Editor’s Note: “Inside Insights” is a column designed to address ongoing issues of interest to building owners, managers and operating engineers who use district energy services.
I probably should let sleeping dogs lie, but this topic has come up several times in the past few months after this column’s last installment, and I decided I’d better address high-temperature differentials in terminal units (baseboard, convectors, etc.). My current crusade is to reduce the water temperature to reheat coils in variable air volume (VAV) systems to save energy, mitigate corrosion and optimize the heating water system. Once again, it’s the old “how low can you go?” dance. (Check out my
“Inside Insights” column from first quarter
2009 at www.districtenergy-digital.org/ districtenergy/20091Q/.)
For those readers who don’t know what a reheat coil is or who live in a cave, it is a serpentine coil (usually copper) with fins (usually aluminum) to increase heat transfer. The coils are mounted in a metal frame suitable for duct mounting to reheat the supply air temperature for zone control.
Reheat coils are also know as ‘booster coils.’
I will start by saying that with the exception of tight humidity control (i.e., dehumidification), I continue to scratch my head as to why hot water heating systems in southern and temperate climates use 180-degree F
106 District Energy / Second Quarter 2009 supply water temperatures when they can get by with something less? I mean really, where do they think they live, Minnesota?
I decided to conduct a little experiment in reheat coil selections using various temperatures and delta T’s to see when I would literally run out of hot air. Does this make sense? Am I all wet? Can I succeed? You have to read on.
Before we can get to the results of my little exercise, we must have a little primer in sizing a simple reheat coil for a non-fanpowered VAV box as well as highlighting some design parameters that may not be fully known. Here are the key reheat coil inputs:
Room air temperature = Typically between 72 F and 75 F.
Entering air temperature (EAT) =
Standard supply air temperatures of around 55 F.
Leaving air temperature (LAT) =
Dependent upon load and flow, but less than 90 F (more on this later).
Entering water temperature (EWT) =
Used various temperatures starting at 180 F.
Leaving water temperature (LWT) =
EWT minus delta T.
Fins per inch (FPI) = A coil attribute.
More fins, more heat transfer area. Typically an attempt is made to keep below 10 FPI for ease of cleaning.
Rows = Also deals with heat transfer area. The more rows, the greater the heat output.
Coil heating load = Capacity of coil to satisfy heat load, MBH (1,000 Btuh).
For the heating load input, let’s use the New York City sample room from my first quarter 2009 column. As you may recall, the room was 25-ft long x 12-ft deep with
14-ft-high walls (600 sq ft – almost the size of my office – not!). The estimated component heat losses are summarized in table 1 and include roof loss, just in case I get promoted, and my office moves to the top floor.
The ventilation load was calculated by bringing the air flow (estimated at 1 CFM/sq ft) from the standard supply air temperature of 55 F from the air handler to the room temperature (75 F). This example assumes all heat loss is handled via the overhead air system, and there is no baseboard radiation.
We now have the major coil inputs, but we have to step back and look at the engineering selection parameters. There is a maximum LAT that is feasible for use in reheat coils. Since 1983, the ASHRAE
Handbook of Fundamentals has provided specific guidance on the maximum room discharge-temperature difference (not to exceed 15 F [8 C]) for effective control of the perimeter environment. So if the space temperature is 75 F, the maximum discharge temperature should be 90 F; many times it can be accomplished with 85 F air. Air that is warmer than 90 F will not make it down to the occupied space and will stratify in the room closer to the ceiling. In my office this cooler air also is required to offset all the hot air that I generate.
Furthermore, the minimum air flow on the VAV box has to be great enough so that the velocity of the air leaving the diffusers has a chance of making it to the
Table 1.
Load Summary Space (Btuh).
Wall
Load
550
Window
Load
3,325
Roof
Load
480
Ventilation
Load
2,770
Infiltration
Load
3,000
Total
Heat
Loss
10,125
© 2009 International District Energy Association. ALL RIGHTS RESERVED.

occupied zone and mixing with the room air even if the LAT is low enough. This is important, as minimum flows greater than 30 percent may be required to provide enough mixing of air to the space. For my example I used 50 percent flow as a minimum for heating flow, or 300 CFM. Full flow through the coil must be kept in mind since the air pressure drop should not be excessive. Accordingly, the air velocity through the coil should be less than 1,000 ft per minute (FPM) and typically is less than
750 FPM to keep the pressure drop of the reheat coil low (typically below 0.3” of H
2
O).
The water velocity in the coil tubes should be greater than 1 ft per second (FPS) to keep the heat transfer in the turbulent range and to achieve ARI Standard 410 Certification.
The water-side pressure drop usually is well below 5 ft of H
2
O.
To obtain the capacity of the coils I need to start with an approximate size. Using a velocity of 600 FPM requires a minimum size of 0.5 sq ft or approximately a 9”x 8” coil (coils only come in certain height increments, hence the odd-sized duct). A larger size would reduce the air pressure drop and increase the heat transfer area.
Using a computerized selection program from USA Coil, I started selecting coils using the above input parameters for the base case
(300 CFM, 55 F EAT, 180 F EWT and 160 F
LWT, two rows, 10 FPI and 10.1 MBH) and progressively reduced the EWT in 10 F increments while using three different delta T’s
(20, 30 and 40) until I could no longer meet the capacity. Then I modified the FPI and coil size until I met the capacity requirements.
Table 2 summarizes the selections.
The majority of the selections had LATs below or close to 90 F, which was deemed a success, but to meet the capacity requirements, I continually had to adjust the heat transfer area (coil size and FPI) in all cases.
For EWTs below 150 F, the maximum 10 FPI was exceeded and the coils were larger than
18
19
20
21
14
15
16
17
22
23
12
12
24
Source: USA Coil.
12
12
12
12
12
9
9
9
12
10
11
12
13
8
9
6
7
4
5
2
3
Table 2.
Reheat Coil Selection Samples.
Item
1
Height Length
(inch) (inch)
9 8
Oversizing
0%
9
9
9
9
8
8
8
8
0%
0%
0%
0%
9
9
9
9
9
9
9
9
8
8
8
8
8
12
12
12
0%
50%
50%
50%
0%
0%
0%
0%
12
12
14
14
14
14
14
14
14
14
14
50%
50%
75%
133%
133%
133%
133%
133%
133%
133%
133%
LAT (°F)
86.6
85.7
71.4
68.8
86.3
87.6
87.0
86.5
72.9
69.0
66.5
87.2
86.4
87.0
89.2
88.0
86.6
88.0
87.3
90.9
90.5
87.5
90.7
87.4
Load (MBH) EWT (°F)
10.4
10.1
5.4
4.3
10.3
10.7
10.5
10.3
5.9
4.6
3.8
10.6
10.3
10.5
11.2
10.8
10.4
10.8
10.6
11.8
11.6
10.7
11.7
10.6
130
120
120
120
140
140
130
130
110
110
110
150
150
150
140
170
160
160
160
180
180
180
170
170
LFT (°F) Flow (GPM ) Rows FPI
109.6
97.7
109.5
99.7
88.9
100.3
104.0
102.0
98.5
96.5
95.2
127.1
139.7
128.0
118.0
129.4
119.4
108.5
118.0
156.6
145.5
137.7
146.9
138.5
0.5
1.0
0.7
0.5
0.7
0.5
1.0
0.7
1.0
0.7
0.5
1.0
0.7
0.5
1.0
0.5
1.0
0.7
0.5
1.0
0.7
0.5
1.0
0.7
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
12
10
12
14
10
12
10
10
14
14
14
10
8
10
10
10
8
10
12
8
8
8
8
8
© 2009 International District Energy Association. ALL RIGHTS RESERVED.
District Energy / Second Quarter 2009 107

the base case. The fourth column labeled
‘Oversizing’ is an indicator of the additional coil surface area above the base case required to meet the load; it is a direct indicator of additional costs. Therefore, I was only partially successful below 150 F EWT, since a minimum of 50 percent more heat transfer area was required.
With these oversizing adjustments, the selections met the capacity requirements until the EWT fell below 120 F, and the delta T was greater than 20 F (as indicated in bold numbers in the ‘Load’ column, selection items 20 to 24), and the capacity could not be met. I had hit the wall, and I ran out of proverbial steam, or in this case hot water.
My initial perception was that there should be a few single-row coils in the selection mix especially for the base case of high temperature and high flow. That confused me, so to further refine the selections,
I reduced the coil heating load to just the ventilation with an assumption that there would be baseboard radiation handling the wall heat loss. I was surprised again to see that selections were still two rows; however, all options but one met the load requirements, with the exception being 110 F EWT with 40 F delta T criteria.
So, what did we learn? Lower heating supply-water temperatures and greater system delta T’s are achievable down to
150 F without any increase in coil size.
Furthermore, both temperatures can be dropped further if the coil size is increased, but eventually the limit of practicality will be reached around 120 F. While the selection results may not be identical for all load scenarios or climates, they should be scalable for larger air flows and loads. I think we all can save a little energy if the systems are designed accordingly from the start without the sacrifice of human comfort. Maybe I am not so wet after all! Lower heating supply water temperature and higher delta T’s do make $en$e.
Based in Madison, Wis.,
Steve Tredinnick, PE , is vice president of energy services for Syska Hennessy
Group, which has more than
16 locations across the U.S.
He has more than 26 years’ experience related to building heating, ventilation and air-conditioning systems. The past 15 years of his work have been focused on district energy systems.
Tredinnick is a graduate of Pennsylvania State
University with a degree in architectural engineering. He is a member of IDEA and
ASHRAE and is currently immediate past chair of ASHRAE TC 6.2 District Energy. Tredinnick currently serves on IDEA’s board of directors.
He may be reached at stredinnick@syska.com.
108 District Energy / Second Quarter 2009 © 2009 International District Energy Association. ALL RIGHTS RESERVED.