From the editor...
ASHRAE Standard 62, “Ventilation for Acceptable Indoor Air Quality,” recommends that the relative humidity not exceed 60 percent at any load condition. This can be problematic because the Standard increases the minimum outdoor-air requirement. Many HVAC designers prefer a low-cost constant-volume solution, believing that it also simplifies ventilation and inherently provides sufficient dehumidification.

This newsletter reveals the flaw in that belief. Dennis Stanke, Trane staff engineer and member of ASHRAE SSPC 62.1, uses psychrometric analyses to demonstrate the difficulty of providing proper dehumidification—particularly at part load, when dry-bulb temperature determines system capacity. He also discusses several design options that improve the latent capacity of a constant-volume system and compares their effectiveness.

Table of Contents

• The Difficulty with CV Dehumidification
  • An Example
  • Packaged Air Conditioning Compounds the Problem
• Enhancing Indirect Dehumidification
  • Total Energy Recovery
  • Mixed-Air Bypass
  • Return-Air Bypass
• Direct Dehumidification
  • Separate Paths
  • Supply-Air Tempering
• To Recap

  Sidebars:

• Sensible versus Latent Design
• What About VAV Systems?

Trane believes the facts and suggestions presented here to be accurate. However, final design and application decisions are your responsibility. Trane disclaims any responsibility for actions taken on the material presented.