HVAC - Mixture Question
Posted 10 April 2012 - 12:11 PM
"An air handler that is equipped with a cooling coil receives 2,000 cu. ft/min of outdoor air that is maintained at 71 deg F and 50% relative humidity. The air is mixed with 4,000 cu. ft/min of outdoor air at 95 deg F dry bulb having a humidity ratio of 0.012 lb of moisture per lb of dry air prior to entering the cooling coil. The enthalpy of the mixture entering the cooling coil (Btu/lb dry air) is most nearly?
I found the enthalpy from the psych chart for the 2 conditions listed and then tried to use Eq. 38.21. Is this equation incorrect to use to find the change in enthalpy? I thought it could be used to find all the properties of a mixture as long as the mass or volumetric flow rates are known. Thanks for any input.
Posted 10 April 2012 - 02:48 PM
T1 x (CFM1/CFMtotal) + T2 x (CFM2/CFMtotal)
In your problem, you first need to find the point of the mixed air. Here's the steps I would take:
1. Find the two given points on the psych chart.
2. Draw a line between the two points. The mixed air temperature will fall somewhere along this line.
3. Find the dry bulb temperature of the mixed air:
Using the equation above, 71 x (2000/6000) + 95 x (4000/6000) = 23.67 + 63.33 = 87.0 degree F dry bulb.
4. Find the point on the line you drew in step 2 that corresponds to 87 deg. F dry bulb. That point represents your mixed air.
From there, you can find any mixed air property. In your problem it would give me an enthalpy of roughly 32.8 BTU/lb.
Posted 10 April 2012 - 03:01 PM
Plot and draw a line between the two state points
Then lever rule (weighted average) your OA & RA ... (2000*71 + 4000*95) /6000 = 87
Having your line drawn between the two state points, you could project up from the bottom the 87 and see where it intercepts the line between the two, then project that to the saturation line to get your enthalpy.
You have you mixed db temp same as above. You have one of your humidity ratios, get the second by projecting the point (71/50%) it to the right. You can use these two humidity ratio and the same lever rule / weighted average to get (2000*.012 + 4000*.0081) /6000 = .0094
Now that you have your humidity ratio, you could really use the formula hmix = Hdry air + W*Hg.... H dry air from the table in ASHRAE at 87 and Hg is from the steam table at 87 degrees saturated vapor. Should look something like this. ..20.905+.0094*1099.11 ~ 31.2 a little on the light side, but straight out of the ashrae book
Also, one other formula for arriving at the enthalpy of the air is (.24*T) + W(1061+.444t)
I've attached a cheat sheet that might help as well. Check it for yourself thought. I don't want to be responsible for causing someone to miss a problem.
Posted 10 April 2012 - 03:04 PM
Posted 10 April 2012 - 03:06 PM
Posted 10 April 2012 - 03:21 PM
Posted 10 April 2012 - 03:37 PM
Tenths of degrees marks off 35.1, 35.2, or 35.3. So if you're looking for the enthalpy of 60.5 deg. F wb, the answer is 26.8 BTU/lb
Posted 10 April 2012 - 04:49 PM
Posted 10 April 2012 - 05:10 PM
Posted 10 April 2012 - 05:34 PM
Now, I seem to remember Mr. Lindeburg stating in the MERM that the Tmix equation that Krak references can be used for anything that changes linearly with temperature. I'm not saying that enthalpy is one of those items, but there are a few he mentions in the paragraph just above or below the equation. Likewise, there are a few things he states it can't be used for. And I don't remember what those are! Enthalpy must be one.
But, thinking in thermo terms, Cp*deltaT is equal to enthalpy, so if Cp doesn't change for a fluid over various temps, then deltaT would be linear to enthalpy, correct? Isn't Cp for air almost always 0.24 Btu/lbm-degF? Does humidity ratio or water vapor in the air screw up being able to use the Tmix equation with enthalpy?
Either way, it's too close to test time for me to switch my process and to be honest, I don't think it matters that I understand why enthalpy can't be used in the equation, so please feel free to completely ignore. Just curious.
Posted 10 April 2012 - 05:43 PM
Posted 10 April 2012 - 06:05 PM
my method for these problems is to:
1) find both points on a psych chart
2) find the ratio of type of air i am looking for / total air
3)measure the distance between the points in cm
4) multiply this distance by the calculated ratio.
5) measure that distance on the chart from the reference point
6) get desired info about that point
Posted 10 April 2012 - 06:19 PM
At 87 degrees, Cp*T = about 20.6 + enthalpy of the vapor. Your exactly right on the dry air side. I think the CpT is mean for a "Ideal gas". Not sure. I'd let someone else chime in to be certain I'm right.
Your also correct about the linear thing the lever rule (weighted average) can be used for all things linear, but not things like relative humidity or wetbulb don't use it for those. I've seen people make the mistake of figuring wetbulb temps this way. It doesn't work quote right.
Posted 10 April 2012 - 06:42 PM
Seriously, here are three examples right now that I'm working on at work.
Given (Not much) 3 direct replacement McQuay Multizone units approximately 15 tons each.
Design conditions - 93db/73wb OA - 75db/62.5wb RA
MZ-1 4000 cfm total - 3,615 cfm RA / 385 cfm OA
MZ-2 4,600 cfm total - 4,125 cfm ra / 475 cfm OA
MZ-3 4,200 cfm total - 2,025 crm RA / 2,175 cfm OA
What is the mixed air condition to the DX coil disreguarding any heat gains from the blower motor?
Posted 10 April 2012 - 06:44 PM
Posted 10 April 2012 - 06:57 PM
1 plot the two state points
2 draw the line between.
3 lever rule the dry bulb temperature (weighted average)
4 project db temp from the bottom of the chart up to where it intersects the line between the two points. (The mixed condition will always lie on that line)
4. Project up to the enthalpy scale.
Easy peasy lemon squeesy.
I will post a couple of my cheat sheets that may help with the long hand method too.
Posted 10 April 2012 - 07:01 PM
http://dl.dropbox.co...blem - Copy.xls
I don't know if it will help anybody at this point, but figured I'd give an alternative to the linric download.
ksprayberry, I'm not sure about the ideal gas law, but it sounds like something I read during my studies. I'll see if I can find it.
krakosky, have you tried to figuring out the enthalpy with the mixture equation using mass instead of mass flow? The equation you pointed out had mass flow instead of volumetric flow, so maybe you have to use the specific volume of the air and calculate the mass flow to get the enthalpy? I could be completely wrong though.
You may be over thinking this too. If you stick with using the mixture equation for dry bulb and then using the psych chart for everything else, you should be good.
Posted 10 April 2012 - 07:05 PM
Posted 10 April 2012 - 07:17 PM
Posted 11 April 2012 - 12:11 AM
Man, I went away to a meeting this afternoon and responses started flying! I think we're all just a tad anxious, eh?
Posted 11 April 2012 - 12:20 AM
Posted 11 April 2012 - 01:02 PM
Posted 11 April 2012 - 02:15 PM
I think the most important thing is to breathe and realize we have studied and should be well prepared for this exam. If the problem looks too complicated, you're probably looking at it the wrong way. It should only take you a couple of equations to solve it.
Now I hope I can use my own advice and not panic
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