Surprising HVAC Lesson: Air Conditioning Costs in Philly vs. Vegas

[russ_watters]

One of the guys I work with is taking a course in HVAC and since my boss used to teach it, we got a little lesson today (helping with the homework). The result of the conversation was the realization that it costs significantly more to air condition a building in Philadelphia than it does in Las Vegas. Surprising, huh? Here's the math though:

Indoor design conditions are for 75F and 50% relative humidity. That corresponds to an enthalpy of about 28.5 (btu/lb). In Philly, the design conditions (from NOAA data) are 92F dry bulb and 75F wet bulb (that's about 48% relative humidity). That corresponds to an enthalpy of 39.0. In Vegas, the conditions are 112F wb and 68F db (about 8% rh). Thats an enthalpy of about 33.

So the difference between the two enthalpies means its 18% hotter! in Philly than in Vegas on a typical August day and requires 57% more energy (10.5 vs 4.5 btu/lb of air) to cool outside air (your house is more complicated since much of the load is actually solar).

 

[mdk42]

Not bad on your explanation. When I design systems and calculate annual energy costs, a good rule of thumb to use is the closer the dry and wet-bulb temperatures are to each other, the more energy you'll use to condition the space. Air conditioning systems cool 'sensibly' first, ie they change the dry bulb temperature. The last thing that they do is remove moisture, ie they drastically change the enthalpy, humidity, etc. Here in Houston, the design dry bulb is 96 and the wet bulb is 79 (depending on the source)--a resulting enthalpy over 42 btu/lbm!

Let me know if you have any further questions on the matter. The subject is still pretty fresh in my mind--I just took my Mechanical PE exam a few months ago.