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value for cp Specific heat of air

asked 2021-11-26 10:29:47 -0600

mdengusiak's avatar

updated 2022-02-16 13:59:40 -0600

What is the value for cp Specific heat of the air used in energy plus?

We normally use for air in Europe cp= 1.0005 kJ/kgK but I guess might be different on other side :)

ps. I think there is an error in Units for cp (J/kg)

also, I found cp= 1.0005 kJ/kgK value in some parts of documentation for cp, but not sure if this is unified approach?

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@mdengusiak great catch on $c_p$ using the wrong units ($J/kg$ should be $J/kg-K$) in the EnergyPlus documentation. You can mention this in the ongoing documentation issues list on Github where the EnergyPlus source code is hosted.

Aaron Boranian's avatar Aaron Boranian  ( 2021-12-01 10:00:25 -0600 )edit

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answered 2021-12-01 14:13:58 -0600

In general, EnergyPlus uses variable specific heat of air. The psychrometric function PsyCpAirFnW calculates specific heat as a function of humidity ratio.

But there are a few instances of 1005. lurking in the code, in the ecoroof module (as noted above), the Mundt room air model, baseboard heat standard rating calculations, and the VRF DX controls.

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this is great! especially link to the source in code thanks! so when I want to calculate manually infiltration based on the mass flow Q = m * cp * dt [W] Q = Zone Infiltration Mass Flow Rate * cp * (Zone Outdoor Air Drybulb Temperature - Zone Air Temperature) I should recalculate cp for each hour from Zone Outdoor Air Drybulb Temperature and Site Outdoor Air Humidity Ratio ? or a should use different variable? I am referring to

mdengusiak's avatar mdengusiak  ( 2021-12-01 14:22:40 -0600 )edit

See answer here.

MJWitte's avatar MJWitte  ( 2021-12-02 10:00:39 -0600 )edit

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Asked: 2021-11-26 10:29:47 -0600

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Last updated: Dec 01 '21