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

•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.

## No comments:

## Post a comment