BuffaloWings
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Would someone be able to draw me a schematic for this problem to help me understand the solution? I’m stumped.
2019 NCEES Problem #34
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MikeGlass1969
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Does this help?
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BuffaloWings
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Absolutely! Thank you
Audi Driver P.E.
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MikeGlass1969
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OOPS!!!! YEP... Mybad! Didnt change the answer though...
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BuffaloWings
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Just reviewed this problem again and noticed your leaving air temperature is wrong. The solution manual had it as 55F DB / 51.7F WB. I think you forgot to include the outside heating load?
MikeGlass1969
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AHHH... Crap!!!! I totally butchered this problem... Diagram and Solution attached.
I apologize to those that I may have screwed up.
Thanks for the heads up BuffaloWings ...
I apologize to those that I may have screwed up.
Thanks for the heads up BuffaloWings ...
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BuffaloWings
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I think the solution they have does it a little different. They use the outside enthalpy and calculate the outside heating load.
MikeGlass1969
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They are correct. I did not carefully read the problem. They gave room load conditions and I left out the outside air load. Odd that I got the exact answer even though I did it wrong. again. and again.
MikeGlass1969
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BuffaloWings,
I do not believe their solution is correct either. I believe, Outside Air Load is from Outside Conditions to Room Conditions for the Outside CFM. Please correct me if I am wrong. The solution appears to use the Mix Air Enthalpy, instead of the Return Air Enthalpy .
I do not believe their solution is correct either. I believe, Outside Air Load is from Outside Conditions to Room Conditions for the Outside CFM. Please correct me if I am wrong. The solution appears to use the Mix Air Enthalpy, instead of the Return Air Enthalpy .
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Unfortunately the attachment from this post has disappeared. Can someone help me with a visual for this problem please?
Edit: here's what I came up with, some of the NCEES steps are funky
Edit: here's what I came up with, some of the NCEES steps are funky
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Can this problem's second half be solved by using enthalpies and a control volume around the room, rather than considering a control volume around the coil looking at heat loads of the room+the outside air?
Using Control Volume of the Room:
Mass leaving coil*enthalpy leaving coil + LH+SH= mass leaving room*enthalpy leaving the room, solving for the enthalpy leaving the coil to find Twb? LH and SH given, enthalpy leaving the room and mass flow rate leaving the room using the psychrometric chart at 78da,45%rh (specific volume = ~14.2ft^3/lbm, h= ~40). In that way, I have a mass flow rate of 15710lbm/hr, and the mass leaving coil*enthalpy leaving coil = 331874btu/hr. Using the same mass flow rate of 15710 lbm/hr, i got an enthalpy of the air leaving the coil as 21.1 btu/lbm. And then given that and our 55da, used the psychrometric chart again to find the wet bulb of 51, same as the solution guide.
By doing this, you don't have to deal with the outside air, but just the mixture of air at a constant flow rate.
I get the same answer as NCEES solutions, and I don't think i'm making any wrong assumptions.
In the NCEES way, am I correct in that we drawing a control volume around the coil, with a heat removal of the load of the room+the 700cfm from the outdoors, since we know the heat that must be removed 2900cfm (the SH+LH)? Essentially splitting up the mixture into outside heat load and room heat load, and then our final state (leaving the coil) is the combined. Am I understanding that right? I think going about it the way I did takes longer, but is more intuitive for me. I have a power plant/boiler background, and not much HVAC experience, so some of these HVAC topics are a new way of thinking for me.
Using Control Volume of the Room:
Mass leaving coil*enthalpy leaving coil + LH+SH= mass leaving room*enthalpy leaving the room, solving for the enthalpy leaving the coil to find Twb? LH and SH given, enthalpy leaving the room and mass flow rate leaving the room using the psychrometric chart at 78da,45%rh (specific volume = ~14.2ft^3/lbm, h= ~40). In that way, I have a mass flow rate of 15710lbm/hr, and the mass leaving coil*enthalpy leaving coil = 331874btu/hr. Using the same mass flow rate of 15710 lbm/hr, i got an enthalpy of the air leaving the coil as 21.1 btu/lbm. And then given that and our 55da, used the psychrometric chart again to find the wet bulb of 51, same as the solution guide.
By doing this, you don't have to deal with the outside air, but just the mixture of air at a constant flow rate.
I get the same answer as NCEES solutions, and I don't think i'm making any wrong assumptions.
In the NCEES way, am I correct in that we drawing a control volume around the coil, with a heat removal of the load of the room+the 700cfm from the outdoors, since we know the heat that must be removed 2900cfm (the SH+LH)? Essentially splitting up the mixture into outside heat load and room heat load, and then our final state (leaving the coil) is the combined. Am I understanding that right? I think going about it the way I did takes longer, but is more intuitive for me. I have a power plant/boiler background, and not much HVAC experience, so some of these HVAC topics are a new way of thinking for me.
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