value for cp Specific heat of air

asked 3 years ago

mdengusiak
133 ●5

updated 3 years ago

__AmirRoth__

4446 ●5 ●17 http://bleedinggreenna...

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)
1. https://bigladdersoftware.com/epx/doc...

also, I found cp= 1.0005 kJ/kgK value in some parts of documentation for cp, but not sure if this is unified approach?
https://bigladdersoftware.com/epx/doc...

Comments

@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 ( 3 years ago )

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answered 3 years ago

MJWitte

9790 ●6 ●11 http://www.gard.com/

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.

link

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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 https://unmethours.com/question/65256...