Steam cooler
Steam coolers and steam saturators are mainly used in pipeline networks in the steam generating and consuming industries, e.g. B. Systems of energy supply companies (EVU) or combined heat and power plants are used. In high pressure and medium pressure lines with strongly superheated steam as well as in low pressure lines with only slightly superheated steam or saturated steam , steam coolers and steam saturators keep both the pressure and the temperature of the steam constant.
The load reduction and power consumption on a turbine require temperature increases, e.g. B. in a power plant, to be compensated by cooling via an injection cooler. However, discontinuous working methods of the steam consumers have a changing effect on the steam status of the individual rails of a steam network. In both cases, the changes in the steam state can be compensated for via bypass stations with pressure reduction and cooling. Especially when steam is used for heating purposes, a steam saturator is usually integrated directly in front of the consumer, since saturated steam or only slightly superheated steam ensures the best heat transfer.
Injection of cooling water or condensate
The injection of finely divided cooling water or condensate into the steam flow is an effective method for steam cooling, even up to the temperature of saturated steam. In this mixing process, the injected cooling water is heated by the steam, so that the cooling water is evaporated and superheated while the steam itself is cooled. The required injection quantity is determined from the equality of the enthalpies and the sum of the mass flows (Fig. 1).
The process of evaporation and mixing takes time and takes place with steam flowing in the pipeline downstream of the injection point. In conventional steam power plants , the injection cooling system is used to control the steam temperature and to protect downstream temperature-sensitive components. Since the injection water has to be brought to high pressure, but does not take part in the absorption of heat and energy conversion, the use of an injection cooler reduces the efficiency.
Regulation of the injection cooler
Once the injected cooling water has completely evaporated and mixed with the superheated steam, the temperature can be measured and used as an actual value to regulate the injection quantity. For this purpose, the necessary distance between a temperature transmitter and the injection nozzle must be determined for the "injection cooler" control circuit. Droplets that have not yet evaporated or hot steam strands can lead to incorrect measurements. In order to avoid this, the distance between the temperature transmitter and the injection nozzle is selected to be greater than the actual evaporation and mixing section (Fig. 2) in all operating states.
For the dimensioning of the distance between the temperature transmitter and the injection nozzle, based on practical experience, the maximum steam speed in the steam line is usually decisive. The evaporation and mixing section must be determined for them, as this results in the distance between the temperature sensor and the injection nozzle.
The distance between the injection point and the temperature measuring point results in a “ dead time ” for the control loop that is dependent on the steam speed and is inversely proportional to the steam speed. If the steam speed decreases, the dead time increases. As a result, the control loop tends to develop undesirable oscillations. The permissible deviation from the temperature setpoint therefore determines the minimum steam speed in the low-load range .
Influences on evaporation and mixing
Several parameters influence the necessary length of the evaporation and mixing section. The droplet size generated by the nozzle during the injection and the droplet distribution over the pipe cross-section is such an influencing variable: the smaller the droplets and the more evenly their distribution over the entire pipe cross-section, the shorter the evaporation path.
Pressure atomizer nozzles with a hollow cone or full cone spray pattern are often used for use with injection coolers. With these nozzles, the water is initially set in rotation around the nozzle axis by a swirl body and then reaches the nozzle opening via a conical vortex chamber. The droplet size generated is determined by the following factors:
- Generated spray angle
- Pressure drop across the nozzle
- Diameter of the nozzle opening
While the spray angle in the range from 45 ° to 135 ° has only a minor influence on the droplet size - a larger spray angle produces only slightly smaller droplets under otherwise constant conditions - the pressure drop across this nozzle has a decisive effect on the size of the droplets. The droplet size generated is roughly inversely proportional to the pressure drop and throughput.
In contrast, the droplet diameter generated is approximately directly proportional to the diameter of the nozzle opening. Here is an example: The mean droplet diameter for a nozzle with a diameter of 2 mm with a pressure drop of 30 bar is approximately 0.2 mm. With a nozzle with a diameter of 5 mm and a pressure drop of 0.5 bar, the droplet diameter is almost 1 mm.
Shortening the evaporation distance
In order to achieve small droplet diameters and thus to shorten the evaporation distance, two-substance nozzles with a gaseous propellant are usually used (Fig. 3). In injection coolers for steam cooling, steam with a higher pressure is used as motive steam, which reduces the droplet diameter by more than a tenth compared to pressure atomizer nozzles.
Even with a high steam speed at the injection point with a strongly turbulent steam flow, the evaporation and mixing distance can be shortened, since the injected droplets are further broken up by the turbulence. To shorten the evaporation and mixing section, the pipe cross-section can be reduced by a short distance.
In contrast, if the steam speed in the low-load range is below about 5 m / s, there is a laminar steam flow, which greatly slows down the evaporation. There is also practically no mixing over the entire pipe cross-section. In such a case, there is a possibility that the distance between the sensor, which is sufficient for maximum steam velocity, is too small.
Extension of the evaporation route
Unintentional lengthening of the evaporation path is caused by wetting the pipe wall by outwardly directed nozzles - especially when the water or condensate exits at high speeds, because a film of water on the pipe wall or a puddle of water on the pipe base with horizontal pipes evaporate very slowly. Applications are known in which even a sensor distance of 30 meters from the injection nozzle did not lead to a satisfactory control result. Such problems are, however, with additional mixer components such. B. diversions, perforated cages or orifice rings and an early and effective drainage of the pipeline to get a grip.
Calculation of the evaporation and mixing section
Various calculation methods are available to determine the necessary length of the evaporation and mixing section. Very complex applications justify the use of a special computer program, as it was developed at the Technical University of Delft and which enables, for example, the calculation of the droplet size, the droplet path and the droplet evaporation time for certain nozzle shapes.
For other applications, calculation diagrams in accordance with the publication “VGB R 540” (steam cooling in thermal power plants) have proven effective. In addition, with the help of the diagram shown in Fig. 4 (applicable to specific nozzle assemblies for a steam speed of 40 m / s), the required distance between the temperature sensor and the injection point for a specific operating case can be determined very easily with the required accuracy.
The diagram is applied as follows: To determine the sensor distance from the injection point, follow the inclined line from the superheating temperature of the uncooled steam to the bottom right to the intersection with the horizontal line of the superheating temperature of the cooled steam. If you follow the perpendicular to the x-axis, you can read off the required minimum distance between the injection point and the measuring sensor - in this case a little more than 14 meters. For other maximum steam velocities, the distance between the sensors increases or decreases in the ratio of the speed to 40 m / s. The determined value can also be adjusted for other nozzle and cooler types using additional correction factors.
Injection cooler designs
The injection cooler usually consists of a protective tube located in the steam line and several nozzles that open radially and in several rows one behind the other into the protective tube. The protective tube is used to prevent erosion damage to the inner wall of the pipeline from injected water droplets. The injection water is taken from behind or from an intermediate stage of the boiler feed pump and fed directly to the nozzles.
Different injection cooler designs cover different operating and application conditions. The optimal solution for the respective application is determined by structural conditions and also economic considerations. Often a simple solution turns out to be completely sufficient; On the other hand, continuous operation, high thermal stress, rapid changes in load cases and the need for a wide control range justify higher expenditure.
Another criterion is the question of whether a pressure reduction should also take place with the steam cooling. Solitary injections with practically no or negligibly small pressure loss are suitable for this. With regard to shorter evaporation and mixing distances, some designs accept a slight pressure loss. Steam cooling can often also be combined directly with pressure control (steam forming station).
literature
- H. Bartscher: Industrial valves . 3rd edition. Vulkan-Verlag, 1990.
- IF Gaballah: Investigations into Optimal Evaporation. In: warmth. Vol. 85, Issue 1.
- P. Krebs: Diameter is crucial. Measurement possibilities and limits of superheated steam cooling by injecting water. In: MM machine market. Issue 28, July 1990.
- P. Krebs: Complex. System components for superheated steam cooling. In: MM machine market. Issue 32, August 1990.
- JJC van Lier, CAA van Paarsen: Overview of the research work “Injection Cooling ” at the Technical University of Delft. In: VGB power plant technology. Issue 12, December 1980.
- VGB guideline steam cooling in thermal power plants, VGB R540, edition 2004.
Web links
Steam and Condensate Plant Manual - Steam Cooling