Question-and-Answer Resource for the Building Energy Modeling Community
Get started with the Help page
Ask Your Question
1

value for cp Specific heat of air

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

mdengusiak gravatar image

updated 2021-12-01 03:01:31 -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)
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...

edit retag flag offensive close merge delete

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 gravatar image Aaron Boranian  ( 2021-12-01 10:00:25 -0600 )edit

1 Answer

Sort by ยป oldest newest most voted
1

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.

edit flag offensive delete link more

Comments

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...

mdengusiak gravatar image mdengusiak  ( 2021-12-01 14:22:40 -0600 )edit

See answer here.

MJWitte gravatar image MJWitte  ( 2021-12-02 10:00:39 -0600 )edit

Your Answer

Please start posting anonymously - your entry will be published after you log in or create a new account.

Add Answer

Training Workshops

Question Tools

1 follower

Stats

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

Seen: 37 times

Last updated: Dec 01 '21