Water temperature increase in tank with steam

[rAuben]

Hello Everyone,

I have been asked to find out what it takes to bring 775 gallons of water at 70 deg (room temperature) to a rolling boil (approx 212 deg) via 25psig steam.

I have 68 square feet of heat transfer surface (stainless steel tank) with no agitation.

I haven't been successful finding a formula that can work this out, is there anyone out there that can help?

Thanks!

 

[russ_watters]

You are missing some info. Do you want to find out how fast, how much steam you need, etc?

 

[rAuben]

Ah yes, I knew I had forgotten something - but when the girlfriend yells at you...

I need to find out how long it takes to heat the water up to 212 deg from the 70 deg.

 

[russ_watters]

68 square feet is a lot of surface area - how much steam (flow rate) is available? Is this the only thing on the steam system? If so, what is the boiler size?

 

[rAuben]

25 PSIG is the only heating point in the system.

It is basically the same as heating water through a kettle. Looking through a steam table, the steam will have a temp of 267 deg at 25 PSIG.

 

[russ_watters]

25 psi is not a capacity. What is the boiler capacity in Btu or the pipe size?

 

[rAuben]

The pipe size is 1 1/2"

I do not think the boiler size is required to determine this

 

[Travis_King]

You need a capacity of steam. lb/hr.

 

[russ_watters]

It is if the boiler can't supply the required flow!

But anyway, with our still limited information, we still have to assume:

1. Boiler sizing is not an issue.
2. Heat exchanger pressure drop is not an issue.

Other assumptions, probably not critical:
3. Heat exchanger area is sufficient for transferring as much heat as we can throw at it.
4. Natural convection will sufficiently mix the water in the tank (apparently not critical since you are trying to boil it anyway).

Spirax Sarco has a steam pipe sizing calculator on their site: http://www.spiraxsarco.com/resources/calculators/pipes/sizing-new-pipes.asp

Typically, a steam pipe is sized for around 7500 fpm velocity. The calculator I linked doesn't do that calc directly, but you can use a guess-and-check method to enter in the knowns, then try different mass flow rates of steam until you match the pipe size. Try it and see what you get (or do some googling to find a table or other calculator that fits your needs better).

Then take from a steam table the heat of vaporization of steam and multiply to find the heat input rate.

Then calculate the heat you need to add to raise the temperature of water to boiling.

Then divide the heat needed by the input rate to find the time.