Creating Comfort – Part 1


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We are the HVAC industry.  We are the creators of indoor comfort.  We know that human comfort is determined by many variables.  We know that dry-bulb temperature, relative humidity, and air velocity are of key importance to the comfort that people experience in their homes and work places.  We know that if we properly control these three variables, then we can deliver a comfortable environment. 

With that in mind, we calculate an ACCA Manual J load on our building.  We use ACCA Manual D to design a ducted distribution system so that we can deliver all of the Btu’s to where they need to be delivered.  We consult ACCA Manual S to choose the equipment for this particular job.  Next, we need to choose the registers and grilles and locate them where they can best accomplish our goal of human comfort.  So, we will consult ACCA Manual T. 

We know that The American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) has actually defined what is “comfortable” for the majority of people.  ASHRAE has established comfort envelopes that, when plotted on a psychrometric chart, as in Figure 1, take the shape of legal envelopes.   

ASHRAE has established a winter envelope and a summer envelope.  According to their research, if you should find yourself in a 70° room with 30% relative humidity, in the winter, you are, by definition, comfortable. So stop complaining, already. 

But, that’s only the intersection of two variables: DB temperature and relative humidity.  What about air velocity? Now take that very comfortable 70° air with 30% RH and blow it over someone’s bare skin at a velocity of 60 fpm and you’re going to get a complaint of cold, drafty air coming out of the supply outlets. You can show this effect by blowing on your own forearm.  The air coming out of your mouth is relatively warm at about 98.6° F, and yet it feels cool as it passes over your skin.  That’s the principle of evaporative cooling at work.   

Evaporative cooling is the reduction in temperature resulting from the evaporation of a liquid, which removes latent heat from the surface from which evaporation takes place. In other words, whenever something boils or evaporates, it takes heat away with it. When that heat removal is realized, the chill is felt. So, when you roll up your sleeve and blow on your forearm, that high velocity air causes a rapid increase in the evaporation rate of the moisture on your skin. When that latent heat is removed, you feel cool. 

The “feels like” and “wind chill” temperatures that we see on the weather map as we watch the evening news are the result of evaporative cooling. If you’re in Milwaukee in the wintertime and the air temperature is 20° and the wind is blowing at 20 mph, the local meteorologist is going to tell you that the wind chill temperature is 0°F. Conversely, if you’re in Tampa in the summertime and the air temperature is 90°, but it feels like 96°, then the humidity is so high that no more moisture can be absorbed by that 90° air so it feels warmer, muggier. 

That’s why air velocity within the occupied zone should fall between 25 and 50 feet per minute (fpm).  A velocity of 60 fpm will blow a piece of paper off of a table. The occupied zone consists of the space between the floor and 6 feet above the floor in the vertical direction and the space that is more than 2 feet from the wall in the horizontal direction. See Figure 2. 

It is within the occupied zone that comfort conditions must be maintained. Therefore, we know that we do not distribute conditioned air into the occupied zone.  Because if we did, we would surely get complaints of “drafty air”.   

We know that conditioned air should be delivered into the mixed air zone of a room. Typically, that is at the periphery of the room along the outside walls or parallel with the ceiling.  After all, that is where we are losing and gaining all of the heat in the room.  So why not attack the heat loss and heat gain where it is happening?  Throwing air out into the middle of a room, or worse than that, delivering conditioned air directly onto the occupants of a room does nothing except annoy the occupants. We are supposed to deliver comfort, not discomfort. 

We will continue our quest for comfort in upcoming editions of NOW Magazine. So, keep an eye out for more to come in this column.   

Jack Rise
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Posted In: ACCA Now, Technical Tips

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