ASHRAE Journal - August 2008 - (Page 64)

to drop its water, and when you are done, you still have cold air. So now what? Well, you have to heat it back up if you want to use it. This is sometimes called cooling with reheat. There are other tricks we can do, but at the end of the day we must add energy back to the air that we cooled. There is no other practical way to do it. It gets worse when we overventilate with humid air, especially when it is cloudy and not so hot. Now, all you are doing is bringing in moisture. But, you are doing it when there is not much of a sensible load (heat gain across the enclosure and heat gain from lights and appliances and people—the load you can sense) to run the air-conditioning system. This moisture you are bringing in with the ventilation air is also a load. But, it is a different kind of load that we have pretty much been ignoring up until now and getting away with it. The operative phrase to note is getting away with it. Mechanical engineers such as me call this the hidden load or the latent load (as in latent defect or hidden defect), and our heads explode when we try to explain this problem to the indoor air crazies who think that only dilution is the solution to indoor pollution. I mean how can too much outside air be bad?II To take the water out of the air, you need to use energy. To make the air comfortable/useful, the air can’t be too cold after you have taken the water out of it. Typically, heat (energy) must be added back to the air. Traditionally, this heat (energy) was available through lousy glazing systems, too much glass, black roofs with no insulation, poorly insulated walls, energy inefficient lights, and crappy appliances. Well, this traditional reheat due to inefficiency and poor practice is disappearing. Now, we are stuck. If we don’t warm up the air after we dry it, we are going to be in trouble. We haven’t been, and we are getting into trouble. The buildings are now getting too cold or too humid or worse, both. Ah, this is easy, you say. Just make the air-conditioning systems smaller—right size them. Don’t make me scream. I am getting tired of being told by energy weenies that my equipment is too big.¶ I still need to satisfy the full-load requirement. If you make the equipment too small, it doesn’t satisfy the cooling requirements under full load. Let the people be uncomfortable, you say. People should suffer, you say; we have had it too good, you say. Yeah, sure, another greenie value judgment that makes me scream. I don’t think we should suffer. I like it being good. I don’t have a problem with using energy. I have a problem with IISome folks wasting energy. Until now we have become good at dealing with this part-load problem with face-bypass, runaround coils, hot gas bypass, and heat pipes, but we have reached our limits. Building enclosures have gotten so good, glazing systems so good, interior load management so good, that we can’t use the same equipment to handle the sensible load and the latent load. Did I mention just how good the glazing systems have become? Did I mention low solar heat gain coefficients? In the past, we installed air conditioners and only cared about making it cold inside. We used one piece of equipment to handle the sensible and latent load. The reason this worked is that the latent load was small compared to the sensible load. Not anymore. The sensible load has decreased a lot. And, much to my chagrin, the latent load has increased due to overventilation. The old systems can’t handle the new sensible to latent ratios. We must separate the two loads. We need separate systems for the sensible load and the latent load. This is a big deal, and it results from the low heat gain in the modern enclosure. In other words, energy conservation and good construction practice is biting us on the butt. We sometimes refer to this second separate latent load control system as supplemental dehumidification. We need it when we have a low heat gain enclosure and efficient lighting and appliances. Others call it preconditioning of outside air since the largest component of the latent load is the ventilation air or the outside air. Whatever you call it, we need this second system. We don’t often get it, and it hurts us. But, these systems use energy. Yes, as the saying goes: there is no thermodynamic free lunch. You need to use energy to make air cold, and you need to use energy to make air dry. We have been good at making air cold. We now have to become good at making air dry. Think of it this way. For every 100 units of energy you save on the efficiency and on the cooling side, you will need to give back about 20 units of energy to be dry. You are still 80 units ahead. The problem is that if you are greedy and want the entire 100 units, your building fails and your occupants become uncomfortable and probably annoyed. It is amazing to me that the hotel industry figured this out first. It had the problem first, so it makes sense that it would also figure it out first. Think about the typical hotel room. The drapes are usually closed, and the room is empty during the day. You also know the unit has been sized for the full load: don’t seem to care about energy either. I mean health is paramount, and we should always err on the side of caution. How can we argue against health? Easy, argue when the healthy measures are unhealthy. Overventilation can be unhealthy and energy wasteful. This is leading to interesting discussions in the Standard 62 committees. Apparently, the Standard 62 committees are not about energy. Apparently, energy security is not a 62 problem nor is climate change nor operating costs. Apparently, it is some other committee’s problem. How about the energy committee (Standard 90.1)? Except the energy committee does not want to touch ventilation. Cowards. My insider’s perspective (on Standard 62.2 at least) is that there is a lot of mileage to be made by scaring people about underventilation, and folks are rising to the occasion. Unfortunately, overventilation in hot, humid climates and in mixed humid climates has led to more indoor air problems due to mold resulting from part-load issues than underventilation anywhere else (in my not-so-humble opinion). And, speaking of health and underventilation, where are all the dead bodies? Where is the causal link between health and specific levels of specific indoor contaminants? In my not-so-humble opinion, all of the rates have been just wild guesses without a sound epidemiological basis. But, the resulting mold from overventilation is real and demonstrable. Just to make it clear, I have been guessing at rates as well, but I have not been afraid to call them guesses or tried to hide the guesses behind a lot of bad science. And, one more thing, this LEED stuff is making me crazy, especially the part where you get extra points when you ventilate at an even higher rate than what Standard 62 calls for. What a bunch of green sustainability hypocrisy. Doesn’t anyone at the U.S. Green Building Council know anything about energy and part-load humidity? go there. Just don’t go there. ¶Don’t 64 ASHRAE Journal ashrae.org August 2008 http://ashrae.org

Table of Contents Feed for the Digital Edition of ASHRAE Journal - August 2008

ASHRAE Journal - August 2008 Contents Commentary Industry News Letters Meetings and Shows Maintain to Sustain—Delivering ASHRAE’s Sustainability Promise Ultraviolet Germicidal Irradiation: Current Best Practices Improving Humidity Control With Energy Recovery Ventilation Single- or Two-Stage Compression Data Center Cooling: Using Wet-Bulb Economizers Building Sciences InfoCenter Practical Pointers Products Emerging Technologies Washington Report People Special Products Classified Advertising Advertising Index

ASHRAE Journal - August 2008

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