Having trouble using Psychometric chart....

[christyu]

Hey guys, I'm actually studying vapor compression refrigeration cycle and would like you guys' help in understanding Psychometric chart. There are no calculations involved but need reasoning and guide.

Given (temperatures)
dry bulb entering condenser: 69oF
dry bulb leaving condenser: 75oF
Wet bulb entering evaporator: 59oF
wet bulb leaving evaporator: 58oF
dry bulb leaving evaporator: 63oF
wet bulb leaving condenser: 61oF

I was asked to use psychometric chart to find All air enthalpies.

I know that in order to find enthalpy you just need to match up dry bulb and wet bulb on the chart and see which value of enthalpy it lines up to. I hope to understand how things work with these problems I got.

1. I asked someone online for help the other day and he used condenser and wet bulb entering evaporator for finding enthalpy of condenser(Entering), and for finding enthalpy of condenser (exiting), he used dry bulb leaving evaporator and wet bulb leaving evaporator.
why did he choose these 4 values and left the other ones? What about evaporator?(entering,exiting)?2. When he used psychometric chart to find enthalpy of condenser (Entering), he got the value as 30 Btu/lbm. When I tried it myself I got like a little bit over 25 btu/lbm and even checked it out with online psychometric chart calculator which provided similar value. did he do it wrong?

link to psychrometric chart (I checked it with my chart just in case if there's any difference):
http://web.uconn.edu/poultry/NE-127/NewFiles/psychrometric_inset.html

link to online psychrometric calculator (change unit to IP since it's Fahrenheit): http://www.sugartech.co.za/psychro/

3. One thing I would like to know is that, does value of dry bulb matter? I mean in the end, wetbulb already lines up with enthalpy side so as long as I have the value of saturation temperature/wet bulb temperature, dry bulb temperature can be neglected? or am I misunderstanding something...

 

[nilesh_pat]

Thanks in advance for the help!1. He chose these 4 values because they are the temperatures that you have given in the problem. The evaporator enthalpy can be found by using the dry bulb entering condenser and wet bulb leaving condenser temperatures.2. It is possible that he did it wrong. The psychrometric chart is an approximation, so there may be slight variations between different charts or calculations.3. The dry bulb temperature does matter. The psychrometric chart is used to find the enthalpy of the air, which is a combination of both the dry bulb and wet bulb temperatures. The dry bulb temperature can not be neglected, as it is needed to calculate the enthalpy.