ASHRAE Journal - April 2021 - 24
TECHNICAL FEATURE
Increasing Efficiency By
Using Four-Pipe Water
Source Heat Pumps
BY NABIL NASSIF, PH.D., P.E., MEMBER ASHRAE
Most buildings in the United States are equipped with traditional centralized variable
air volume (VAV) systems that mix air returning from different zones and centrally
recirculate a large portion of it back to zones. During the coronavirus pandemic, this
recirculation could make the spaces less safe and increase the possibility of coronavirus transmission between zones in buildings.1 On other hand, distributed water
source heat pumps (WSHPs) or any chilled water terminal unit integrated with a dedicated outdoor air system make the spaces safer by eliminating return air mixing and
also may provide great energy efficiency if they are designed and operated properly.1
Thus, this article focuses on improving the design and operation of distributed WSHPs
to achieve that desired higher energy efficiency.
Typically, WSHPs use single- or two-pipe water loop
configurations. The latter could be either a directreturn or a reverse-return arrangement. In all cases,
the relatively warm water leaving the heat pumps
operating in cooling mode and the cold water leaving
the heat pumps operating in heating mode are mixed
before being delivered back to those heat pumps. This
reduces the efficiency of the WSHPs because they reject
or absorb heat to or from the water at the average loop
temperature rather than at the " best possible " attainable temperature.
Figure 1 shows a typical two-pipe configuration (2PC)
for WSHPs with a direct-return arrangement. For the
sake of discussion, it is assumed that the loop contains
only three WSHPs; one operates in heating mode, the
second one is off and the third operates in cooling
mode. A water tank may be installed. A boiler and cooling tower (CT) are added to the loop to keep the water
loop temperature within its low and high limits (e.g.,
50°F and 90°F [10°C and 32°C], respectively).
The figure shows an example when each WSHP needs
to reject or absorb heat of 12 kBtu/h (3.5 kW) to or from
the water at a flow rate of 2.4 gpm (0.15 L/s), causing a
temperature rise or drop of 10°F (5.6°C). In this perfect
Nabil Nassif, Ph.D., P.E., is associate professor of Architectural Engineering at the University of Cincinnati.
24
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ASHRAE Journal - April 2021
Table of Contents for the Digital Edition of ASHRAE Journal - April 2021
Contents
ASHRAE Journal - April 2021 - Intro
ASHRAE Journal - April 2021 - Cover1
ASHRAE Journal - April 2021 - Cover2
ASHRAE Journal - April 2021 - 1
ASHRAE Journal - April 2021 - Contents
ASHRAE Journal - April 2021 - 3
ASHRAE Journal - April 2021 - 4
ASHRAE Journal - April 2021 - 5
ASHRAE Journal - April 2021 - 6
ASHRAE Journal - April 2021 - 7
ASHRAE Journal - April 2021 - 8
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ASHRAE Journal - April 2021 - Cover3
ASHRAE Journal - April 2021 - Cover4
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