Shell And Tube Heat Exchangers

Abstract

A process for exchanging heat in a shell and tube gas-to-gas heat exchanger between a plurality of gases, said process comprising passing a cold first gas in parallel flow to a second hot gas to provide a warmer first gas; and passing said warmer first gas in counter-current flow to a hot third gas to provide a cooler said third gas. The invention provides increased minimum tube wall temperature within the exchanger for given process conditions while maintaining a high log mean temperature differential allowing for the prevention of corrosion from entrained corrosive vapours or entrained corrosive mist with a minimal increase in effective area.

Description

FIELD OF THE INVENTION[0001]This invention relates to shell and tube heat exchangers, more specifically to exchangers operating in service where a standard counter flow shell and tube heat exchanger would not be able to meet the required process conditions without experiencing dew point corrosion, more specifically to exchangers using a combination of counter flow and parallel flow throughout the exchanger to reduce the potential for dew point corrosion while being able to maintain a high thermal efficiency with only a minimal increase in effective area, and most specifically to gas to gas heat exchangers used to cool hot gases containing sulphur trioxide and / or acid vapour or heating cold gases containing sulphur dioxide and / or entrained acid mist.BACKGROUND OF THE INVENTION[0002]The invention relates to heat exchangers operating in potentially corrosive or high fouling conditions where said corrosion or fouling rates are highly dependent upon the tube wall temperature throughout t...

AI Technical Summary

Benefits of technology

[0013]The present invention provides a shell and tube heat exchangers utilizing an improved flow combination of parallel flow and counter flow to retain a high LMTD and increase the minimum tube wall temperature in comparison to a counter flow heat exchanger operating under identical process conditions. This exchanger is particularly well suited for the prevention of dew point corrosion and damage from entrained acid mist.

[0014]The exchanger generally comprises two main sections with one section having a generally parallel flow arrangement and the other having a generally counter flow arrangement between the two fluids. Partially cooled hot gas transfers heat in a generally parallel flow manner to a cold gas through the tube walls in the colder of the two sections, while hot gas transfers heat in a generally counter flow manner to a partially heated cold gas through the tube walls in the hotter of the two sections. These two sections may be separated by a transition section where the flows alternate between shell side and tube side. This transition is particularly beneficial in plants that have unsteady process conditions. Dividing the exchanger into two sections allows for full control over the thermal design of the system, the inclusion of a partial gas by-pass or intermediate gas addition, and easier repairs if necessary. By alternating the shell side and tube side gas streams the overall difference in thermal growth between the shell and tubes is reduced, which in turn reduces the stresses caused by differential thermal growth and reduces fatigue stresses from thermal cycling.

[0044]Having the cold inlet gas flow in parallel to partially cooled hot gas maintains a higher LMTD throughout the exchanger and reduces thermally induced differential stresses when compared to a standard parallel flow design. Additional uses of this exchanger design including alternating the hot and cold gas streams to prevent against high temperature corrosion will become apparent to a person skilled in the art of exchanger design and operation following a review of this disclosure.

[0045]The parallel flow section maintains a more consistent tube wall temperature than the counter flow section, which allows for further heat to be transferred between the two gas streams while maintaining the tube wall temperature above the dew point. Maintaining the tube wall temperature above the dew point prevents corrosive vapours present in the hot gas stream from condensing. This in turn allows for the use of standard materials of construction as opposed to corrosion resistant materials, thus reducing the capital cost of the exchanger while simultaneously extending its expected life.

[0048]An exchanger designed according to the invention may be designed to have the gases alternate sides of the tube wall during the transition between the parallel flow and counter flow sections. Doing so maintains the average shell wall temperature of the exchanger closer to the average tube wall temperature, thus reducing differential thermal growth. Dividing the tubes into two separate sections allows for the differential growth between the shell and tubes to be absorbed in stages, which reduces the forces on the tube sheets. The combined reduction in differential growth and thermally induced stresses from alternating the shell side and tube side flows can be substantial in exchangers with a high temperature differential between their hot and cold gas streams, especially in plants with fluctuating process conditions; however, this is not required to realize the corrosion resistance benefits and relatively high heat duty capabilities of the exchanger design.

Problems solved by technology

When the shell or tube wall temperature on the corrosive side of the heat exchanger falls below said corrosive vapour's dew point, there is a potential for corrosion.

This can lead to fouling which causes a decrease in performance, an increase in pressure drop, and premature failure of the heat exchanger.

In sulphuric acid manufacturing, both gas streams in a heat exchanger are often potentially corrosive.

This entrained acid mist can rapidly corrode the heat exchanger when it comes into direct contact with the tube walls, especially when said tube walls are directly impacted with droplets of entrained acid mist within the cold inlet gas.

Therefore, these preheat exchangers often experience dew point corrosion issues similar to those in the production of sulphuric acid.

Counter flow exchangers have the highest LMTD of the standard heat exchanger arrangements and therefore require less effective heat transfer area to attain an equivalent heat duty to other heat exchanger designs with equivalent process requirements.

Limiting the heat duty can prevent the minimum tube wall temperature from falling beneath the dew point but may prevent the exchanger from being able to meet its process requirements.

An alternative to limiting the effectiveness of the exchanger is to increase the gas inlet temperature, although for process reasons this may not be desirable or feasible.

Use of these materials reduces the effects of corrosion but does not prevent the formation of dew within the exchanger.

The capital cost of the exchanger can increase significantly by using corrosion resistant materials depending on the materials required, and yet the material may still experience a considerable amount of corrosion and fouling.

The temperatures of the two streams approach asymptotically and converge towards a common temperature which in turn limits the maximum heat duty and exit temperatures.

There is also a potential for significant thermal differential stresses in the inlet region where the temperature difference between the hot and cold gases is greatest.

Because of these traits, pure parallel flow exchangers are not preferable when there is a requirement for high heat duties or where there is a large inlet temperature differential.

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