basic BTU questions
basic BTU questions
(OP)
ok - sorry this is so basic. i haven't done heat transfer calcs in some 30 years. my old books are greek to me. online calulators are too simplistic and usually based on HVAC of homes.
can't i calc the BTU's of a heater by knowing the flowrate (water), and delta temp of the water? the inlet water & outlet water? and can't i calc how long a volume of water will take to reach a selected temp?
i'm reviewing a heater system. water heater, electric heater, 5500 watts. the water vessel is 600 gallons. how many BTU's am i really putting into the water, and how long will the water reach 100 degrees F?
i seem to recall this was a basic calc, but forgot long long ago.
see ya
steve
steve.graber@sundancespas.com
can't i calc the BTU's of a heater by knowing the flowrate (water), and delta temp of the water? the inlet water & outlet water? and can't i calc how long a volume of water will take to reach a selected temp?
i'm reviewing a heater system. water heater, electric heater, 5500 watts. the water vessel is 600 gallons. how many BTU's am i really putting into the water, and how long will the water reach 100 degrees F?
i seem to recall this was a basic calc, but forgot long long ago.
see ya
steve
steve.graber@sundancespas.com





RE: basic BTU questions
10 gallons per minute X 8.33 lbs/gal X 60 minutes per hour.
8.33 X 60 give you very close to 500.
The shortcut method is GPM X 500, so 10 X 500 = 5,000 lb/hr
One BTU to heat 1 lb of water 1*F, so say your delta-t is 100*, then:
10 X 500 X 100 = 500,000 BTU/hr.
I'd talk to the electric water heater manufacturer for some advice on recovery times for their units, and what they'd recommend for heating water for your particular application.
RE: basic BTU questions
Should I say more ? :0)
HVAC68
RE: basic BTU questions
Convert the 5.5 kW to Btu/h.
Convert the 600 gallons to mass, lb.
Assuming the specific heat of water Cp stays at 1 Btu/(lb*oF), and that no heat is lost to the surroundings, use HVAC68's equation to estimate the heat up of the mass in the vessel in degrees F per hour.
RE: basic BTU questions
For water,
1 Gallon = 3.785litres = 3.785kg = 3.785 * 2.2lb = 8.327lb
i.e. 1 gallon = 8.327 lb
HVAC68
RE: basic BTU questions
I have a 120GL insulated tk on 5kw and it takes 12-20 Hrs to rase 50 deg F.
GB
RE: basic BTU questions
There is something wrong with your system if it takes 12-20hrs for heating up. It should not be morethan 3hrs theoretically.
In OP's case,
Specific heat = 1btu/lbF
Density of water = 62.4lb/cu.ft
600gallons = 80.2cu.ft
5kW = 17072.1btu/hr
So total heat required for a temperature rise of 50F will be
80.2cu.ftx62.4lb/cu.ftx1btu/lbFx50F = 250224btu
So time taken will be 250224/17072.1 = 14.65hrs or 14hr39min
With all other conditions same, you have 1/5th capacity, so time taken should be around 3hrs for perfectly insulated tanks.
Regards,
I wish us all A Happy and Prosperous New Year
RE: basic BTU questions