Storage Tanks

[indoorcomfrtsl]

I have 3- 3300 gallon domestic hot water storage tanks that i am trying to heat and maintain temps of 140. I am trying to estimate how many btu's it will need. I am using 3 steam boilers rated at 4 mbtu/hr. What formula would I use to determine what I need to heat these tanks?

[chillerguy81]

You'll need to have an idea of the flow rate on the domestic side (hot water consumption). Hopefully this formula helps:

hout = q ρ dt cp (1)

where

hout = heating capacity, output (Btu/h)

q = flow rate (gph)

ρ = 8.34 - density of water (lbs/gal)

dt = temperature rise (*F)

cp = 1.0 - specific heat capacity of water (Btu/lb*F)

[JStar]

Don't you need the flow rate to determine the actual tank size, and the btuh load of the storage tank is based on the water mass and temperature difference only?

[rich pickering]

Actually, you need to know the cap[acity of the heat exchanger being used.

[JStar]

I thought storage tanks didn't have heat exchangers, but indirect water heaters did. But then I guess you would need a heat exchanger with steam, right?

[Moose]

To add to Rich's comment, also need the type of heat exchanger.

[chillerguy81]

I was thinking the OP was talking about a steam to hot water heat exchanger but I guess we'll have to find out for sure from him. I'm really not sure how an engineer would size a hx tank for a job becasue I'm not sure how they estimate the hot water usage. I'll see if I can ask one of our engineers and find out.

[flange]

me thinks you are headed down a tricky path. Do you want just btus for the storage tanks, or for demand as well. For storage tanks only, it takes one btu to raise one pound of water one degree. so you have 9900 gallons of water at 8.33 pounds per gallon for a total of 82467 pounds of water. you then want to heat it from ambient to 140. what is ambient? lets say 50, so to go from 50 to 140 is 90 degree rise. so 90 x 82467=7,422,030btus without thinking about demand, standby losses, boiler efficiency, or other mechanical disadvantages such as fouled heat exchangers or undersized pumps and /or piping. your boiler should get you there, but you may need less depending upon time and demand.

[pecmsg]

OOPS

[indoorcomfrtsl]

The heat exchanger is a steam tube bundle,do not know the capacity ,yet. I installed tees on the pumps to get pressures. I did find out engineers design stogage tanks with the assumption that at 800 am all 200 suites will use hot water for showers,drinking etc.
I guess my question is that is there some way to determine if the steam boilers we are running will take care of 3 - 3300 gallon storage tanks. By the way I heard armstrong has a chart for this but have not seen it.

[chillerguy81]

I talked to our engineer yesterday and he said that for the original design, they would add up the flow rate for all the hot water using appliances, and the useage is calculated through software to account for what type of building it is. Which I'm sure could be done manually or just figure the worst case scenairo.

To answer your last question, yes the btu requirement can be calculated but you need to know the flow rate through the heat exchanger. As flange stated earlier it's easy to figure out the btu's required to heat the 9900 gal of water if its sitting still, but it will be moving and changing over, so that changes the formula.

[flange]

typically they will use a diversity factor of up to 75 percent calling at once. very few size for 100 percent demand. they also typically size for "fouling factor" of both the exchanger and the associated piping. systems dont stay new for long.