Today I would like to tell you about water bath heater calculation. i.e. How to design a Water bath heater.

I assume you have enough Idea about water bath heaters BEFORE deciding to design a one, if not that may be just read on Wikipedia.

Read API 12K for design, Its short ( Total 40 pages) and easy but you won’t be able to figure out from where to start for design.

Below is the Figure showing various parts of a Water bath heater:

Here is the steps :

1. What is the duty required i.e. how much process fluid is to be heated, flow, pressure & composition, and check phase

change is occurred or not during temperature rice.

2. Calculate the no of process coils , OD, Length :

a. Assume heat transfer Coefficient h = 100

b. Water bath temperature 90°C

c. We already know process fluid inlet and required out let temperature.

i. Calculate LMTD

d. You will get area ,

e. Now assume diameter of tube and no of tubes

i. check velocity in it ( should be in the range of 10 to 15 m/sec, if fluid is clean , check for erosion

velocity if this contain sand particles.)

f. assume standard length of 6 , 8 of 10 ft and check the total area is more than the area required by heat

transfer in step d.

3. fuel consumption : what is the fuel calorific vale? divide heat duty by this and get mass flow rate of fuel, fuel system

is designed based on this , line size, valve sizes etc.

4. Fire tube design :

a. The heat given to process fluid is divide by the heater efficiency ( typically 80-90%). this will give you the heat

given by fire tube.

b. Assume Diameter and length of fire tube ( i.e. 20” and 10 Ft) , calculate this area

i. Divide The Heat calculated for fire tube from this area and get the heat flux,

ii. This should be in the range of 10000 to 12000 btu/hr/ft2 as per API 12 K , so adjust dia and length.

5. Check fire box rating and as per API 12k

6. Determine Tube thickness as per API 12k

a. Which material to be used and what would be the hoop stress of that (‘S’)

That’s All for beginner, but you really need to do a real life situation design to fully understand the pain of designing this. the above

article is meant to use as a guide path for the same.

Please comment to enrich the same.