In most cities, the hot and humid weather experienced during August carried through September, wrapping up an expensive cooling season. Check out the wet bulb design hours for Miami: there were 270 hours when the wet bulb temperature was equal to or above the ASHRAE 0.4% design condition of 80 degrees F (normally 35 hours for an entire year). This may be an extreme case, but it's a sobering reminder of the potentially large swings in wet bulb design hours from year to year. This can cause serious migraines when dealing with cooling towers that are overloaded.

Right now, you may be thinking, "How do I measure the impact of high humidity levels on my utility bills?" One popular method is to graph monthly energy costs against cooling degree days (CDD's). It is often assumed that if the CDD's were near or lower than normal for a given month, then the weather can be ruled out as a possible reason for a simultaneous increase in energy costs. Not a very good assumption, especially where large ventilation loads are present because CDD's ignore latent cooling (dehumidification) loads. CDD's are calculated by simply subtracting a space neutral temperature (usually 65 degrees) from the average daily dry bulb temperature.

Here is a good case in point: The CDD's in Baltimore were actually lower than normal for September, but the total cooling ventilation load index (VLIc) was almost 30% higher than normal, mostly due to the 41% higher than normal latent VLIc component. So, a perceived increase in September cooling costs for a Baltimore building with large ventilation loads could likely be due to the weather, even though the CDD's demostrate otherwise.

In lieu of CDD's, a graph of energy cost vs the total VLIc would make more sense because both sensible and latent components are represented.

Creating Your Own Heat Recovery Savings Calculator (Continued)

Just to clear up some confusion from last month's column, we never provided the assumed input values in input block #1 that were used to get the results in blocks 6, 7, and 8. These values were elevation, 30 ft; cfm, 5,000; heating Eff, 0.80; cooling Eff, 1.25; heating fuel cost, $0.75/Therm; and electrical cost, $0.09/kWh. Entering these values will allow you to check your results.

Step- 13: Fan and pump energy costs. As we struggle through life, we all eventually learn the fact that you "don't get something for nothing." The same lesson applies to heat recovery systems. Although significant heat energy can be recovered from or delivered to the exhaust airstream, there is a price to be paid in electrical fan and pump energy.

In a sensible heat runaround coil system, the heat recovery coils located at the intake and exhaust airstreams often consist of multiple rows and significant heat transfer surface area. The result is additional static pressure that the exhaust and intake fans need to overcome and additional pumping energy to circulate the fluid (usually a glycol solution) through the coils. Similarly, enthalpy wheels and other air-to-air systems add a significant amount of static pressure to both fans.

The increase in fan energy due to additional heat recovery coils or wheels can be calculated from the cfm flow rate, the rise in air pressure drop (in H2O), and fan efficiency. Similarly for runaround systems, the pumping energy can be calculated from the gpm flow rate, the water pressure drop across the coils (ft of head), and pump efficiency.

We also need to establish the number of hours that the fans and pump (if there is one) are operating. This is accomplished by assuming that the equipment will be "on" or "off" in accordance with the operating schedule defined in input block #5. So in the data worksheet, input the formulas indicated in Figure 3 for cells W4 and X4 and copy them down to the last row and label columns W and X with "Wkdy" and "Wkend". This will flag each operating hour with a "1"or "0" when the equipment is on or off.

In the report worksheet, create blocks 9 and 10 as shown in Figures 2 and 3. These formulas calculate the fan and pump energy costs based on the defined operating schedule. The net heat recovery savings is then provided in figure 10. That's all she wrote folks! If your results are in line with mine, you should be ready to roll. Just replace the data in the "data" worksheet with your favorite city and happy calculating! 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.