ASHRAE Journal - November 2021 - 17

TECHNICAL FEATURE
One should investigate if it is economical to operate
the pump during the cooling mode. My experience is
that if the analysis indicates it saves money not to operate
the pump during the cooling mode, the system has
not been optimized.
8. Variable Speed Pumping
Pump energy plays a signifi cant role in the system economics
and should be reduced whenever possible, especially
in the cooling mode. Using variable speed pumping
can reduce the pump energy by a factor of three or
more and usually has an economic payback of less than
two years. Remember to program the pump speed in the
cooling mode to maintain the minimum cooling mode
tube velocities, which are 52% to 74% lower than the
minimum heating mode tube velocities.
Coil Selection
Many coil options affect system cost and performance.
Following the recommendations above should reduce
the number of options to be considered. The designer
should make a list of other options to be considered,
select the coils for each option and then complete the
energy model and economic analysis for each option
to determine the best selection. The following discussion
should assist in reducing the number of options
considered.
Coil Dimensions. If using commercial air-handling
unit coils, use the dimensions based on the manufacturer's
catalog data. If selecting custom coil sizes, use the
largest aspect ratio (length/height) possible to reduce
the coil cost and the system fl ow rate based on reducing
the number of circuits. A practical guide is usually an
aspect ratio between two and four.
Number of Tube Rows. For coil velocities less than
275 fpm (1.40 m/s), use eight-row coils. For higher face
velocities, consider both six- and eight-row options.
Fin Design. Consider using the maximum number
of fi ns possible without exceeding 0.6 in. w.c. (149 Pa)
air pressure drop. Fin thickness has a small impact on
coil performance but does affect coil cost. Studies have
found that the economic benefi t of using thicker fi ns can
range from a small improvement to a negative impact.
The conclusion is that fi n thickness has little impact and
that 0.0075 in. (0.19 mm) fi ns should be used to reduce
coil costs.
The actual system energy recovered per hour (Qactual)
is determined by coil selection software. The maximum
available sensible heat recovered per hour is:
Q max = scfm × 1.08 × (tea - toa)
where
Q max = Maximum sensible heat transfer rate (Btu/h)
scfm = Standard exhaust air fl ow (cfm)
1.08 = 60 min/h × ρ × cp
ρ
= Exhaust air density (0.075 lb/ft3)
cp
tea
toa
= Specifi c heat of dry air (0.24 Btu/lb · °F)
= Exhaust air temperature (°F) entering the
exhaust coil
= Outdoor air temperature (°F) used in calculating
Q actual
The value of SERR is relatively constant and is independent
of outdoor air temperature as long as the coil
U-factor remains relatively constant.10 Once this value is
calculated at one outdoor air temperature, the amount
of energy recovered can be easily calculated for other
outdoor air temperatures. The SERR value will change as
the coil U-factor changes when using variable airfl ow or
variable fl uid fl ow and/or when fl uid properties change
between heating and cooling modes. Depending on how
energy modeling software or bin data spreadsheets are
set up, values of SERR may need to be recalculated for
N O V E M B E R 2 0 2 1 ashrae.o rg ASHRAE JOURNAL
17
System Design
Once the coils have been selected, piping can be sized
and pump heads calculated based on the system layout.
The pump brake horsepower or effi ciency should be one
of the inputs into the energy model. After the piping is
sized and pumps have been selected, calculate the estimated
incremental construction cost. Repeat the above
process for each option to be analyzed.
Energy and Economic Analysis
One of the main inputs into modeling coil loop performance
is the sensible energy recovery ratio (SERR). This
is defi ned by Standard 90.1-20199 as the actual energy
recovered at a given outdoor air temperature divided by
the maximum energy available at that same outdoor air
temperature.
SERR = Q actual /Q max
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ASHRAE Journal - November 2021

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