NCEES Morning # 134-136

[patioshep]

#134 comes right out MERM 38-5 and equation for the T mix (Dry Bulb). But how do you use the Psych chart to find the T-dry bulb mix if you dont have the equation? I realize you plot the T dry bulb and the RH that's given, and draw a line to connect the two points, but how do you know where it crosses to get the t mix? Also, #135, is there an equation to calculate T wet bulb ? Can't find it but the solution magically calculates it. The solution also uses the Psych chart to find it as an alternative solution. HVAC is a pain.

thanks

 

[mechdaddy]

I would appreciate an explanation too. I am not good with these charts, and if anyone had almost a "cheat-sheet" of how to use the charts in an efficient way, I would be grateful.

 

[M.E. Nebraska]

#134 comes right out MERM 38-5 and equation for the T mix (Dry Bulb). But how do you use the Psych chart to find the T-dry bulb mix if you dont have the equation? I realize you plot the T dry bulb and the RH that's given, and draw a line to connect the two points, but how do you know where it crosses to get the t mix? Also, #135, is there an equation to calculate T wet bulb ? Can't find it but the solution magically calculates it. The solution also uses the Psych chart to find it as an alternative solution. HVAC is a pain. thanks

The only way I came up with this answer was using Eg. 38-21 for T mixture (dry bulb). I then followed this T mixture dry buld temperature up to the drawn line that is connecting your two initial points (T mixture dry bulb should be inbetween your initial points). This will now be your T Mixture point. Using this T mixture point location on your psych chart you can find everything else you need to know (T wet bulb, RH, Humidty ratio, etc.)

Make sure you are using the mixture streams of outside air and conditioned air. Instead of air that enters air conditioning system.

I hope this helps. I am glad I passed in April and don't have to take it this Fall. Good Luck. Keep working practice problems.

 

[patioshep]

ok.. plot the points 58 F & 90% RH , and 95F & 47% RH , draw a diagonal line to connect the points. Now, draw a vertical for T dry mix of 64 F (that we calculated from the equation) that intersects this line. However, how are you reading a T wet of 60 F from this diagram -which is way at the top right of the Psych chart ? Are you assuming a cubic foot of lb of air ? It looks it would be about 27 F from the diagram we drew?

Thanks.

(Merm says that if any 2 points are known, all other parameters can be read directly from the psch chart. Trying to find a good example problem to see how it works...)

 

[M.E. Nebraska]

ok.. plot the points 58 F & 90% RH , and 95F & 47% RH , draw a diagonal line to connect the points. Now, draw a vertical for T dry mix of 64 F (that we calculated from the equation) that intersects this line. However, how are you reading a T wet of 60 F from this diagram -which is way at the top right of the Psych chart ? Are you assuming a cubic foot of lb of air ? It looks it would be about 27 F from the diagram we drew?
Thanks.

(Merm says that if any 2 points are known, all other parameters can be read directly from the psch chart. Trying to find a good example problem to see how it works...)

I believe your are reading enthalpy instead of wet bulb temperature. 27 Btu/lb of Dry Air is approximately 60 F Wet Bulb.

I don't think I can answer your second question. Looking at MERM 38.9 the Lever Rule is what I became comfortable with. I am sure there are many different ways to solve this problem. You just have to decide what works best for you and stick with it. If you try to figure out every different possible way to solve it, you may be making it more difficult then it needs to be.

MERM 38.5 states that if you have any two parameters (dry bulb, wet bulb, R.H) then you can find the remaining parameters of that one point. I think you have confused parameters and a state point.

 

[patioshep]

You were right ! I was reading enthalpy instead of T wet. Thanks .

That freaking chart is hard to read.. the lines are so small. I guess blowing it up is a good start... HVAC is my weak subject...really need to get up to speed.

Did you find the morning exam divided up pretty evenly between the 3 areas (MD, Termo/Fuilds, HVAC) or was one area hit harder than the other ?

Thanks

 

[M.E. Nebraska]

It seemed like it was a complete mix. More than likely it will change from test to test. But I will say, I was glad I went through the MERM completely except for Chapters 60-68. (I just skimmed these chapters and practice problems.) I took the thermo/fluids afternoon section so I spent extra time in this area. But I was suprised on the afternoon section how much HVAC and MD showed up.

It is hard to remember exactly what was on the test. After you take the exam you try to forget the problems or else you will drive yourself crazy trying to remember which ones you got right and wrong during your 10-16 week wait for the results.