basic BTU questions 1

[stevegraber1]

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

 

[TBP]

You need to flip the flowrate (GPM) in lbs, normally per hour. For US gallons, for say, 10 GPM, then:

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.

 

[HVAC68]

Q = m * Cp * Delta T

Should I say more ? :0)

HVAC68

 

[25362]

For the second question:
Convert the 5.5 kW to Btu/h.
Convert the 600 gallons to mass, lb.
Assuming the specific heat of water C_p 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.

 

[HVAC68]

1 KW = 3415 Btu/hr

For water,

1 Gallon = 3.785litres = 3.785kg = 3.785 * 2.2lb = 8.327lb

i.e. 1 gallon = 8.327 lb

HVAC68

 

[GenB]

You will need a few days to heat your water if no lost of heat considerably
I have a 120GL insulated tk on 5kw and it takes 12-20 Hrs to rase 50 deg F.
GB

 

[quark]

generalblr,

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

 

[25362]

BTW, have you all noticed the small differences in the Btu/h values ? The most common are the International and the Thermochemical Btu values. Then we may find the medium (or average) Btu resulting from the energy spent to heat one pound of water from 32 to 212 deg F divided by 180, etc., etc.