Take a look at the ASHRAE design data. In many cities, the normal annual cooling and wet bulb design hours were far exceeded during the month of July. Recall that 0.4%, 1.0%, and 2.0% design represent 35, 88, and 175 hours per year that each design condition is exceeded.
In Houston, the 1.0% wet bulb design condition of 79 degrees F was exceeded for 260 hours. That's almost three times the normal 88 hours. Take this into consideration when selecting your next cooling tower because, although the ASHRAE design data represents the average condition, wet bulb temperatures can vary widely year to year.
Creating Your Own Heat Recovery Savings Calculator (Continued)Last month, in Steps 6, 7, and 8, we created input blocks for thermal effectiveness and a schedule of operation. In this issue, we will tweak the thermal effectiveness input block and start generating the hourly calculations.
As always, if you would like to follow along and still have not obtained a copy of the sample Boston text file, send me an e-mail and I'll send you a copy.
Step 9 - Input data: Enter 5,000 into the cfm input (cell B10) in the "report" worksheet. This value is needed to check your results after Step 11.
Step 10 - Block #4 Revision: Recently, an ES reader suggested that we follow the ARI 1060 Standard for air-to-air energy recovery ventilation equipment (AAERV) when defining the thermal effectiveness. I agree that this would be a good idea because it would allow for direct comparison between different types of heat exchangers as well as manufacturers.
The ARI 1060 directory is a very useful document, providing the sensible and total effectiveness at standard heating and cooling conditions for all ARI-certified manufacturers. You can obtain a copy of the certification directory at www.ARI.org.
Let's modify input block #4 as shown in Figure 2 and fill in the values listed. Remember to make sure you locate block #4 in the same cell addresses shown in Figure 2 because other cells will refer to these addresses.
Step 11 - Hourly Calculations: Click on the "data" worksheet and input the column titles and formulas shown in Figure 3. Note that Figure 3 starts at column K, row 2. Columns A through J contain the hourly data from the Boston.txt file. Starting at column K, type in the column titles in rows 2 and 3 and fill in the formulas given by Figure 4 into row 4.
These formulas calculate the sensible and total heating and cooling heat recovery for every hour based on the input conditions on the report worksheet. Note that the formulas refer to specific cell addresses on both the report and data worksheets, which is why it is so important to make sure you use the same cell addresses as you follow along.
Although the formulas look complicated, there really isn't a lot of substance there. They mainly consist of symbols used by Microsoft Excel to refer to a given cell address.
For example, "Report!$F$7" refers to cell F7 on the report worksheet. Most of the IF/OR/AND statements have to do with the operating schedule. You may also have noticed that the formulas incorporate a built-in heat recovery system "dead band" where the system is scheduled off when the outside air conditions are between the winter and summer space design conditions.
After you double-check your inputs (your results in cells Q4 and T4 should be the same as in Figure 3 if you follow Step 9), copy row 4 down to row 8,763. Buried in all this Excel mumbo jumbo are the formulas used to calculate sensible and total heat recovery. Here they are in English (or, I guess you could say math). We will expand a little more on these formulas in next month's issue.
Sensible cooling (Ton-hr) = (1.08 x cfm x (Toa - Tsummer) x Thermal effectiveness)/12,000
Sensible heating (Therms) = (1.08 x cfm x (Twinter - Toa) x Thermal effectiveness)/100,000
Total cooling (Ton-hr) = (4.5 x cfm x (Hoa - Hsummer) x
Total heating (Therms) = (4.5 x cfm x (Hwinter - Hoa) x Thermal effectiveness)/100,000
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.