Dec. 1, 2011
Would the author of the article “Evaporative Pool Dehumidification” (October 2011, comment on a few issues? The IDECVAV system doesn’t appear to address the air-distribution recommendations provided in Section 4.6 of ASHRAE Handbook—HVAC Applications

Would the author of the article "Evaporative Pool Dehumidification" (October 2011, comment on a few issues?

The IDECVAV system doesn't appear to address the air-distribution recommendations provided in Section 4.6 of ASHRAE Handbook—HVAC Applications:

• ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) recommends four to six air changes of total air movement within a pool space. Code minimums for outdoor air usually result in one to one-and-a-half air changes. Controlling outdoor air based on space humidity as described in the article will result in a supply-air volume of minimum outdoor air for six to seven months in Marathon, Wis.

• ASHRAE recommends that a portion of air movement be directed across a pool surface. If total airflow is modulated from minimum (say, 20 to 25 percent) to 100 percent based on space humidity, how is distribution effectively accomplished?

The IDECVAV system is not capable of controlling space humidity on humid days during summer (about one to two months in Marathon, depending on actual pool design). For a typical pool designed at six air changes of air movement, pool humidity can be controlled to about a 63°F outdoor-air dew point, if 100-percent outdoor air is used with an 85/60-percent-RH space. The design dew point for Marathon is 71°F. More supply-air volume could be used, but this would increase operating costs and handle only a few more degrees of dew point. The article doesn’t mention that the dehumidification unit (DHU) could maintain humidity all year, while the IDECVAV system cannot.

I agree that the DHU is the wrong system for the Marathon climate because of high energy costs and it being complicated mechanically and likely to cause operational problems.

The IDECVAV system certainly can be used to handle space cooling in a pool space in Marathon, as long as there is not too much solar load. There is a great health benefit to using more outside air, as the article states. However, the inability to provide adequate total air movement and the inability to control space humidity for the entire year seems to make the comparison of the two systems somewhat questionable.
Gary Lochner
Unison Comfort Technologies
Minneapolis, Minn.

Author's response:
Because of word and figure limits, an author always has choices to make regarding information presented. Mr. Lochner's inquiry provides the opportunity to present additional supporting data.

The IDECVAV system meets or exceeds all relevant ASHRAE recommendations by employing 100-percent outdoor air. Consider:

• The recommendation for pool-space air movement in Section 4.6 of the 2007 edition of ASHRAE Handbook—HVAC Applications is limited to systems employing mechanical refrigeration; it does not apply to 100-percent-outdoor-air approaches. Other than outdoor-air ventilation, no minimum flow rates are required by model mechanical codes or ASHRAE standards. The International Mechanical Code and 2004 and later editions of ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality, require 0.50 and 0.48 cfm of outdoor air per square foot of pool and deck area, respectively. The six- to seven-month minimum-airflow claim is no truer than saying Marathon continuously sees -25°F temperatures for six to seven months. Even if it were, so what? Ventilation and humidity-control requirements are met.

• ASHRAE does not recommend that air be directed across pool surfaces. It recommends air motion across pool surfaces be limited to velocities from 10 to 30 fpm. Directing air at a pool surface is poor design, as it increases pool evaporation rates and disperses chloramines, making capture by exhaust less effective.

By promoting vertical displacement of air and limiting air turbulence, the IDECVAV system improves the effective capture and removal of contaminants at low flows. Airflows are lowest during cold weather; however, the IDECVAV system provides superior humidity control over almost the entire range of ambient weather conditions, deviating from set point only under the most extreme conditions, which usually are experienced for only hours or, occasionally, a few days per year, rather than months on end.

Figure 1 shows space-temperature and relative-humidity performance, measured at 15-min intervals, from March 1-7, 2011. The supply-air-delivery rate was controlled proportionally between minimum and maximum flows from space relative humidity of 50 to 60 percent. During that period of time, the average daily standard deviation from relative-humidity set point was only 1.13 percent. This is well within the accuracy limitations of the typical controlling humidity sensor of ±2 percent. Figure 1 shows even better control over temperature, set to 85°F, ±2°F. During the same period of time, the average daily standard deviation from temperature set point was only 0.4°F.

As the humidity ratio of outdoor air increases with increasing outdoor-air temperature, the volume of outdoor air required to satisfy dehumidification needs increases.

During cooling season, airflow increases to its maximum. The indirect evaporative (sensible) cooling process typically has no problem maintaining space-cooling requirements. Figure 2 shows space-temperature and relative-humidity performance, measured at 15-min intervals, from July 1-7, 2011, and illustrates what happens during both "normal" summer and "extremely" humid conditions. During normal summer conditions, the IDECVAV system has no problem maintaining temperature or relative humidity; the average daily standard deviation from relative-humidity set point is only 2.10 percent. During extremely humid weather conditions, inside temperature essentially is unaffected, but relative humidity in the pool space rises. This, however, is a relatively rare event. A review of AFM 88-29 or bin weather data demonstrates that mean coincident wet-bulb temperature typically declines when temperatures increase to above 80°F. As such, the operational periods when humidity cannot be controlled effectively are rare and very intermittent, measured in hours per year, rather than months at a time.

System psychrometrics were evaluated over a full year (8,760 hr) for the closest community using normalized TMY-2 weather data. This information is readily available from the Gas Research Institute and can be output in text-delimited format and imported into Excel spreadsheets for detailed analysis. Our experience with this technique is that it produces accurate representations of energy savings and performance. This allows us to evaluate operating economics and determine appropriate maximum air-delivery requirements under reasonably anticipatable conditions for any locale. Total air-delivery rates are not a function of temperature, but of the difference between indoor and ambient humidity ratios.

When properly designed and set up, the IDECVAV system uses airflow rates similar to those required by dehumidification units. It provides superior control of relative humidity, easily meeting ±2 percent under almost all ambient conditions, as compared with the ±10-percent or greater swings usually seen with refrigerant dehumidifiers. This provides our clients significant and predictable performance advantages. Whereas the IDECVAV approach has been limited to the Midwest and Northeast United States, careful psychrometric modeling has shown it is applicable to all areas of the continental United States. High ambient temperatures and/or relative humidity have not proven problematic in almost 17 years of operation. The primary limiting factor is operation in extremely cold weather (below -25°F), when there is a need to prevent the formation of ice in the primary heat exchanger.
Mark S. Lentz, PE
Lentz Engineering Associates Inc.
Sheboygan Falls, Wis.

Letters on HPAC Engineering editorial content and issues affecting the HVACR industry are welcome. Please address them to Scott Arnold, executive editor, at [email protected].

About the Author

Scott Arnold | Executive Editor

Described by a colleague as "a cyborg ... requir(ing) virtually no sleep, no time off, and bland nourishment that can be consumed while at his desk" who was sent "back from the future not to terminate anyone, but with the prime directive 'to edit dry technical copy' in order to save the world at a later date," Scott Arnold joined the editorial staff of HPAC Engineering in 1999. Prior to that, he worked as an editor for daily newspapers and a specialty-publications company. He has a bachelor's degree in journalism from Kent State University.