Today, there are two main types of heaters. The first type is called surface, and the second - mixing. The first type of device is manufactured by the Saratov Power Engineering Plant . They make PND for turbine units, the power of which is in the range from 50 to 300 MW. If the heat exchange surface is up to 400 m 2 , then these are low pressure heaters for thermal power plants. If you increase this parameter to 800 m 2 , then in this case they can be used for turbine installations at nuclear power plants.
Device Overview
It should be noted that if the surface of the HDPE is more than 550 m 2 and they are of the surface type, then they can be operated in conjunction with installations designed for 300 MW or more. Mixers with the same dimensions can be operated with installations ranging from 200 MW.
It can be added that the Krasny Kotelshchik software company produces equipment of both the surface type and the mixing one. Low pressure heaters for nuclear power plants from this manufacturer have a surface area of 500 m 2 .
The main purpose of this device is to heat the condensate in a regenerative way. Most often, the condensate is taken from the steam spent in the turbine. However, it is also possible to take condensate, which is formed after the stage of cooling the superheated steam.
Device Performance Criteria
Like any other technical device, PND has a main parameter characterizing its operation. Such a parameter was temperature heating, or, in other words, under-heating of water. The value of this characteristic is influenced by several other parameters. A rather strong influence will be exerted by such a factor as contamination of the tubes of the heating surfaces of the low-pressure heater. The second significant factor is the presence of unwanted inclusions in the steam space of the heater. Such inclusions are non-condensable gases, as well as air. Most often, these volatile substances can penetrate into the heater through leaks in the connection of pipelines, for example through water-indicating glasses and other nodes that are under vacuum. As for gases, they enter the heater along with the selection from the turbine.
PND zones
Low pressure heaters have special systems that are designed to remove all unwanted volatile substances. The main among them is the suction system.
The steam that enters the turbine of the heater from the selection may have an elevated temperature. In order to use these excess heat and cool the condensate of the heating steam below the saturation temperature, the entire heat transfer surface is divided into several structural zones:
- The first zone is steam cooling. In this section of the HDPE, the wall temperature will be higher than the saturation temperature. In addition, it is in this zone that the process that occurs when the steam is cooled, which is called convective heat transfer, will take place. Another characteristic feature of the zone is the heating of the liquid, which flows in the tubes higher than the saturation temperature.
- The next section is the condensate cooling area. In this case, the site is characterized by the fact that convective heat transfer is carried out, but already when the condensate is cooled, which was released when steam was introduced.
- The last section is the vapor condensation zone. Everything is simple here, heating steam condenses on this site.
Description of PND with SZEM
If the principle of operation of the low-pressure heater is simple and understandable, then the combined system is more complex, but at the same time more optimal design. In this case, the distinguishing feature is that the device is connected by means of a vacuum pipe to the place of extraction of steam, after which they enter the equipment of the mixing type. Most often, such models are used only in sufficiently powerful plants with a capacity of 200 to 800 MW.
A distinctive feature of all devices manufactured at SZEM is that they all have a vertical design, as well as a flange-type connector on the body. It can be added that since 1985 on all products that were manufactured, flat-type flanges were replaced by collar ones.
PND TKZ device
Low pressure heaters (PND) are produced in accordance with TKZ for turbines that use organic substances as fuel. Their design features are as follows:
- First, all parts of the pipe type are connected to the housing by welding.
- Secondly, the location of the flange type connector may vary. These connectors may be located on water chambers. At the same time, they are usually located higher than the tube boards, and also higher than the nozzles for both supply and removal of condensate. This is quite advantageous, since it allows not to disconnect the pipeline if repairs are necessary. The second arrangement is lower than the tube boards. It is also worth noting that some heaters are made even without such connectors.
- Thirdly, the fact that anchor-type bonds do not extend beyond the boundaries of the water chamber is particularly prominent.
It is also worth highlighting such a feature that some heaters go along with the built-in sections of the OP and OK.
Mixer Heater
High and low pressure heaters are not exactly the same thing. PND mixing type significantly differ in their design. The main difference is that the mixing type does not have a surface that is designed for heat transfer. In addition, it becomes possible to more effectively use the heat of steam, since there will be no such criterion as underheating caused by the temperature difference.
VD Heaters
It can be noted here that the LDPE system can be single-threaded or multi-threaded. In the case of a single thread, everything is simple. Liquid heating will be carried out in one group of heaters. As for the multithreaded system, most often these are two, rarely three groups of LDPEs that are located in parallel. The main characteristic for such devices was the working pressure of water in the pipes. It is determined using the full pressure of the feed pumps. For example, thermal power plants are characterized by a maximum working pressure of steam in the LDPE of 7.0 MPa, feed water pressure can reach 38.0 MPa. As for nuclear power plants, here the indicators of these characteristics are respectively 2.8 MPa and 9.7 MPa.
The design of the LDPE is somewhat similar to a surface-type low-pressure heater, since the system has one housing, which is also divided into three working zones. To date, only four systems that use high-pressure heaters in their composition have received special distribution.