Heat Recovery with Steam
As you may remember, our current series of articles revolves around the topic of heat recovery. In previous issues we have described the different conditions for heat recovery with water and with organic heat transfer fluids (the so-called “thermal oil”), which is designed to transport extreme temperatures in a liquid state and without pressure.
Therefore we would like to give you an overview of heat recovery with steam today.
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 fed to the heat exchanger at elevated temperatures. When the liquid gives off heat 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 the gaseous state at a temperature corresponding to the steam pressure. It is therefore also called “saturated steam” (when heated further, it becomes superheated steam).
The main difference between steam and water or thermal oil is its vaporous state. This, or the fact that steam is water heated above boiling point, has several advantages and disadvantages for the processes, apparatus and equipment that generate and control the process.
In both steam and liquid heat recovery systems, the inlet temperature of the secondary fluid to the heat exchanger can change over time, requiring control equipment in the system. This means that in order to keep the outlet temperature of the secondary fluid 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 to steam and fed to a device or process where it can be used as effective, economical and environmentally friendly thermal energy.
Generally means heating system with steam;
- The steam is supplied to the heat exchanger in a 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 heat transfer at a constant temperature, which is not possible with heat transfer in the liquid state.
Some advantages and disadvantages of steam systems compared to heat transfer systems with thermal oil are described below.
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 flow naturally against the heat transfer surface and support the heat transfer. In contrast, hot water and thermal oil systems transfer heat by convection heating, which does not cause any change of state or phase when the medium is heated.
A further advantage of steam is that energy is required for evaporation, which can be recovered when the steam changes state from steam 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)
Another advantage is that the steam is pumped without the need for pumps, either by gravity due to the low density or by the pressure difference during steam generation and expansion.
This eliminates the need for pumps and thus reduces the consumption of electrical energy.
If, on the other hand, the water is to be reused, 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.
There is also a very small temperature difference between the inlet and outlet of the vapour. This can be a significant advantage wherever small temperature differences are required over a specific heating surface (e.g. press plates). This is put into perspective 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
The 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 pressureless.
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 space above the water level in which the steam can establish itself. This results in the need for a larger apparatus and, in addition, steam generators are usually installed horizontally, since a steam space in vertical design is difficult to realize. If the heat is recovered from hot air or gases, this requires more work, higher costs and more space for the installation of piping etc.
Furthermore, steam is compressible. Therefore, a lot of mechanical energy is accumulated in the steam space, which is caused by compression. This leads to an increased damage potential and may require additional safety devices compared to liquid heat carriers.
Another disadvantage is that during evaporation, minerals and oxygen contained in the water are released and concentrated on the water surface. This can lead not only to corrosion, but also to deposits on the heating surface, the so-called scale, which reduce the heat transfer within the heating surface and the water.
Depending on the temperature on the hot side, this can lead to corrosive or thermal decomposition of the surface material and even damage the heat exchanger tubes.
In combination with the high mechanical energy that accumulates in the steam chamber, a high damage potential can be covered. This means that the feed water quality must be continuously monitored and in most cases demineralisation and degassing of the water is necessary. This in turn causes additional costs for apparatus and operating materials (see chemicals).
In addition, even if chemical water treatment reduces the risk of deposits, minerals are released during evaporation. These minerals have a higher density than water and are concentrated in the form of sludge at the bottom of the steam generator.
The nitrates, which have a slightly lower density than water, are concentrated on the water surface. The deposits at the bottom of the steam generator often have to be released by blowing them off.
The nitrate concentration is measured with a conductivity electrode and accordingly released 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, demineralised and deoxidised water, which leads to energy losses, as the fresh water must be heated.
The return of the condensate requires additional care. The use of condensate separators, the correct inclination of the pipes, etc., represents an additional operating risk. If the system is not properly designed, steam hammering (steam absorbs the water, forms a “lump” and throws it at high speed into a pipe fitting, causing a loud hammering noise and putting a lot of stress on 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 planning of the process, the apparatuses, but also the operation of a steam system requires highly qualified and experienced specialists to ensure safe and trouble-free operation.
Examples of processes where steam heat recovery systems can be used;
- Steam jacket heaters are commonly used in process plants to heat equipment such as tanks, boilers, dryers, reactors and glass lined vessels. With direct steam injection, the heater provides 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 method 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 creating a vacuum by means of a vacuum pump to reduce the chamber pressure below the vapour pressure of water.
Humidification by means of steam humidifiers is used when a certain level of humidity must be maintained in order not to preserve the material properties and to ensure a pleasant 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 stream.
- The steam generated by waste heat recovery can be used directly in a steam turbine to generate electricity.
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