SMS HVAC Problem #20 Pressurized Tank??

#1 Seafever

Seafever

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Posted 15 July 2011 - 12:53 PM

I am working thru the SMS HVAC am stuff and do not understand the SMS solution for problem #20. Why does their solution include atmospheric pressure? The tank is pressurized with a bladder at the top and they give a gage pressure of 15psi and there is 2' of water in the tank and they are asking for the Pressure at the tank bottom in psi.

There is a similar example in merm page 15-14 that shows the P as the gage pressure + the hydrostatic pressure. Why does SMS add atm pressure? Am I missing something?

#2 jamiecta

jamiecta

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Posted 13 August 2011 - 12:57 PM

QUOTE (Seafever @ Jul 15 2011, 01:53 PM) <{POST_SNAPBACK}>
I am working thru the SMS HVAC am stuff and do not understand the SMS solution for problem #20. Why does their solution include atmospheric pressure? The tank is pressurized with a bladder at the top and they give a gage pressure of 15psi and there is 2' of water in the tank and they are asking for the Pressure at the tank bottom in psi.

There is a similar example in merm page 15-14 that shows the P as the gage pressure + the hydrostatic pressure. Why does SMS add atm pressure? Am I missing something?

I sort of remember this problem from last weekend and if I remember correctly, they are asking for the pressure at the bottom in psia.

For example, if the tank were open to the atmosphere, the pressure from the fluid alone would be pgh/gc (US). If the available answers were in psig, you would be done. If they were asking for the answer in psia you would have to add local atmospheric pressure to pgh/gc to find the total or absolute pressure psia.

Same concept here, except the water is "open" to a pressurized bladder instead of the atmosphere. So "Patm" in this problem is now the pressure of the bladder above the water. The important part to note however, is that they give the bladder pressure in psg and the question is asking for the pressure in psia. So, the total pressure at the bottom of the tank, in psia, is the bladder gage pressure adjusted by the local atmospheric pressure to get a psia value and then added to the pgh/gc value.

Someone else feel free to correct me if I am wrong.

Edited by jamiecta, 13 August 2011 - 12:58 PM.

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