ASHRAE Journal - November 2021 - 10
TECHNICAL FEATURE
Optimizing Coil Loop
Energy Recovery
Systems
BY GENE NELSON, P.E., LIFE MEMBER ASHRAE
A coil loop (runaround) energy recovery system is sometimes the only practical solution
for exhaust air energy recovery. And the selected system options should ideally
provide the highest amount of net energy recovered at the lowest first cost. However,
published articles on how to select coil loop energy recovery coils, working fluids and
system flow rates to optimize the system performance are not easily found. This article
provides basic system configuration and performance characteristics and a practical
guide on optimizing a traditional coil loop energy recovery system.
Basic System Configuration and Performance
Characteristics
The basic configuration of a coil loop (runaround)
energy recovery system includes an energy recovery coil
(ERC) in the exhaust airstream and a supply (preheat)
ERC upstream of the final heating coil in the outdoor
airstream. If a system has return air, the supply (preheat)
ERC should be in the outdoor air stream to provide
maximum energy recovery. The system also includes a
piped closed loop between the two coils and a pump to
circulate a heat transfer fluid between them.
The performance of a system varies depending on
the control limits applied. Common controls include a
low-temperature limit to prevent frosting of the exhaust
coil and supply air temperature control to limit the
amount of heat transfer to meet the maximum supply
Gene Nelson, P.E., is retired from Affiliated Engineers, Inc., Madison, Wis.
10
ASHRAE JOURNAL ashrae.o rg N O V E M B E R 2021
coil leaving air temperature setpoint. The upper limit
in the heating mode is required because the required
leaving supply air temperature is usually lower than the
entering exhaust air temperature. This control usually
involves varying the fluid flow through the supply coil
using a three-way valve and/or varying the total system
flow using a variable speed pump. Varying the airflow
through the supply coil using face and bypass dampers
is also an option but can be more difficult to control.
Figure 1 illustrates how the relative capacity of the system
in the heating mode changes with respect to the
outdoor air temperature. The capacity of the exhaust
coil varies with the outdoor air (OA) temperature
between points A' and E. The capacity of the supply coil
varies with the OA temperature between points D and F.
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ASHRAE Journal - November 2021
Table of Contents for the Digital Edition of ASHRAE Journal - November 2021
Contents
ASHRAE Journal - November 2021 - Intro
ASHRAE Journal - November 2021 - Cover1
ASHRAE Journal - November 2021 - Cover2
ASHRAE Journal - November 2021 - 1
ASHRAE Journal - November 2021 - Contents
ASHRAE Journal - November 2021 - 3
ASHRAE Journal - November 2021 - 4
ASHRAE Journal - November 2021 - 5
ASHRAE Journal - November 2021 - 6
ASHRAE Journal - November 2021 - 7
ASHRAE Journal - November 2021 - 8
ASHRAE Journal - November 2021 - 9
ASHRAE Journal - November 2021 - 10
ASHRAE Journal - November 2021 - 11
ASHRAE Journal - November 2021 - 12
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ASHRAE Journal - November 2021 - 19
ASHRAE Journal - November 2021 - 20
ASHRAE Journal - November 2021 - 21
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ASHRAE Journal - November 2021 - Cover3
ASHRAE Journal - November 2021 - Cover4
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