Simple, constant-volume HVAC systems inevitably cause high zone humidity at sensible part-load conditions. Any of several design options can improve part-load dehumidification. Table 1 ranks the dehumidification enhancements considered for our example classroom from “best” (separate paths) to “poorest” (basic cooling only) based on how well they control humidity.

Table 1. Comparison of Dehumidification Enhancementsa
		Sensible Design		Latent Design
Enhancement	Effectivenessb	Zone RH	Cooling Req’d	Zone RH	Cooling Req’d
Basic CV system	6 (poorest)	52.4%	4.78 tons	66.9%	3.68 tons
Indirect: total energy recovery mixed-air bypass   return-air bypass	4   5   3	50.4%   52.4%   52.4%	3.50 tons 4.78 tons 4.78 tons	65.0%   64.5%   55.2%	2.47 tons 3.74 tons 3.92 tons
Direct: separate paths   supply-air tempering	1 (best)   2	51.6%   52.4%	4.81 tons 4.78 tons	52.4%   52.4%	4.16 tons 5.21 tons
aComparison is based on a 30-person, 10,000-cu-ft classroom in Jacksonville, Florida. Supply airflow is 1,500 cfm of which 450 cfm is outdoor air for ventilation.
b“Effectiveness” ranks the dehumidification enhancements based on the lowest relative humidity (RH) in the zone and fewest cooling tons.

Ultimately, however, the “best” system choice must also consider first cost and operating cost as well as climate and zone loads.

To design a constant-volume system for effective dehumidification:

• Limit zone relative humidity at all load conditions.
  

  Analyze system performance at both sensible and latent design conditions when sizing the cooling coil. The maximum coil load may occur at the wettest, not the hottest, outdoor condition.
  

  Determine the need for dehumidification during unoccupied periods. Moisture from infiltration, wet-process cleaning, or vapor pressure diffusion can significantly increase the relative humidity indoors.

• Consider the advantages and disadvantages of each dehumidification enhancement. Remember that indirect dehumidification, which depends on a sensible load, is often ineffective during unoccupied periods.

• Pick the dehumidification enhancement that works best for the application, given the budget.

By Dennis Stanke, applications engineer, and Brenda Bradley, information designer, Trane.

You can find this and other issues of the Engineers Newsletter in the commercial section of www.trane.com. To comment, send a note to Trane, Engineers Newsletter Editor, 3600 Pammel Creek Road, La Crosse WI 54601, or e-mail us from the Trane Web site.

For more information about humidity management, refer to Managing Building Moisture (SYS-AM-15). To review the fundamentals of psychrometric analysis, see Air Conditioning Clinic: Psychrometry (TRG-TRC001-EN). You can order either publication from www.trane.com/bookstore.

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