MERM, Chapter 39, Practice Problem #4

The answer shows that some of the return air bypasses the cooling coil. How are you supposed to know this? Are you supposed to assume only the return bypasses the coil and mixes with the air leaving the coil? Because the air leaves the coil at 50.8F and enters the room at 55F?

I don'thave my book on my-its at the office. But I think I remember something about this problem from April. Did you check the errata? I believe (if I'm remembering correctly) that the MERM was supposed to give the temp of 50.8 off the coil in the problem statement, but didn't.

I'm not sure if that is what you're asking b/c my book is not on me. I'll try to remember to check it out on Monday if you haven't gotten better answers by then.

Ed

You're correct about the missing discharge temp of 50.8F. From that info, are you supposed to assume some of the return air bypasses the cooling coil, since the supply air enters the room at 55F?

Can't quite remember. I'll have to check out my book Monday. If I happen to remember and find my notebook at the house this weekend, I'll post.

Wasn't it something to do with bypass factor? You know, where some of the air squeezes between the fins unnaffected by the cooling coil?

Ed

I just looked at it.

It appears they just assume that you should assume that some of the return air bypasses the coil. Kind of silly. What if the duct was not very well insulated and it gained all that heat from the surrounding air (ie, no return air bypassed the coil)?

Its just not a very well worded question.

In the solutions book, it even throws in that there is 2000 CFM of outside air. That's just a distractor and has no relvance to the problem.

All they want is for you to analyze how the return air that bypasses the coil (again, we just need to totally guess that's whats happening) mixes with the return and outside air that has gone through the coil.

Weird problem. Awkward wording.

Ed