energy consumption by chillers. Variable speed pumps are essential tools for minimising pumping energy use; however, variable speed pumps cannot ensure good controllability to all circuits at all loads, because they are “global” devices. Local differential pressure controllers are required for this task. When differential pressure control is applied consistently in a plant, the sensor of VSP can be placed on the index branch or zone, thereby leading to enhanced pumping energy savings while providing an excellent controllability to the system.

The author is the Head of the Hydronic College, the knowledge development unit of Tour and Andersson, an Indoor Climate business of IMI plc. He may be reached at <jc.carette@ tahcollege.com>

1 Ratio between the cooling output delivered at the evaporator and the electrical power required for running the compressor.

2 For cost-efficiency and depending on the hydronic structure of a plant, one approach can be used for some parts of the system while the other approach will be used for the other parts.