Mechanical Dehumidification, Comfort, and Cost

ATLANTA, GA — We’ve heard it so many times: It’s not the heat, it’s the humidity that makes us uncomfortable. But are system designers “doing the right things” to achieve optimum comfort and system performance?

The ASHRAE forum on dehumidification got down to basics, and at the same time became quite complex, rather quickly. “What is the impact on comfort and health?” asked an engineer. Do any current studies draw direct, measurable correlations between comfort and humidity?

An overseas participant noted that “Heat pipes are OK, but not satisfactory for control. How do you dehumidify economically, so that you offset reheat costs?”

A consulting engineer from Europe noted that his firm designed what may be the “largest desiccant commercial cooling system in the world,” in a climate that is “mostly humid, not hot. Comfort is a humidity problem.” The system uses waste heat to regenerate the desiccant. “You have to convince owners to use these systems,” he pointed out.

Adding to the problem is the fact that comfort is often perceived. Temperatures can feel more or less tolerable depending on the degree of humidity.

As one participant noted, “100°F is OK in the desert, but not in the rain forest.”

A health study from the World Health Organization cites absolute humidity, asthma, and dust mites, but this does not address relative humidity and comfort.

“We changed design condition, raised temperature from 70° to 75°, and relative humidity to 40% maximum,” said the engineer. “In summer we raise that to 78° or 80°, but keep humidity in check.” It has been effective, but “We need studies” to verify these designs.

He added that there once was a 1930s ASHRAE guide showing “lines of equal wetness.” While it has been a valuable resource, “It is not correlated into a fashion that we now need it.”

Health and Cost

One fairly well-known chart shows the relative humidity window where bacterial growth is minimized. Participants knew of no current charts on rh and comfort.

How do engineers define “healthy?” “I’m talking health and comfort,” said a forum participant. “If you’re not healthy, you’re certainly not comfortable.”

Air velocity has an impact as well in the temperature-humidity-comfort equation. “It takes less energy to reduce humidity than to reduce space temperature.”

“Are we not doing enough to control humidity, because it costs more energy?” asked an engineer.

Actually, it costs less, but the general perception is that it costs more. It was pointed out that mechanical dehumidification can use hot gas reheat for regeneration.

“Do we have equipment available?” asked another. “DOE is pushing to produce more cooling with lower kW consumption. Manufacturers are pushing more air through the evaporator, but can’t get the humidity we’re looking for.”

A manufacturer responded that “Unitary manufacturers are working on lowering head pressure, etc. Part load is another issue. It depends on the design of the indoor coil.”

“Most buildings are not addressing humidity directly during cooling season,” said an engineer. “We have some tools. We don’t know all the tools.”

“There’s not one single system,” responded another. “Engin-eers must study load patterns, then design for it. There is too great a propensity to use what was used before.”

Publication date: 02/19/2001

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