Heat loss

MJC

I'm doing a heat loss calculation for a small (325sf) room in a water treatment plant. About 40 ft. of 12" diameter piping carrying cold (40 degree) water will run through the room. There is concern that this pipe will act as a "massive heat sink" resulting in a considerable heat load on the room. Anyone have any idea how to determine how much of a heating load this pipe will have?

hydronicsmike

Is it not possible....

...to insulate the pipe sufficiently?

MJC

Heat loss

Insulation is certainly an option but I would still like to know how much of an impact the uninsulated pipe will have on the heating load.

Mike T., Swampeast MO

http://epb1.lbl.gov/thermal/chapter2.pdf

Is a VERY good source of info regarding both panel cooling and heating!

Will try to come up with a guestimate based on radiative+convective "input" of chilled ceiling where:

"Input" (in watts/sq.meter) = 8.92 * (airtemp-coldsurfacetemp)^1.1

TEMPS IN CELSIUS!

Assuming 21° air and 4° surface we have 17 ^ 1.1 = 22.6

22.6 * 8.92 = 201.6 watts/sq.meter

201.6 watts/sq.meter = 688.5 btu/hr/sq.meter

688.5 btu/hr/sq.meter = 64 btu/hr/sqft for a 17° celsius difference in temperature between cold object and air.

Now to Fahrenheit:

64 btu/hr/sqft/30° F
--------------------------------------------

Believe I got all those conversions right but you might want to verify. Thanks Dan for the link to the little "convert.exe" program!

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If pipe is truly 12" in diameter it has about 3.14 square feet per lineal foot so each foot of pipe would be absorbing about 201 btu/hr (30°F temp difference).


So, 40' of 12" pipe would absorb about 8,040 btu/hr (30°F temp difference).

As you suspected this seems HIGHLY significant.

-------------------------------------------------------

Of course this data is only a ROUGH estimate and THEN for a cooled ceiling. My [guess!] as to compensation for a pipe near the floor?

If no forced convection directed towards the pipe: about 0.8 times the above value as I believe natural convection wouldn't be as much as with a chilled ceiling.

If forced convection directed towards the pipe: about 1.2 times the above value.

If floor is radiantly heated: about 1.3 times the above value. SORRY! About "par" with the above value--was thinking of radiant floor WITH forced convection!

ALL assume that the temp of the pipe will be above the dewpoint of the air during the heating season. If below the dewpoint you start extracting latent heat and it gets complicated beyond my ability to even guestimate.

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For different air/surface temperature differences RECALCULATE! Remember--temps must be in celsius in the equation:

"Input" (in watts/sq.meter) = 8.92 * (airtemp-coldsurfacetemp)^1.1

Then for "U.S. friendly" comparison:

First convert watts/sq.meter to btu/hr

Then convert sq.meters to sq.feet.

Mike T., Swampeast MO

ABOVE IS WRONG FOR NOW

Temp is in celcius.

Will correct momentarily!

Mike T., Swampeast MO

Should be fixed now

but I would SINCERELY verify the math!!!! LOTS of conversions going on there and easy to screw up!!!!

Mike T., Swampeast MO

Found another math error

> http://epb1.lbl.gov/thermal/chapter2.pdf

>

> Is a

> VERY good source of info regarding both panel

> cooling and heating!

>

> Will try to come up with

> a guestimate based on radiative+convective

> "input" of chilled ceiling where:

>

> "Input" (in

> watts/sq.meter) = 8.92 *

> (airtemp-coldsurfacetemp)^1.1

>

> TEMPS IN

> CELSIUS!

>

> Assuming 21° air and 4° surface we

> have 17 ^ 1.1 = 22.6

>

> 22.6 * 8.92 = 201.6

> watts/sq.meter

>

> 201.6 watts/sq.meter = 688.5

> btu/hr/sq.meter

>

> 688.5 btu/hr/sq.meter = 64

> btu/hr/sqft for a 17° celsius difference in

> temperature between cold object and air.

>

> 64/17

> = 3.8 btu/hr/sqft/1° celsius temp

> difference

>

> Now to Fahrenheit:

>

> 3.8

> btu/hr/sqft/1° celsius = 2.1 btu/hr/sqft/1°

> Fahrenheit difference so...

>

> 2.1 * 30 = 63

> btu/hr/sqft/30° Fahrenheit difference (assuming

> 70° air, 40° pipe

> temp)

>

> -----------------------------------------

> ---

>

> Believe I got all those conversions right

> but you might want to verify. Thanks Dan for the

> link to the little "convert.exe"

> program!

>

> --------------------------------------

> ------

>

> If pipe is truly 12" in diameter it has

> about 3.14 square feet per lineal foot so each

> foot of pipe would be absorbing about 198 btu/hr

> (30° temp difference) or 6.6 btu/hr per 1° temp

> difference.

>

> So, 40' of 12" pipe would absorb

> about 7,920 btu/hr (30° temp difference) or 264

> btu/hr per 1° temp difference.

>

> As you

> suspected this seems HIGHLY

> significant.

>

> ----------------------------------

> ---------------------

>

> Of course this data is

> only a ROUGH estimate and THEN for a cooled

> ceiling. My [guess!] as to compensation for a

> pipe near the floor?

>

> If no forced convection

> directed towards the pipe: about 0.8 times the

> above value as I believe natural convection

> wouldn't be as much as with a chilled

> ceiling.

>

> If forced convection directed towards

> the pipe: about 1.2 times the above value.

>

> If

> floor is radiantly heated: about 1.3 times the

> above value. SORRY! About "par" with the above

> value--was thinking of radiant floor WITH forced

> convection!

>

> ALL assume that the temp of the

> pipe will be above the dewpoint of the air during

> the heating season. If below the dewpoint you

> start extracting latent heat and it gets

> complicated beyond my ability to even guestimate.



Hadn't removed all of the celsius numbers--duh!

Mike T., Swampeast MO

Found another math error

Hadn't removed all of the celsius numbers--duh!

Also the exponent in the output equation made the output per 1° changes invalid! I've removed reference.

MJC

Mike, thanks for your help. I looked at it a little more simplisticly and used the basic heat loss method. 40ft. of 12" pipe has an area of about 134sf (assumes pipe o.d. of 12.825"). Assuming an R value of .68 for the air film and a 30 degree delta T, I get a heat loss of about 6,000 BTUH which is in the ball park of your calculation. Thanks again.

Mike T., Swampeast MO

No problem

If using forced air for heating though keep air from being blown over or pulled across as much as possible--no more air film for "insulation"!

If radiant floor the underside of the pipe will be drawing lots of heat from the floor!

If left uninsulated a light glossy enamel paint should cut down on radiant transfer a bit--at least till it gets dirty

MJC

Heat loss

Thanks, Mike, I'll keep that in mind. We'll probably insulate the piping. It'll cut down on the heat loss in winter and condensation on the piping in the summer.