October is really the first month that economizer cooling systems can be initiated, at least in the Northeastern and Midwestern cities, and this year proved to be fruitful for some. Chicago, Minneapolis, and St. Louis all experienced at least 100 more airside economizer hours compared to normal. On the waterside, Boston, Detroit, and Minneapolis benefited the most.
Heat recovery calculator wrap up: runaround coil example
We tried to finish strong last month but as some of you have discovered and promptly pointed out, there were some errors in the formula table (which were likely attributed to the translation to print). Figure 2 is the same table with the corrected formulas highlighted in red. Another pesky bug surfaced while writing this column. When modeling a runaround coil, the savings are zeroed out. To fix this, change the formulas in Report-F47, G47 as shown in Figure 2 (blue). This will sum the sensible and latent components instead of reporting the total enthalpy savings.Now we can take the spreadsheet out for a test drive. Consider a runaround heat recovery loop for a four-story hospital building with four 70,000-cfm, constant-volume, 100% outside air systems located in the basement and four central exhaust fan systems of equal capacity on the roof. The heat recovery system would consist of a supply heat recovery coil in each air handler upstream of the steam preheat coil, an exhaust heat recovery coil at each exhaust fan inlet, and a pump to circulate a glycol solution between the supply and exhaust coils.
Obviously, the thermal effectiveness for this type of system is not published information because it's a custom engineered system. So the first step is to calculate the effectiveness. In Boston, we typically shoot for at least a 35 degrees F rise across the supply heat recovery coil during design winter conditions. Assuming 0 degrees entering supply air (Ts1), 35 degrees leaving supply air (Ts2) and 70 degrees entering exhaust air (Te1), we can calculate the thermal effectiveness as follows: Effectiveness = (Ts2 - Ts1) / (Te1 - Ts1) = (35 - 0) / (70 - 0) = 50%.
With the effectiveness known, the leaving exhaust air temperature can be calculated as: Te2 = Te1 - Eff x (Te1 - Ts1) = 70 - 0.5 x (70 - 0) = 35 degrees. The total coil MBtuh = cfm x 1.08 x DT = 280,000 x 1.08 x 35 = 10,584 MBtuh. Assuming a heat recovery loop water temperature differential of 12 degrees, the required pump capacity gpm = MBtuh / (500 x 12) = 1,765 gpm. The supply and exhaust coils are then selected to achieve the design conditions. Using coil selection software, eight row coils are selected with an air pressure drop of 0.70 inches each. So the total added resistance in the two airstreams is 0.7 inches x 2 = 1.4 inches. The total system water pressure drop (pump head) is 10 ft for each coil, plus piping loop losses of 20 ft = 40 ft.
Figure 3 lists all of the above assumptions to be plugged into the spreadsheet. The results show that the gross savings would be $4,900 for cooling and $252,305 for heating. The added fan and pump operating costs would be $65,684 for a net annual savings of $191,521. Note that the savings during the cooling season is minimal, usually not sufficient enough to justify running the pump during the summer in Boston. If your results match mine, you should be in business! ES
EDITOR'S NOTE: The images associated with this article do not transfer to the Internet. To review the figures, please refer to the print version of this issue.
