Water bath vs. T&S HX - pros and cons

[MortenA]

Would anybody like to give me a few pros & cons when comparing a water bath and a T&S HX for the following applications:

Heating natural gas (single phase but water saturated) from approx 20-30 deg C to 60-70 deg C upstream a pressure let-down valve. The gas comes from a cavern storage and pressure is reduced before drying. Due to expected JT cooling the temperature is increased so that the gas will be at least 20 deg C after pressure reduction.

Flow: max 450 kNm3/hr @ 70-190 barg T approx 30 deg C.
P after after pressure let-down min 50 barg
Expected max duty: 11 MW

All operation points in-between are considered - so that you may have a large volume flow with a low heat input (low carvern pressure, low dP) or a fairly small actual flow rate with a high heat input (high arvn pressur high dP). Just gimmy "punch lines" then i will be happy

Best regards

Morten

 

[speco]

MortenA,

Shell and tube exchangers are typically used in this application. However, I am not familiar with the term "water bath". Can you further explain your question. I suspect that I'm not the only one scratching his/her head over this one.

Regards,

Speco

 

[MortenA]

Indirect fired heater may be another term?

Best regards

Morten

 

[Montemayor]

Morten:

The first inclination is to apply the well-known and well-utilized, direct-fired water bath heater. This type of heater is field proven and rugged - as proven by its many decades of use out in the "oil patch" where it was developed for the expressed purpose of heating gas well production lines.

11 mW (37.5 MMbtu/h) is a lot of heat for one "normal" sized unit. The biggest I've seen is 10 MMbtu/h. However, the units I've seeen and used are designed for heating and hydrate control so that there is a lot of other baggage - like choke valves and separate coils. I suspect you could design a 40 MMbtu/h unit as a reasonable size.

I think the design is very appropriate - depending largely on your scope of work and design basis. Some noted advantages may be:

1) It is, by far, the most economical to justify in a remote area where you have to generate your own reliable heat source;
2) The heat source it uses is the same process gas you treat, so you have direct control over your field autonomy;
3) It is by far much cheaper and simpler to construct than a TEMA shell & tube. There really is no need for a shell & tube unless you demand it or you think it's cute or avant guarde.
4) It is inherently very rugged and safe. The gas in the tubes is in the best of all locations. Simple level and temperature controls ensure the consistant operation. This is field proven by decades of history.
5) It is portable and adjusts itself to cheap, skid design. You can bring it in on a flat bed and roll it on or off.
6) It is much simpler and fail-safe than a shell & tube since there is no pump or flow of heating fluid. It therefore requires much less instrumentation and maintenance.
7) These units operate all by themselves in remote, isolated places where no one goes - or wants to go. They just operate --and operate --and operate.

These are just some quick, down-and-dirty thoughts. There are probably more advantages I could list if I took more time.

 

[dcasto]

The EG/water bath is perfect and off the shelf. A lot of them will have a split design where the gas is first heated by 1/2 the tubes and the gas exits the heater and the pressure cut is taken, then re enters the heater for a final heating.

I get 54 MMBTU/hr and at least a 12" line by 1" tick by 2250 feet long (in multiple passes) and the outlet line that is a 16". This will require multiple heaters. 5 nominal 10 MMBTU/hr should do well with 6" sch 120 in 8" sch 40 out.

 

[MortenA]

dcasto

The heat input im pretty certain off (HYSYS) but thanks anyway.

Best regards

Morten

 

[krossview]

MortenA

1. Inlet temperatures not beneficial for S&T operation here.
Benefits over line heater are fuel gas savings.
2. I assume worst case heat input is based on 20C Inlet Gas at 190 Bar to get 11MW. Inlet gas at 30C & 190 Bar requires 16 MM (54 MMBtu/Hr). Also above 80 Bar gas is in Critical Phase region.

4. Why not use Molecular Sieve for dehydrating gas, Lineheater and two (2) step JT process. This requires 600# or 900# ASME design vessel, but reduces line heater duties considerable resulting in considerable fuel gas savings over time.
Line heater duty: 9 MW with JT outlet at 70Bar or 3.5 MW with JT at 110Bar, based on 20C/190Bar Inlet Gas and reheating gas to reheating to 20C

Hope this helps and Good Luck

Krossview

 

[MortenA]

I may have misquoted the temps slightly. I wasnt trying to get help sizing the thing - just pros&cons. However actual size may matter since this is a lareg water bath as i understand it from the posts - so i will be more carefull with the spec sheets ;-)

Re mol sieve: Quotes from vendors indicates a +30% price premium over TEG.

Best regards

Morten