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Calculation for converting GPM to BTU
- Thread starter techsup01
- Start date
Not sure what your application is, so I will take a guess. The following formula is used in the HVAC industry and is accurate enough for flow requirements in a building heating system using water for heat transfer.
q BTU/hr
deltaT Temperature drop of water in degF
Qw Quantity of water in Gallons per Minute
q=500*Qw*deltaT
q BTU/hr
deltaT Temperature drop of water in degF
Qw Quantity of water in Gallons per Minute
q=500*Qw*deltaT
there is no formula to convert gpm to btu/hr as the two units of measurement are not equivalent. gpm - volumetric flow rate and btu/hr - a rate of energy per hour.
further description or explanation of the problem is needed to provide a succinct response.
-pmover
further description or explanation of the problem is needed to provide a succinct response.
-pmover
TechSup01
As you can tell from pmover's correct response, it is impossible with the information you gave, and easy with more information.
The BTU/gal factor for water is zero, so multiplying that factor times any volume flow rate yeilds zero BTU/hr. On the other hand, deisel has several thousand BTU/gal and you can get meaningful answers.
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
As you can tell from pmover's correct response, it is impossible with the information you gave, and easy with more information.
The BTU/gal factor for water is zero, so multiplying that factor times any volume flow rate yeilds zero BTU/hr. On the other hand, deisel has several thousand BTU/gal and you can get meaningful answers.
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
Montemayor
Chemical
The "formula":
q=500*Qw*deltaT
is a misnomer. It is the equation for the transfer of sensible heat via the use of heat capacity, as 25362 has noted.
It is not a conversion "formula" for converting liquid flow rate to energy flow rate. One has exactly nothing to do with the other. There is a major problem in the logic and communication of techsup01's question.
If he wants to convert liquid flowrate into energy flowrate, he needs a miracle, not a formula. If he wants help to find the sensible heat transfer from/to a liquid flowrate, he has to be able to state or express it correctly - otherwise, we don't know what he needs.
You guys are a tough lot.
Random House Webster’s Dictionary defines a formula as, “any fixed or conventional method or approach.” Which is exactly what q=500*Qw*deltaT is in the HVAC world. I have never seen this formula written as a dimensionally homogeneous equation within the HVAC engineering groups I have worked with. I have worked for several consulting engineering firms and the standard convention for HVAC is q=500*GPM*deltaT
There is a lot more missing than what 25362 has noted: Cp of 1 BTU/(lb*degF)
Also missing is the conversion of mass to volume and minutes to hours.
I put my psychic hat on and tried to guess at what techsup01 was looking for. Note that techsup01 never requested a unit conversion for GPM to BTU/hr. He asked for a formula to convert GPM to BTU/hr which is what I posted. I said that I had to guess at what techsup01 wanted. I prefaced the formula describing its use in the HVAC industry. I have never seen the dimensionally homogeneous equation for water heat transfer written out by the HVAC guys as follows:
q=1BTU/(lb*degF)*64.5*lb/ft^3*Qw*gal/min*60*min/hr*0.134*ft^3/gal*deltaT*degF
Where
deltaT is the change in temperature of the fluid in degF
Qw is the flow in gallons per minute
q is the heat tranfer in BTU/hr
Today I did see where techsup01 posted the same question in the HVAC forum. So techsup01, did I guess correctly?
Random House Webster’s Dictionary defines a formula as, “any fixed or conventional method or approach.” Which is exactly what q=500*Qw*deltaT is in the HVAC world. I have never seen this formula written as a dimensionally homogeneous equation within the HVAC engineering groups I have worked with. I have worked for several consulting engineering firms and the standard convention for HVAC is q=500*GPM*deltaT
There is a lot more missing than what 25362 has noted: Cp of 1 BTU/(lb*degF)
Also missing is the conversion of mass to volume and minutes to hours.
I put my psychic hat on and tried to guess at what techsup01 was looking for. Note that techsup01 never requested a unit conversion for GPM to BTU/hr. He asked for a formula to convert GPM to BTU/hr which is what I posted. I said that I had to guess at what techsup01 wanted. I prefaced the formula describing its use in the HVAC industry. I have never seen the dimensionally homogeneous equation for water heat transfer written out by the HVAC guys as follows:
q=1BTU/(lb*degF)*64.5*lb/ft^3*Qw*gal/min*60*min/hr*0.134*ft^3/gal*deltaT*degF
Where
deltaT is the change in temperature of the fluid in degF
Qw is the flow in gallons per minute
q is the heat tranfer in BTU/hr
Today I did see where techsup01 posted the same question in the HVAC forum. So techsup01, did I guess correctly?
David and Art,
thanks for "rising above the occasion" and attempting to clarify the problem or at least attempt to solicit further information from techsup01.
CRG, no, we really are not a tough lot. we are just applying fundamental principles we all learned or practical advise to requests and try to avoid guesswork.
also, techsup01, you should only post your question in one forum, hopefully the forum suited to the matter at hand. thanks for your cooperation.
good luck!
-pmover
thanks for "rising above the occasion" and attempting to clarify the problem or at least attempt to solicit further information from techsup01.
CRG, no, we really are not a tough lot. we are just applying fundamental principles we all learned or practical advise to requests and try to avoid guesswork.
also, techsup01, you should only post your question in one forum, hopefully the forum suited to the matter at hand. thanks for your cooperation.
good luck!
-pmover
- Thread starter
- #13
Wow, what a thread. Actual energy flow is BTU/hr. This takes the form of Q = q*h*dT.
q = gpm as a flow rate.
h = enthalpy of the fluid moving the energy, or its capacity to move energy. h is temperature and fluid type dependent and as such there are many tables for determining the h value.
and delta T describes the two end states.
I hope the calculation doesn't need to be accurate like for sizing of components. I presume the 500 is the "nominal" value used by the HVAC industry for simplification.
Bill
Bill Swain
Ultra Electronics Precison Air Systems
Technical Coordination Manager-US Programs
swainw@asme.org
q = gpm as a flow rate.
h = enthalpy of the fluid moving the energy, or its capacity to move energy. h is temperature and fluid type dependent and as such there are many tables for determining the h value.
and delta T describes the two end states.
I hope the calculation doesn't need to be accurate like for sizing of components. I presume the 500 is the "nominal" value used by the HVAC industry for simplification.
Bill
Bill Swain
Ultra Electronics Precison Air Systems
Technical Coordination Manager-US Programs
swainw@asme.org
That doesn't make sense. Enthalphy has units of BTU/lb (or whatever system of units you want to use). If q is gpm, then you need a conversion factor in there to convert to a mass flow like lb/hr. There's not need for the dT term.
Now, if you replace h (enthalphy) by a heat capacity (BTU per lb per deg F), then you need the dT term.
Enthalphy is simply the energy content of the fluid relative to some basis, I don't understand your statement "enthalpy of the fluid moving the energy, or its capacity to move energy"
Now, if you replace h (enthalphy) by a heat capacity (BTU per lb per deg F), then you need the dT term.
Enthalphy is simply the energy content of the fluid relative to some basis, I don't understand your statement "enthalpy of the fluid moving the energy, or its capacity to move energy"
Sorry,
I was trying to take a comprehensive answer involving enthalpy and put it into what seems to be someone looking for a generic answer of two different fluid flowing terms. What I meant by "ethalpy of the fluid moving the energy, or its capacity to move the energy" was to try to illustrate in words the concept that the fluid has an energy content related to BTU/lbm of moving fluid. Versus, the concept of GPM which is only a volumetric flow rate. The answer given with volumetric flow rate (GPM) * 500 * dT to get to BTU/hr seemed rudimentary as BTU/hr relates to the the amount of BTU/lbm of the liquid and that is temperature dependant.
Regards,
Bill
Bill Swain
Ultra Electronics Precison Air Systems
Technical Manager-USA
swainw@asme.org
I was trying to take a comprehensive answer involving enthalpy and put it into what seems to be someone looking for a generic answer of two different fluid flowing terms. What I meant by "ethalpy of the fluid moving the energy, or its capacity to move the energy" was to try to illustrate in words the concept that the fluid has an energy content related to BTU/lbm of moving fluid. Versus, the concept of GPM which is only a volumetric flow rate. The answer given with volumetric flow rate (GPM) * 500 * dT to get to BTU/hr seemed rudimentary as BTU/hr relates to the the amount of BTU/lbm of the liquid and that is temperature dependant.
Regards,
Bill
Bill Swain
Ultra Electronics Precison Air Systems
Technical Manager-USA
swainw@asme.org
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