The old expression, "It's not the heat, it's the humidity," definitely applies for August 2002. Although dry bulb temperatures and sensible cooling ventilation load indexes (VLIc's) were higher than normal for most of the country, it was the long duration of simultaneous high humidity levels that really had an impact on ventilation costs.

In fact, Boston, Chicago, Cleveland, Memphis, Detroit, and Washington, DC all saw latent VLIc's that were at least 40% higher than normal. Boston topped off the list at +56%. What was the impact? In Boston, with an assumed blended electric rate of $0.10/kWh and 0.75 kW/ton cooling efficiency, the total ventilation cooling for August 2002 was 2.15 (Figure 1) x 0.75 x $0.10 = $0.161/cfm. Compare that to the norm for August of 1.58 (Figure 1) x 0.75 x $0.10 = $0.119/cfm.

In addition to the increase in ventilation cooling costs, cooling towers around the country were being pushed to the limit in August. Just during this one month, all three 0.4%, 1.0%, and 2.0% ASHRAE design wet bulb hours of 35, 88, and 175 were exceeded in Boston, Dallas, Houston, Memphis, Miami, New York, and Washington, DC. Add these wet bulb design hours to the hours we saw in July and combine that with the higher than normal ventilation loads experienced in both months, and you are looking at a long summer if your cooling towers were selected "on the margin."

On the positive side, total heat recovery (sensible + latent) systems were big winners due to the higher humidity levels. Total heat recovery savings were as much as two or three times the norm in many cities. Boston again topped the list at +203%. Using the same electric rate and cooling efficiency, the total heat recovery savings for Boston in August was 0.612 (Figure 1) x 0.75 x $0.10 = $0.046/cfm vs. the norm of 0.202 (Figure 1) x 0.75 x $0.10 = $0.015/cfm.

Creating Your Own Heat Recovery Savings Calculator (Continued)

Last month, in Step 11, we entered the heat recovery formulas used to calculate the sensible and total heating and cooling heat recovery for every hour based on the input conditions on the report worksheet. It should be noted here that the intent of this spreadsheet is to provide accurate, dependable results that allow the user to evaluate the payback associated with installing heat recovery systems and to make comparisons between different types of systems and manufacturers.

It is not intended to pinpoint the cost savings down to the penny. So the formulas do not take into consideration the effects of frost control, heat transfer between the heat exchanger and its surroundings, or heat energy from cross leakage or fan motors. Now that the formulas have been input, we can sum up the data and calculate the cost savings.

Step 12 - Calculating heat recovery cost savings: Create blocks 6,7, and 8 as shown in Figure 2, making sure they are located in the cell addresses identified in the figure. The formulas for blocks 6,7, and 8 are given in Figure 3. These formulas calculate the sensible and latent components of the heat recovery savings using the input data we have defined in blocks 1 through 5.

Once completed, your results should match the values shown in Figure 2. If they don't match, double check your formulas and make sure you have entered all of the same values that have been entered into the other input blocks. Note that the results in our example represent the savings for a 5,000-cfm total heat recovery system located in Boston based on the efficiencies, utility rates, winter and summer indoor space conditions, and operating schedule input to blocks 1 through 5.

To run the calculation for a sensible heat recovery system like a flat plate or runaround coil loop, try entering zero into the "net total" thermal effectiveness for heating and cooling in block 4 or leave both cells blank. That will zero out the latent and total values (blocks 7 and 8).

The end of this project is approaching, and we should be able to wrap it up in the next column or two. This undertaking has benefited from input from readers with many different backgrounds and perspectives, so keep up the useful comments as we enter into the home stretch. ES

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.