Lord Fin Tube-Heat recovery with steam

Heat recovery with steam

When transferring heat from a liquid medium, such as hot water or thermal oil, the sensible heat of the medium is used. The liquid is supplied to the heat exchanger at increased temperatures. When the liquid gives off thermal energy, the temperature drops and leaves the heat exchanger at a lower temperature. Steam is generally divided into saturated (wet) and superheated (dry) steam. Steam is produced when all its water molecules remain in a gaseous state at a temperature corresponding to the vapor-pressure. It is therefore also called "saturated steam" (when heated further it becomes superheated steam).

The main difference of steam compared to water or thermal oil is its vaporous condition. This, or the fact that steam is water heated above boiling point, has several advantages and disadvantages for the processes, the apparatus, and equipment that generate and control the process.

For both steam and liquid heat recovery systems, the inlet temperature of the secondary fluid to the heat exchanger can change over time, requiring a control device in the system. This means that in order to keep the outlet temperature of the secondary medium constant, the heat supplied to the heat exchanger must also vary. This can be achieved, for example, by a control valve on the primary side of the heat exchanger.

Heat recovery with steam can be defined as the process by which heat that would normally be wasted is captured and transferred to a steam generator. The waste heat is converted into steam and supplies to a device or process where it can be used as effective, economical and environmentally friendly thermal energy.

Generally, heating system with steam means Steam is supplied to the heat exchanger in gaseous state. In heat transfer with saturated steam, the latent heat of the steam is used, and a large amount of energy is released during condensation (transition to the liquid state).

Steam enables a heat transfer at a constant temperature, which is impossible to do in a liquid state heat transfer.

In the following, we will describe a few advantages and disadvantages of steam systems compared to heat transfer systems with thermal oil.

Advantages and Disadvantages of Steam Systems

A. The Advantages of Steam Systems

First of all, the amount of latent heat released is 2 to 5 times greater than the amount of sensible heat available after the condensation of hot water (saturated water). This latent heat is automatically released and transferred to the product to be heated via the heat transfer surface. Through the condensation, steam and liquid condensate naturally flows against the heat transfer surface and supports the heat transfer. In opposition to that, hot water and thermal oil systems transfer the heat by convection heating, which does not cause any change of state or phase when the medium is heated.

Another benefit of steam is that evaporation requires energy that can be recovered when the state of steam changes from vapor to liquid (condensation), i.e. the energy accumulated during evaporation is used by being released again during condensation. (The energy that can be released during condensation is called latent heat)

Also an advantage is that steam is conveyed without pumps, either by gravity due to the low density or by the pressure difference during steam generation and expansion.

This eliminates the requirement of pumps and thus reduces consumption of electrical energy. On the other hand, if the water is to be re-used, the condensate must be pumped back into the feedwater system.

However, heat transfer in a heat transfer system with liquid heat carriers would be extremely slow due to the sole natural circulation. Therefore, a pump must be used to create a flow against the heat transfer surface to increase the speed of heat transfer, which is called forced convection heat transfer.

Further, there is a very small temperature difference between steam inlet and outlet. This can be a significant advantage wherever small temperature differences over a certain heating surface are required (for instance for press plates). This is relativized as soon as the latent heat of condensation is to be used, as the condensate temperature is below 100°C.

B. The Disadvantages of Steam Systems

A disadvantage, however, is that high pressure is required at high temperatures. For example, the pressure of saturated steam at 300°C is already over 85 bar, while thermal oil can transfer temperatures up to 400°C, typically pressure-less.

The fact that the entire system must be designed for steam pressure can lead to enormous additional costs when high temperatures are required.

Another distinct disadvantage is that steam generation requires an additional room above the water level, in which the water vapor can establish itself. This results in the necessity of a larger apparatus and, besides, steam generators are usually installed horizontally, since installing in vertical execution is difficult to realize a vapor room. If the heat is recovered from hot air or gases, this requires more work, higher costs, and more space for the installation of ducting, etc.

Furthermore, steam is compressible. Therefore, a lot of mechanical energy is accumulated in the vapor room, which is caused by compression. This leads to an increased damage potential and may require additional safety devices compared to liquid heat transfer media.

Also, minerals and oxygen contained in the water are released during evaporation and concentrated on the water surface. This can cause not only corrosion, but also deposits on the heating surface, known as boiler scale, which reduce the heat transfer within the heating surface and the water.

Depending on the temperature on the hot side, corrosive or thermal decomposition of the surface material can occur, even damage to the heat exchanger tubes can occur.

In combination with the high mechanical energy that accumulates in the steam chamber, it can cover a high damage potential. This means the feed-water quality must be monitored continuously and in most cases demineralization and deaeration of the water is required. In turn, it causes additional costs for apparatus and operating materials (see chemicals).

On top of that, even if chemical water treatment reduces the risk of deposits, minerals are still released during evaporation. These minerals have a higher density than water and are concentrated in the form of mud at the bottom of the steam generator.

The nitrates, which have a slightly lower density than water, are concentrated on the surface of the water. The deposits at the bottom of the steam generator must be frequently released by blowdown.

The nitrate concentration is measured with a conductivity electrode and released accordingly at the water surface.

These measures lead to continuous water losses, even if the condensate is completely returned. The water losses must be compensated by a continuous supply of fresh, demineralized and de-oxidized water, which leads to energy losses, since the fresh water must be heated.

The return of condensate requires additional care. The use of condensate traps, the correct inclination of the pipes, etc. poses an additional operational risk. If the system is not properly designed, steam hammering (Steam picks up the water, forming a "slug", and hurls this at high velocity into a pipe fitting, creating a loud hammering noise and greatly stressing the pipe) can cause minor damage to the pipes or even complete damage to a heat exchanger and/or a waste heat boiler and pumps.

Finally, steam systems are relatively complicated due to the facts described above. Therefore, the engineering of the process, the apparatus, also the operation of a steam system, require highly qualified and experienced specialists to ensure safe and trouble-free operation.

Examples for processes, where steam heat recovery systems can be used;

Steam jacketed heating is often used in process plants for heating devices, such as; tanks, kettles, dryers, reactors and glass-lined vessels. With direct steam injection, the heater offers the most precise and energy-efficient method of heating the jacket water to the desired temperature. This system works by injecting steam directly into the jacket to ensure efficient energy transfer of the steam, which is immediately absorbed by the liquid.

Vacuum steam drying is a heat transfer process for removing moisture from a wet solid or product. It is generally used for heating and drying hygroscopic and heat sensitive substances and is based on the principle of generating a vacuum by means of a vacuum pump to lower the chamber pressure below the vapor pressure of water.

Humidification by steam humidifiers is used when a certain level of humidity must be maintained to prevent the material properties from being maintained and to ensure a comfortable and healthy environment for workers or residents. When the cold air is heated by the steam coils, the relative humidity of the air decreases and must then be adjusted to normal levels, adding a controlled injection of dry saturated steam into the downstream air flow.

Steam generated by waste heat recovery can be used directly in a steam turbine to generate electricity.