ASHRAE Journal - August 2019 - 31

TECHNICAL FEATURE

FIGURE 1-A Partial heat recovery on P-H diagram.

FIGURE 1-B Full heat recovery on P-H diagram.

Total Heat Rejection

Total Heat Rejection
Superheat

Saturated
Liquid

Saturated
Vapor

Condensing

Pressure

Condensing

Saturated
Liquid

Saturated
Vapor

Compression

Refrigeration Effect

Enthalpy

study shows a net reduction in operating costs when
implementing heat recovery chillers compared with
the base case of chiller-only and boiler-only systems. The energy cost savings may reach up to 26%.
This study will also be helpful for decision makers
to promote recovery chillers in similar applications
when both heating and cooling are simultaneously
required.

CCH System Description
In many applications, cooling and heating demands
are required simultaneously. In these cases, the heat
released by the cooling process, which is usually wasted
to the atmosphere directly through air-cooled condensers or indirectly through water-cooled condensers via
cooling towers, might potentially be recovered and used
for heating or hot water production.
A heat recovery chiller aims to achieve an optimum
balance between cooling and heat recovery to maximize
the unit efﬁciency by energetically recovering useful
heat from the thermodynamic cycle that would otherwise be rejected to the atmosphere. An additional heat
exchanger (usually brazed plate type) is mounted in
series between the compressor and the condenser.6,7
Depending on the temperature requirement for the
hot water production, this exchanger will act either as
a desuperheater for partial heat recovery (15% to 20%
heat recovery), allowing higher temperature levels to be
reached, or as a condenser for full heat recovery (90% to
100% heat recovery) allowing lower temperature levels
to be reached.
In partial recovery mode, only a small quantity of

Superheat

Enthalpy

heat is available since only superheat is removed from
the refrigerant, which is about 15% of compression
work. High grade heat comes from desuperheating the
refrigerant between the compressor and the condenser
(Figure 1-a). A heat exchanger (the desuperheater) is
installed, with the refrigerant on one side and the ﬂuid
to be heated on the other side. Not only does this desuperheat the refrigerant, it reduces the cooling water
or cooling air needed by the condenser. The size of the
desuperheater is generally designed to meet hot water
demand. The high grade heat recovered is beneﬁcial for
high temperature hot water production between 140°F
(60°C) and 176°F (80°C).
In contrast, in a full recovery mode, a larger quantity of
heat is available since all heat coming from the refrigerant condensing process is recovered, and the heat recovery exchanger will perform as full condenser (Figure 1-b).
Consequently, low grade heat from the refrigerant being
condensed is provided with temperatures ranging from
86°F (30°C) to 140°F (60°C) and diminishes the cooling
water or cooling air needed by the condenser.
The plant under study is a CCH system using a commercially available recovery chiller for simultaneous
production of cooling and useful heat with possible
chiller-only operation mode (Figure 2). Chillers to be considered are electrically driven compression type. Boilers
are fueled by liqueﬁed petroleum gas (LPG).

Methodology
During this study, in-house developed software has
been adopted to provide comparison of energy utilization efﬁciency to minimize both fuel consumption and
A U G U S T 2 0 19

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ASHRAE Journal - August 2019

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