HEATING EFFICIENCY

Indirect Bath Heater

Indirect bath heaters are mainly used in the oil and gas industry to heat process fluids. The mechanism takes place indirectly through a bath (utility bath) as opposed to heating directly by flame or furnace (direct fire-tube heaters).  The use of a bath promotes uniform heating and reduces the chance of hotspots. Due to the indirect nature of bath heaters the safety risks are much lower (combustible streams isolated) compared to direct fire-tube heaters, they can therefore be used in hazardous areas where safety is a concern.

The “indirect” name comes from the fact that a fire-tube is submerged in a bath that provides heating through the bath medium to a submerged coil containing the process stream. Usually the bath fluid is water but depending on the climate and heating requirements it can also be oil (capable of heating process fluids to higher temperatures) or a mixture of water and glycol (used in colder climates to lower the freezing point).

Hotspots often occur in direct fire-tube heaters as a result of carbon build up on the fire side of the tubes. If left unchecked this build up can be a source of process fluid degradation and may lead to ruptures, explosions and leaks.

Typically, FARAN’s indirect bath heaters achieve around 50-55% heating efficiency. This relatively low efficiency compared to direct fire-tube heaters is a result of the number of heat transfer steps involved. Typically indirect bath heaters are used where the flow rate of the process stream is low and operations may be unattended, they could also be used to prevent coke formation on the tubes when low heat fluxes are required.

Variety of applications:

1.     Gas dew point heating: high-pressure natural gas heating upstream pressure regulation stations preventing condensation phenomena due to Joule-Thompson effect.

2.     Heating of high-pressure natural gas upstream pressure regulation in order to prevent external icing formation.

3.     Fuel gas super-heating upstream gas turbines.

4.     Gas heating downstream low temperature storages.

5.     Hydrate prevention.

6.     Viscosity reduction: Crude oil heating treatments to facilitate degassing and dewatering.

FARAN’s Indirect Bath Heaters main parts:

-       Main body

-       The coil or tube bundle

-       The water expansion tank

-       The fuel feed line

-       Burner (gas type, oil type or bi-fuel)

-       The fire-tube

-       Chimney

-       Burner management system panel

-       Skid frame and service platform

-       Insulation

The heater shell contains the fire-tube that allows a rapid heat transfer (both radiant and convective) from the flame to the hot medium. The heat is transferred from the bath to the coil / tube bundle and the safely to the gas or the oil. The expansion tank allow the hot medium to be contained for the thermal expansion due to the temperature increase. Level gauges and transmitters advise when additional water or hot medium is required and give alarms in case of overfilling.

Forced draft Vs natural draft:

Natural draft burner relies on a stack to create negative pressure at the burner inlet, which induces as much air as is required for combustion. These heaters are popular for remote locations and/or locations with limited or even no power supply available.

Forced draft heaters are ideal whenever there is a low emission requirement, utilizing small bore fire-tubes in conjunction with a combustion air fan. High velocities in small bore fire tube reduce the dead gas film region along the fire walls. This results in a reduced tube areas compared to their large bore counterparts, therefore, overall heater size is minimized creating a significantly smaller footprint while offering superior burner control compared to natural draft style heaters.

FARAN Process & Energy engineering company is able to design and manufacture different size of indirect water bath heater  which vary from 90 kW up to 10MW.

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