Sensible Heat Load Calculation issue 1

[whisperinghill]

I have a room that is supplied by 60 F air at 570 CFM and it is maintaning 74.5 F. With no people or lights on in the room.

If I calculate the Sensible Heat at CFM * 1.08 * Delta T or 570*1.08*14.5= 8926 BTU/Hr

Now if I want to convert this to watts to determine the room load. I divide the BTU/Hr by 3.415 equaling 2614 watts

Now if I divide the watts by 120v, it provides 22 amps. Actual circuits amps are 26.

But if I reverse the calclations to determine CFM required at 26 amps, I get 680 CFM required to maintain the current temperature of 74.5 F.

The room is surrounded by other conditioned spaces, no more than 1 deg cooler and the room has insulated.

Flow hood and VAV Box info shows 570 CFM and tempertures and loading has been verified. What am I not taking into account?

Thanks

 

[MintJulep]

The room is surrounded by other conditioned spaces, no more than 1 deg cooler and the room has insulated.

Right there.

 

[DrRTU]

Your logic is correct, but you are off on your application. You will need to improve accuracy by increasing the significant figures. You need multiple precession space sensors inside the room to ensure the bulk space is 74.50 and the same holds for the duct sensors. Look at the output if you change the temp 0.5 degrees. Flow rate from a flow cross will have 5 to 10% accuracy and you need to compensate for air pressure. You can get the flow hood accuracy from the manufacture. The 1.08 conversion is from specific heat of air (.24 btu per pound per degree F). It takes .24 Btu of heat to change the temperature of one pound of air by one degree Fahrenheit. The 1.08 contains the specific density of air (.075 pounds per cubic foot) extent these numbers with more decimal places from known tables. You will need to correct for humidity & density (elevation). Run the numbers with a spread sheet that include the accuracy. Sometimes you have to say its close enough.

 

[whisperinghill]

Thanks DrRTU, can you provide me a link to a good spreadheet? "Run the numbers with a spread sheet that include the accuracy."

And I believe you are correct regarding:" Sometimes you have to say its close enough."

It is always interesting to ensure verification of your calculations.

Thanks for the reply

 

[DrRTU]

I do not have a link, just run the math with a calculator or sheet with max and min and that should paint the box.

 

[ChrisConley]

Unless the equipment you are using is 100% efficient, I would assume that some/most of the disparity is efficiency losses.

 

[imok2]

Are you sure you are only removing sensible heat. what is the room RH

 

[whisperinghill]

Relative Humidity is 45%. Can you give me a hint on adjustment?

 

[whisperinghill]

I have created an excel spreadsheet that attempts to provide a cooling needs analysis.

Can some of you test drive it and make some recommendations?

Is there away to attach files?