ASHRAE Journal - November 2021 - 12

TECHNICAL FEATURE
Optimizing System Performance
Eight design parameters can greatly affect the annual
efficiency of a coil loop energy recovery system:
1. Coil heat transfer characteristics (avoiding laminar
flow conditions for both air and fluid sides).
2. Working fluid used.
3. System fluid flow rate.
4. Coil surface area.
5. Coil air pressure drop.
6. Coil options, including tube wall thickness.
7. Control sequences.
8. Variable speed pumping.
Each of these parameters is discussed in detail below.
In addition to making decisions about these parameters,
the designer should also compare options using energy
modeling software or bin data models to predict annual
performance and construction costs so he or she can
evaluate options not only on energy performance but
also on economic benefit.
1. Avoiding Laminar Flow
Laminar flow occurs when the combined effects of
fluid velocity and fluid properties result in nonmixing
conditions that create larger boundary layers. When
this occurs, heat transfer can be unpredictable. For the
air side, avoid coil face velocities below 200 fpm (1 m/s)
per AHRI Standard 410.1 For the fluid side, the Reynolds
number is the best parameter to predict when laminar
flow occurs. AHRI Standard 410 requires Reynolds
numbers to be above 3,100.1 Figure 42 illustrates how heat
transfer can change with Reynolds number.
2. Most Efficient Working Fluid
Water is the best option in terms of heat transfer characteristics
but is only suitable if the lowest expected outdoor
temperature is above freezing. For freeze protection,
glycol/water solutions are usually used. The most
popular are propylene glycol (PG) and ethylene glycol
(EG). PG is nontoxic and is safer to handle and dispose
of. EG is more toxic and disposal concerns exist if there
is a leak or when fluid is replaced.
The tube velocity required to ensure at least transitional
flow for predictable heat transfer varies with fluid
type, temperature and inside tube diameters. In general,
EG has better thermal performance and lower viscosity
than PG in cold climates. In the heating mode, PG
usually requires much higher tube velocities to achieve
12
ASHRAE JOURNAL ashrae.o rg N O V E M B E R 2021
FIGURE 4 Typical dimensionless representation of forced-convection heat transfer.
0.04
0.02
0.01
0.008
0.006
0.004
0.002
0.001
L/D = 60
L/D = 120
L/D = 180
L/D = 235
Laminar Transition
6 8103 2
3,100
4 6 8104 2
Re =
Turbulent
4 6 8105 2
GD
µ
TABLE 1 Burst temperatures for propylene glycol (PG) and ethylene glycol (EG) fluids.3
BURST TEMPERATURE (°F)
20°F
PROPYLENE GLYCOL
PERCENT BY VOLUME
10°F
0°F
-10°F
-20°F
-30°F
12.6
20.9
25.1
29.3
31.4
34.6
4 6
Plates
Inside Tubes
Friction Factor
Inside Tubes
0.04
0.02
0.01
0.008
0.006
0.004
0.002
0.001
ETHYLENE GLYCOL
PERCENT BY VOLUME
11.5
17.8
23.1
27.3
31.4
31.4
the same Reynolds numbers as EG. For cold climates
(heating mode hours greater than cooling mode hours),
EG is the preferred fluid and should be used whenever
possible. For warm climates, PG will perform better in
the cooling mode.
To enhance system performance, use the lowest concentration
of glycol possible based on burst temperatures
(not freezing temperatures). Concentrations for
burst protection will be lower than for freeze protection.
Table 1 values are suggested concentrations based on lowest
expected outdoor temperature.
3. Lowest System Fluid Flow Rate
Piping and pump size affect the overall system costs
and thus the economics. Keeping flow rates as low
as possible will minimize both first cost of the piping
system and pump energy. The system flow rate
should be the lowest possible to maintain transitional
flow at full coil flow. The equation for calculating the
Reynolds number found in the 2017 ASHRAE Handbook-
Fundamentals4 can be rewritten as:
Vmin = Re(μ/ρ)/d
where
Heat Transfer Factor j
Friction Factor 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
ASHRAE Journal - November 2021 - 13
ASHRAE Journal - November 2021 - 14
ASHRAE Journal - November 2021 - 15
ASHRAE Journal - November 2021 - 16
ASHRAE Journal - November 2021 - 17
ASHRAE Journal - November 2021 - 18
ASHRAE Journal - November 2021 - 19
ASHRAE Journal - November 2021 - 20
ASHRAE Journal - November 2021 - 21
ASHRAE Journal - November 2021 - 22
ASHRAE Journal - November 2021 - 23
ASHRAE Journal - November 2021 - 24
ASHRAE Journal - November 2021 - 25
ASHRAE Journal - November 2021 - 26
ASHRAE Journal - November 2021 - 27
ASHRAE Journal - November 2021 - 28
ASHRAE Journal - November 2021 - 29
ASHRAE Journal - November 2021 - 30
ASHRAE Journal - November 2021 - 31
ASHRAE Journal - November 2021 - 32
ASHRAE Journal - November 2021 - 33
ASHRAE Journal - November 2021 - 34
ASHRAE Journal - November 2021 - 35
ASHRAE Journal - November 2021 - 36
ASHRAE Journal - November 2021 - 37
ASHRAE Journal - November 2021 - 38
ASHRAE Journal - November 2021 - 39
ASHRAE Journal - November 2021 - 40
ASHRAE Journal - November 2021 - 41
ASHRAE Journal - November 2021 - 42
ASHRAE Journal - November 2021 - 43
ASHRAE Journal - November 2021 - 44
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ASHRAE Journal - November 2021 - 47
ASHRAE Journal - November 2021 - 48
ASHRAE Journal - November 2021 - 49
ASHRAE Journal - November 2021 - 50
ASHRAE Journal - November 2021 - 51
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ASHRAE Journal - November 2021 - 53
ASHRAE Journal - November 2021 - 54
ASHRAE Journal - November 2021 - 55
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ASHRAE Journal - November 2021 - 68
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ASHRAE Journal - November 2021 - 70
ASHRAE Journal - November 2021 - 71
ASHRAE Journal - November 2021 - 72
ASHRAE Journal - November 2021 - Cover3
ASHRAE Journal - November 2021 - Cover4
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