Heat Transfer - Controlling Costs During Industrial Processes

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Heat transfer is an important aspect of many commercial and industrial processes, so making it as efficient as possible is vital. Graham Dear, Marketing Product Manager – Heat Transfer at Spirax Sarco highlights some of the advanced techniques available for improving heat transfer efficiency, such as sub-cooling condensate, and the best ways to ensure reliable operation by tackling stalling in the heat exchanger.

Graham Dear, Product Marketing Manager - Heat Transfer, Spirax Sarco — *By Graham Dear, Product Marketing Manager – Heat Transfer, Spirax Sarco*

Transferring heat efficiently from one place or process to another is the basis of a wide range of industrial processes, as well as being fundamental in building services for space heating and hot water. Most energy transfer applications in industry are based on the heat exchanger.

Steam is the most efficient and flexible energy transfer medium and there is a range of heat exchanger technologies available that can provide reliable service across a wide variety of applications.

Essentially heat transfer is concerned with two things: temperature and the flow of thermal energy from a heat source to a heat sink. To be practical, any heat transfer technology must also be efficient and controllable.

Energy efficiency determines what proportion of the energy entering the process ends up where it’s needed, while effective control enables the thermal energy transferred to match the demands of the process. Not only does this link closely with energy efficiency, but it can also influence production efficiency by affecting parameters such as product quality and waste.

For any steam user, it is important to have a sound grasp of the basics of energy transfer and steam plant operation in order to specify systems correctly and prevent problems cropping up later. They also need to understand the differences between the types of heat exchangers currently available (see below).

In the case of users for whom steam lies outside the scope of their core activities, bringing in expertise from external steam specialists can help to ensure that their heat transfer installations provide optimised, reliable service for years to come.

Types of Heat Exchangers – at a glance

Of the three methods of transferring thermal energy, conduction is the most widespread and versatile approach and can be applied by several different heat exchanger technologies.

Shell and tube

– Shell and tube heat exchangers are based on a pressure vessel (the shell), containing a bundle of tubes. One fluid flows through the shell and the other through the tubes. Each fluid enters the exchanger at a different temperature and heat passes between them through the tube walls as they flow through the exchanger.

Plate heat exchangers

– Standard plate heat exchangers (PHEs) are increasingly popular in applications that transfer heat between medium- and low-pressure fluids. In place of tubes passing through a shell, standard PHEs are built from a series of corrugated metal plates held together to form channels through which the two heat transfer fluids flow in alternating layers of the ‘sandwich’. The plates produce an extremely large surface area relative to physical size, which promotes very effective heat transfer.

This design also offers the additional benefit of having a low pressure/volume relationship minimising the level of routine inspection requirements.

Plate and shell heat exchangers

– The plate and shell combination offers high heat transfer, compact size, low fouling and a close approach temperature, which is the temperature difference between the leaving process fluid and the entering service fluid. It is also able to cope with a high pressure/temperature envelope.

Corrugated tube heat exchangers

– A variation on traditional shell and tube exchangers. They have corrugated tubes to create greater turbulence, achieving much greater heat transfer compared to smooth tube heat exchangers, allowing them to be more compact.

Shell and coil heat exchangers

– Built from circular layers of helically corrugated tubes inside a compact shell. The large number of closely packed tubes creates a significant heat transfer surface, while the alternate layers create a swift uniform heating of fluids and increase the total heat transfer coefficient.

Heat pipe heat exchangers

– Vacuum tubes with one end in the ‘hot’ stream and other in the ‘cold’ stream. They contain a working fluid, and it’s the constant cycle of evaporation and condensation as the working fluid moves around the sealed tube that transfers thermal energy from one stream to the other. The big advantage of heat pipes is their great efficiency in transferring heat. For example, a heat pipe can transfer up to 1,000 times more thermal energy than copper, the best known conductor.

*The latest heat exchanger packages feature intelligent monitoring systems to deliver meaningful energy management and system performance data for the user*

Controlling steam heat exchangers

Apart from heat pipes, all the different types of exchanger are generally controlled in a similar way, by sensing the temperature of the secondary fluid (often water) emerging from the unit and using a valve to modulate the primary fluid (the incoming flow of steam) to the exchanger.

An alternative control method to modulating the steam flow into the heat exchanger is condensate control, which keeps the input steam pressure constant and instead adjusts the flow of condensate coming out of the exchanger. This varies the amount of condensate inside the exchanger to control its heat transfer area and hence its heat transfer rate.

Condensate control allows the condensate to be maintained at a sub-cooled temperature to extract the maximum amount of useful heat from the steam and avoids any potential flash steam plumes.

Heat exchangers of any type can stall when the condensate is not removed effectively and builds up internally. Typical symptoms of heat exchanger flooding include banging and crashing noises coming from shell and tube heat exchangers caused by waterhammer.

Stalling happens when the pressure in the heat exchanger is less than or equal to the back pressure on the steam trap, often occurring when demand from the heating process falls due to a change of flow rate. Stalling is also dependent on the rate of change of flow.

When this happens, the control valve reduces the steam pressure accordingly and this may reach a level that’s too low for the steam trap to clear the condensate effectively.

The best protection against stalling heat exchangers is prevention by good system design and by fitting measures to solve the problem. One solution is to fit an automatic pump trap to ensure condensate is always cleared under even the most demanding conditions.

What is clear is that efficient and controllable heat exchange is essential for thermal processes and should be considered crucial for any steam-using operation. As well as providing the heat transfer needed to enable the process to operate correctly and at the right rate, it also keeps energy costs down and reduces the production of carbon.