Equivalent Metric Sensible Heat Formula

[62hog]

Can anybody out there help me with a metric equivalent to the old tried and true formula of:

Q = 1.08 x CFM x delta T

where:

Q - Heat quantity (BTU/Hr)
CFM - Cubic Feet per Minute
Delta T - Change in temperature - Deg F

Thanks in advance.

I don't know much about statistics, but I do know that if something has a 50-50 chance of going wrong, 9 times out of 10 it will. Author unknown

 

[katmar]

I don't know your formula, but it will depend on the temperature and pressure of the air. It would be easy to derive an equivalent in SI units.

The specific heat of air at 20[°]C is 1.05 kJ/kg.[°]C
At 20[°]C and 1 atm abs the density of air is 1.20 kg/m[sup]3[/sup]

So the equivalent of your formula at 20[°]C and 1 atm abs would be

Q = 1.26 x m[sup]3[/sup]/s x delta T

where:

Q - Heat quantity (kW or kJ/s)
m[sup]3[/sup]/s - Cubic meters per second at 20[°]C and 1 atm abs
Delta T - Change in temperature - Deg C


Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[IRstuff]

1.08 seems to be the product of the density of air at RT and the specific heat at RT in English units, but the units are still a bit crazy (BTU/hr)*min/(ft^3*ºF)

TTFN

FAQ731-376

 

[AbbyNormal]

its standard density, specific heat, and minutes to hours

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[62hog]

Katmar,

Is m^3/sec the preferred HVAC airflow units in metric?

During my searches on the internet and in ASHRAE the units I saw were liters/sec.

Any assistance on preferred units for all metric HVAC and plumbing/piping flows, pressures and heat values would be apprciated.

Once again, Thanks in advance.

I don't know much about statistics, but I do know that if something has a 50-50 chance of going wrong, 9 times out of 10 it will. Author unknown

 

[IRstuff]

Shouldn't really matter. Instead of using an untraceable constant with no units attached, you should run the full equation with the correct units and constants, which will then accommodate whatever flow units you choose.

Programs like Mathcad or TKSolver can handle these unit conversions transparently. For the problem stated, the exact same equation can be used with any of the terms applied in a different unit system, e.g., density in lbs/in^3, specific heat in BTU/gm-ºF, and flow in cubic furlongs/nanosecond, if desired.



TTFN

FAQ731-376

 

[ChrisConley]

We use litres/sec for air flow, litres/minute for water flow, kilowatts for heat/cooling, milimeters for pipe (soft convert 100mm = 4") on all of our drawings.

Actually though, we do all of our calculations in imperial units then convert. The contractor's get our drawings, convert to imperial then bid. As a result we try to choose metric units that are 'close' to the imperial...

 

[AbbyNormal]

metric is a farce in Canada until they make metric sized sheet rock

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[ChrisConley]

Try ordering a 150mm pipe.

To be fair to Canada, we do use kilometers, Celsius and litres... Just not in construction.

 

[KiwiMace]

Just a slight correction - typo by Katmar:

Specific heat of Air is generally accepted to be 1.005 kJ/kgK (another 0)

Which makes the formula:
Q = 1.206 x m3/s x dT

Cheers.

 

[AbbyNormal]

I meant construction Chris

need a real metric stud spacing to match up with the size of a sheet is all.

I only saw metric on gov/public jobs

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[katmar]

CinciMace, thanks for the correction. I read the Cp off an old nomograph (in Btu/lb.F !) and converted it. Your value, and therefore your overall formula, is more accurate than mine.
regards
Harvey

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com