In this post I want to share with you how to size indirect fired water bath heater.
Indirect Fired Water Bath Heaters (IFWBH) safely heat various mediums by transferring energy indirectly. In this system, the process coils are submerged in a heated bath solution—typically a water-glycol mixture—which absorbs heat from the firetube and transfers it to the process media.
How Indirect Fired Water Bath Heater Works
Fuel gas burns inside a firebox submerged in the lower section of the water bath, transferring heat through the firebox wall to maintain the bath at the desired temperature. The process fluid (such as well fluids, natural gas, or oil) flows through a coil immersed in the upper section of the bath, where it is heated indirectly through the tube walls. A temperature controller regulates the fuel gas supply to the firebox to keep the water bath at an optimum operating temperature of 190°F. Operating above this threshold is inefficient, as it leads to excessive water loss through evaporation.
Applications
Indirect fired water bath heaters are usually used for the following applications:
- Heating natural gas before pressure reduction to prevent freezing of valve and instrumentation
- Heating well stream fluids before phase separation
- Heating fuel gas for power generation station
- Fuel gas dew point control
- Heating high pressure hydrocarbon gas streams
Standard Feature
Most IFWBH are designed in accordance with API 12K.
Sizing of Indirect Fired Water Bath Heater with Example
The target of sizing of IFWBH are to obtain the following parameters:
- Heat load of firebox
- Number of coils
- Type of coil
The sizing of IFWBH in this post is for academic purposes only. For real application, you need to ask the specific vendors.
Below is the example of IFWBH sizing for well gas heating.
Data:
- Natural gas flow rate = 4 MMscfd
- Gas specific gravity = 0.7
- Gas flowing pressure = 3350 psig
- Shut-in pressure = 4500 psig
- Gas flowing temperature = 85oF
- Heater outlet pressure = 1000 psig
[1] Determination of Firebox Capacity/Heat Load
Determination of firebox heat load is necessary for identifying standard heater sizes. Before determining the firebox heat load, we need to perform the following two steps:
- Whether hydrate is formed at inlet and outlet condition of heaters
- Determine the temperature drop because of choking or pressure reduction
[1a] Hydrate formation checking at inlet condition
To check hydrate formation, we need the following parameters:
- Flowing pressure = 3350 psig (3364.7 psia)
By using the following figure, we can estimate the hydrate formation temperature at the flowing pressure. At 3364.7 psia, hydrate formation temperature is 75oF. Because the flowing temperature is 85oF and the difference between flowing temperature and formation temperature is 10oF (85oF-75oF), therefore it is expected that hydrate will not form.
[1b] Hydrate formation checking at outlet condition
By using the same method as in step 1a, the hydrate formation temperature at outlet condition (pressure 1000 psig or 1014.7 psia) is 64oF. Giving the margin of 10oF, so the minimum required outlet temperature is 74oF (64oF + 10oF).
[1c] Temperature drop checking
Temperature drop can be estimated using the following chart. To estimate, we need the following data:
- Initial pressure (before choking) = 3350 psig
- Final pressure (after choking) = 1000 psig
Based on the chart, the temperature drop is 80oF. When initial temperature or flowing temperature is 85oF and the pressure is 3350 psig, then final temperature will be 5oF (85oF-80oF) when the pressure is reduced to 1000 psig.
[1d] Firebox heat load calculation
To avoid low temperature after the choke, the heater coil will be split into a coil upstream of the choke valve and coil downstream the choke valve. For practical purpose, the temperature of the stream before choke will be raised to 130oF (with water bath temperature 190oF maximum, 130oF is within 60oF of the bath temperature).
The simplified flow will be like this:
Note:
- Based on the data
- Selected value
- 45oF is value in stream 2 minus temperature drop (see Step 1c) (130oF-85oF)
- 74oF is minimum required outlet temperature (see Step 1b)
The illustration of split pass coil is as follows:
To estimate heat load, we need to estimate enthalpy at specific pressure and temperature. Figures below can be used, depending on the gas specific gravity.
We will use gas specific gravity 0.7 since it is the case. For upstream coil, the following operating condition are used to estimate enthalpy:
Heat load at upstream coil is estimated = (Houtlet – Hinlet) x flowrate (in MMscfd)* 1,000,000 / 24 = (7-5.2) x 4 x 1,000,000 / 24 = 300,000 Btu/h
During choking process, it is assumed that movement of natural gas takes place so rapidly so that there is no heat lost or heat gained passing through the choke valve seat. Therefore, there is no changes in enthalpy.
Use the same method to estimate heat load at downstream coil. The following operating condition are used to estimate enthalpy:
Heat load at downstream coil is estimated = (Houtlet – Hinlet) x flowrate (in MMscfd)* 1,000,000 / 24 = (8.2-7) x 4 x 1,000,000 / 24 = 200,000 Btu/h
Heat loss is assumed to be 10% of total process heat load = 10% x (300,000 + 200,000) = 50,000 Btu/h
Therefore, total heat load will be heat load for upstream coil + heat load for downstream coil + heat loss = 300,000 + 200,000 + 50,000 = 550,000 Btu/h
Based on typical commercial IFWBH specification, we will select heat load 750,000 Btu/h.
[2] Coil Heat Transfer Area Determination
To estimate heat transfer area, we need to estimate “U” (overall heat transfer coefficient), mean temperature difference (MTD), and heat load. We already calculated heat load at Step 1.
Because shut-in pressure is 4500 psig, we will use 2” XXtra heavy coil.
Overall heat transfer coefficient can be estimated using the following chart. We need the data of flow rate, coil type, and operating pressure.
For upstream coil, the following data is used:
MTD (mean temperature difference) is estimated using the following chart.
For upstream coil, the following data is used to estimate mean temperature difference (MTD).
Since U, MTD, and heat load are already defined we can estimate the heat transfer area at upstream section coil.
A = Q/(U x MTD)
We get overall heat transfer area for upstream coil is 28.8 ft2.
We do the same method to estimate heat transfer area for downstream coil. The overall heat transfer coefficient for downstream coil is estimated as below.
For downstream coil, the following data is used:
MTD (mean temperature difference) is estimated using the following chart.
If the chart is not available, the MTD can be calculated using arithmetic MTD.
Where:
ΔT1 = Temperature of water bath – final gas temperature (190oF – 74oF)
ΔT2 = Temperature of water bath – initial gas temperature (190oF – 45oF)
We get mean temperature difference is 130oF
Using the same equation, we get heat transfer area for downstream coil is 13 ft2.
Total heat transfer area is 28.8 + 13 = 41.9 ft2.
[3] Selection of Standard Heater Specification
The standard heater specification is as follows.
Because we need heater with heat load 550,000 Btu/h (we select heater with heat load 750,000 Btu/h) with coil type 2”XXtra Heavy Coil, then number of coil will be 10, with selected heat transfer area 58 ft2 (calculated heat transfer area 41.9 ft2).
We also can estimate quantity of coil at upstream coil and downstream, respectively. Table below summarizes number of coil using heat transfer area approach.
[4] Flow Coil Pressure Drop Calculation
Coil pressure drop can be estimated by using the following chart.
Coil length can be checked by using standard specification of heater.
Pressure drop at upstream and downstream coil are estimated below.
[5] Conclusion
Based on case study above, the key specification required for IFWBH are:
- Heat load = 750,000 Btu/h
- Number of coil = 10
- Coil type = 2” XXtra heavy coil
- Type of pass = split pass
Free Spreadsheet
Due to high capacity, please email me if you need the spreadsheet!
I hope you find this post useful.
References: