Reduced flow rates may improve your chiller system efficiency!!

There are many options available for chilled-water-system design; however, in a basic sense, each individual option is a function of flow, temperature, system configuration, and control. It is worthy to remember that flow rates & temperatures are variables. By prudent selection of these variables, chilled-water systems can be designed to both satisfy chilled-water requirements and operate cost effectively.

Chilled-water systems are generally designed using flow rates and temperatures applied in testing standards developed by the Air-Conditioning, Heating, and Refrigeration Institute (AHRI), ARI 550/590–2003 for vapour compression chillers and ARI 560–2000 for absorption chillers. These standard benchmarks provide requirements for testing and rating chillers under multiple rating conditions. But however they are not intended to prescribe any proper or optimal flow rates or temperature differentials for any particular system. In fact, as component efficiency and customer requirements change, these standard rating conditions are seldom the optimal conditions for a real system.

In recent days due to increased focus on improving energy efficiency, some researchers found that reducing flow rates may improve chiller system efficiency.

Currently, the standard rating condition temperatures in ARI 550/5905 and ARI 5609 are:

• Evaporator leaving water temperature: 44°F [6.7°C]

• Water-cooled condenser, entering water temperature: 85°F [29.4°C]

• Air-cooled condenser, entering air dry bulb: 95°F [35.0°C]

And standard flow rates as per ARI 550/590

• 2.4 gpm/ton [0.043 L/s/kW] for evaporator

• 3.0 gpm/ton [0.054 L/s/kW] for condenser

This evaporator flow rate corresponds to a 10°F [5.6°C] temperature difference. Depending on the compressor efficiency, the corresponding condenser temperature difference is 9.1°F to 10°F [5.1°C to 5.6°C].

Now let’s examine what happens when the chilled water & condenser water flow rates are reduced to 1.5 gpm/ton & 2gpm/ton from 2.4 & 3 gpm/ton respectively for an 450 TR chiller;

In this example, notice that the leaving chilled-water temperature decreases and the leaving condenser-water temperature increases. This means that the chiller’s compressor must provide more lift and use more power. At first glance, the design team may decide the chiller power difference is too large to be overcome by ancillary equipment savings. The key question is, how does this impact system energy consumption? Using the following assumptions, we can calculate system energy usage:

•80 feet of water [239 kPa] pressure drop through chilled-water piping
•30 feet of water [89.7 kPa] pressure drop through condenser-water piping

•78°F [25.6°C] design wet bulb

•93 percent motor efficiency for pumps and tower

•75 percent pump efficiency

•Identical pipe size in chilled- and condenser-water loops (either a design decision, or indicating changing flows in an existing system)

The pressure drop through the chiller will decrease due to the lower flow rates. When using the same size pipe, the pressure drop falls by nearly the square of the decreased flow rate. While this is true for straight piping, the pressure drop does not follow this exact relationship for control valves or branches serving loads of varying diversity.

Now lets calculate the combined effect of pumping energy reduction and compressor energy increment to understand the overall effect on the system :

It becomes clear that flow rates can affect full-load system power . Even though the chiller requires more power in the low-flow system, the power reductions experienced by the pumps and cooling tower result in an overall savings for the system.

Another question may arise like the effect of reduced flow rate at partial loading. Even at partial loads savings will be there but the quantum is relatively low.

Even though the concept is attractive this involves some modifications in cooling tower circuit, piping circuit and chilled water coil to be done.

*Data has been taken from a technology suppliers open source document.