Heat transfer in a hot oil system

[Cam0]

I am an HVAC engineer trying to determine the proper amount of OA(outside air) ventilation required for a new addition of a chemical mixing factory. The room will have three 33,000gal tanks of glycerin that will be kept at 150F. The tanks will be heated by a hot oil system that pumps syntherm A-32 heat transfer fluid through 2in diam sched 40 pipe coil through the bottom of the tanks. I have the following info:

density of glycerin = 78.6 lb/ft^3
heat capacity of glycerin = .576 btu/lb-F
glycerin starting temp = 90F
glycerin ending temp = 150F
density of A-32= 47.09 lb/ft^3
heat capacity of A-32= .474 btu/lb-F
A-32 temp (constant) = 150F
flow rate of A-32 = 50gpm (529.2 lb/min)
volume of A-32(in one tank) = 3.5ft^3
total length of pipe in the tank = 160.5ft

I am trying to calculate the amount of time it takes for the A-32 hot oil coil to heat the glycerin from room temp (approx 90F) to 150F. I came up with roughly 36 million BTU in 5.3 hours, which is quite a lot. I was hoping to get some help/double check on my work.
Thank you.

 

[MintJulep]

36 million BTU is about right, considering the glycerin only.

You need to also consider:
Heating of the tank material.
Heat loss from the tank to the surroundings.

A-32 temp (constant) = 150F

That is wrong because:
If the temperature is constant than there is no heat transferred from the A-32 to the glycerin.
If the temperature of your "hot" fluid = the target temperature of your cold fluid then you'll never get there.

 

[IRstuff]

Additionally, if the A-32 is only 150F, you have the additional problem that any heating of the glycerin will result in a reduction in temperature of the A-32, so you don't even really have 150F, making it impossible to get 150F glycerin

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[Cam0]

Thank you all for the help. Taking your advice into consideration, I have gone through this problem several times now in different ways after changes several factors. Glycerin starts to break down at 130F, not sure why the client wants to store it at such a high temp, but 130F is the storage temp of glycerin, not 150F.
The A-32 if being heated to 100C(212F)while running through the coils.
After recalculating, I concluded roughly 6.2 million BTU make it from the A-32, through the sched 40 coil, heat the glycerin, heat the stainless steel tank wall, and escape into the air every hour. reducing the amount BTU/Hr by about 600,000.

"...you have the additional problem that any heating of the glycerin will result in a reduction in temperature of the A-32, so you don't even really have 150F."

I am attempting to calculate for worst case scenario. So leaving out minor loses such as this builds in a safety factor.

 

[georgeverghese]

Most heat transfer texts will discuss ways of calculating external htc from heating coils due to natural convection. See DQ Kern - Process Heat Transfer for example.
Q=UAoLMTD, where

1/UoAo = 1/hiAi + 1/hoAo + Rf, ignoring the minimal resistance at the tube wall
Ao=External surface area presented by the heating coil
LMTD = log mean dT, which in many cases, may be reduced to MTD

Heat loss to the environment through the lagging should account for both natural convection losses and radiant heat loss from the surface of the lagging. Losses would be through the tank roof, tank bottom and tank sides.
These are not simple calcs to run, so get a senior process engineer to countercheck.

 

[IRstuff]

What are you using for:
> tank surface area
> ambient temperature
> air convection coefficient

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