Tank Heating Calculation 1

[g2mike]

We are a manufacturer of Christmas Ornaments. In the process of painting the ornaments, we have excess lacquer which collects on our equipment racks. We have a stripper tank containing hot water and caustic soda for cleaning. We have a boiler that was used for heating the stripper tank and 2 evaporators. The evaporators are off line now so it is overkill to heat just the stripper tank with a 750k BTU boiler so we are trying to cut costs and see if a commercial water heater could to the job.

The tank measures 12'L x 2'W x 4.5'H and holds about 840 gallons. 1" copper pipe comes from the boiler which is on a platform above the tank and about 100 ft away. We would tie into the pipe with a gas water heater about 15 ft above the tank. The pipe comes from above at one end of the tank and extends vertically into the tank along the tank wall. At the bottom of the tank it turns 90 and coils back and forth along the bottom. There are 6 runs 10' in length along the bottom. The pipe at the end of the coil then comes up out of the tank back to the boiler to complete the closed closed loop.

We need to heat and hold the water between 130 and 140 degrees 24/7. Could this be done with a gas water heater and what is the calculation to determine what size water heater we would need and any other changes to the piping, etc.?

Thanks in advance for your help.

 

[Latexman]

g2mike,

Sounds like the stripper tank is the only energy user the boiler has to service now. If you record the energy input to the boiler over time, you’d have the energy requirements of the stripper tank plus inefficiencies. Is the fuel to the boiler continuously monitored and recorded? If so, you have it made! This would enable you to see the peak demand. If you have to manually record the usage, finding the peak demand will be tougher. Be sure you record the energy demands during a cold start-up of the stripper tank, from the coldest temperature it can be at to 135 degrees. Sizing the water heater from steady state data could result in start-ups that take forever.

If you don’t have a good handle on the energy input to the boiler, then you have to estimate the requirements of the stripper tank. How long does a cold start-up take now? This is unsteady state batch heating, so the water heater will need to supply more energy when the stripper tank is cold than right before it reaches 135 degrees. Batch cooling and heating equations can be found in Perry’s Chemical Engineers’ Handbook and other references, like Kern’s Process Heat Transfer. In the Perry’s 6th Edition it’s page 10-38. The steady state requirements will be heat losses plus the heating requirements of your process. How fast are equipment racks put in the tank? How much do they weigh? What’s their heat capacity? From this you can estimate the heat duty of putting equipment racks in the tank.

After you know the energy input the stripper tank needs, I’d call in 2 or 3 reputable M&E contactors or commercial water heater vendors and let them tell you what is needed. If you contract out this kind of work, call M&E contactors. If you have your own crew, call commercial water heater vendors. These guys will also help you understand what’s “doable”.

Now, if you have to size everything, and this just depends on the work process where you are, crack open your fluid flow and heat transfer references and go at it. With a circulating hot water system, you have to assume a flow, calculate the overall heat transfer coefficient, and, then, calculate the temperature of circulating water you need to satisfy your worst case. I’d recommend using a velocity of 4-6 ft/sec in the copper pipe. Lower will silt out and higher will cause erosion. You’ll quickly see if the wetted area of the 1” copper pipe is going to give you a reasonable design or not.


Good luck,
Latexman

 

[Montemayor]

Latex,

Great job! Excellent, detailed rundown on how to identify a heat up requirement using an accurate, practical method. Mike owes you dinner. You've got my star.

Mike:
It's tough to follow Latex's act, and all I can add is the experience that your boiler is probably more efficient than the future, proposed commercial water heater would be. What I mean is that your boiler flue gas is probably cooler than the water heater's will be - assuming both use the same fuel. Therefore, apply a correction factor to the capacity of the proposed, specified water heater to allow for the incremental inefficiency.

You should be able to maintain a hot water temperature within a 5 oF variation with ease, depending on the type of heater and its controls. Latex hit this on the head when he recommends you bring in a proposed contractor and spell out your needs and scope of work to him. He should confirm all we've said and warrant the temperature control.

Hope this helps.