As the world's population increases, there is an increasing demand on our water resources, and we are constantly looking for ways to recycle and conserve. HVAC systems can consume large amounts of water, and it's important to be able to quantify how much water they consume. In the June 2003 report, we reviewed methods for determining the amount of water consumed by cooling towers through evaporation, blowdown, and drift. In this report, we'll take a look at humidifier water usage and cooling coil condensation.

Humidifier Water Consumption

The design capacity of a humidifier is usually determined from the outside airflow rate and the difference between the desired space condition and minimum outside air humidity ratios. For a 100,000 cfm outside air system and 70 degrees F 30% rh space condition, Wsp = 0.0046 Lbw/Lba. The maximum humidification load would be (.0046-0) x .075 x 60 x 100,000 = 2,070 lb/hr. But for most of the year, the actual humidification load is much less than this value, and the design capacity obviously cannot be used to estimate water consumption.

This calculation can be done fairly easily using VLI data. By definition, the VLIlh (latent heating or humidification VLI) is the total annual humidification water consumption per cfm of outside air. The total annual VLIlh is based on the same calculation above for every hour that Woa is less than Wsp, or VLIlh = •(Wsp-Woa) x .075 x 60. To calculate water consumption, it's just a matter of multiplying outside air cfm by the total annual VLIlh. If the above system were located in Cleveland and operated 24/7, the annual water consumption would be 4.03 gal/cfm/year x 100,000 = 403,000 gal/yr.

Cooling Coil Condensate

Many states have adopted the policy of connecting all cooling coil condensate to the storm drainage system, and separating the storm from the sanitary to reduce the load on the sanitary treatment system. However, some older cites are still dealing with older systems that are still interconnected. The impact of cooling coil condensate drainage can be determined using VLI data. The total annual VLIlc (latent cooling VLI) is calculated for every hour that the outside air humidity ratio is greater than the humidity ratio of the supply air temperature setpoint at saturated conditions. Or VLIlc = • (Woa - Wsa) x .075 x 60/8.34. In Boston for example, the total annual VLIlc for 55 degrees supply air temperature and 24-hour operation is 2.81 gal/cfm/yr. For 100,000 cfm, the amount of water condensed would be 100,000 x 2.81 = 281,000 gal/yr.

To determine the peak condensation flow rate, consider the summer design wet bulb condition for the location. For example, the humidity ratio associated with the ASHRAE 0.4% Boston wet bulb design of 75/87 is 0.016 Lbw/Lba. And the humidity ratio at 55 degrees saturated is 0.0092 Lbw/Lba.

So, the peak condensation flow rate for a 100,000 cfm system would be 100,000 x (0.016 - 0.0092) x .075 x 60 / 8.43 = 363 gph, or 6.0 gpm. The peak condensation flow rate is useful if your're interested in determining what the maximum impact would be. It's also useful in sizing the drain piping properly (many popular rule of thumb methods can result in significant oversizing). ES

NOTES - This report is generated from raw data furnished by the National Weather Service (NWS). Normal max/min and degree day values are from the historical record provided by the National Climatic Data Center (NCDC). Normal values for VLI and economizer hours were derived from the TMY2 data set compiled by the National Renewable Energy Laboratory (NREL). ASHRAE design hours are number of hours that meet or exceed the 1997 ASHRAE Fundamentals design conditions. Airside and waterside economizer cooling hours are the quantity of hours that the outdoor air was below 55 degrees F dry bulb and below 40 degrees wet bulb. The cooling ventilation load index (VLI) is the total (sensible + latent) energy in ton-hrs/cfm required to maintain 55 degrees discharge air temperature. Likewise, the heating VLI is the sensible heat energy in therms/cfm required to maintain 55 degrees. The humidification VLI is the amount of water in gal/cfm required to maintain 30% rh at 70 degree space temperature.