The sewage heat exchanger is the most important part of the sewage source heat pump system.

The heat transfer coefficient of the sewage heat exchanger is small or the heat transfer area is insufficient, which cause the heat pump main engine condition to deteriorate, the low efficiency, the insufficient output. The heat exchanger's pressure capacity is insufficient, to cause the Large deformation to internal part connection, stress concentration, fatigue damage, finally the leakage of water. The fact that heat exchanger blocking capacity is not good, or resistance is too large will increase the system pump consumption. After blocked, if heat exchanger structural design has not convenient clean up maintenance measures, it will cause the burden of operation and maintenance.

In fact, the cost of sewage heat exchanger costs about 15% of the entire heat pump room, which is not the main investment part, but plays a vital role. Therefore, this paper suggests that investors do not have to save; because increasing about 20% of the heat transfer area is just to increase their total investment by 3%, but it saves future operating costs.

Recently, the main heat exchanger is shell and tube heat exchanger, wide flow channel plate heat exchanger; wide flow tube tube heat exchanger is this area.

Design Method of Sewage Heat Exchanger.

1 sewage heat exchanger of the basic requirements and characteristics

Due to the special nature of this sewage itself such as: high risk of congestion, viscosity, easy to scale, corrosive, etc. The following basic requirements of sewage heat exchanger are necessary:

(1) Must be slightly larger sewage flow section.

(2) Must be smooth and smooth flow path.

(3) Must ensure that the heat transfer form closer to the pure counter current state, requiring the process cannot be "channeling water", "short circuit."

(4) Require the heat exchanger structure to have the measures of opening, installation and cleaning. Safety and efficiency is the first.

(5) Must have a certain pressure capacity.

(6) Sewage heat exchanger should have a certain degree of corrosion resistance or corrosion resistance. The weld cannot be too long.

Failure to meet the above six requirements of the heat exchanger is not a good sewage heat exchanger.

2 the universal relationship of sewage heat exchanger structure

The first form of heat exchanger used in sewage source heat pump systems is the shell and tube heat exchanger, which complies with the above six basic requirements for sewage heat exchangers. Using 2.5 ~ 3 mm thick ordinary seamless carbon steel pipe, practice has proved that under anaerobic conditions, it can be used for about 15 years without corrosion. The heat transfer tube diameter of the shell and tube heat exchanger is mainly related to the filtration size of the front end resistance machine, which needs to be determined scientifically. Generally, it is about 20 mm. In order to achieve a small temperature difference between the countercurrent, the heat exchanger must also be designed specially. Figure 1 is the practical application of the shell and tube heat exchanger. So far, shell and tube sewage heat exchanger is the most successful sewage heat exchanger.

Some people think that the plate heat exchanger heat transfer coefficient is very high, so they would like to simply apply to the sewage heat transfer, but the fact that the simple improvement of the plate heat exchanger is simply not feasible. Figure 2 is the result of the traditional plate heat exchanger with increased the plate spacing for heat transfer. The traditional plate heat exchanger is a failed sewage heat exchanger. At present there is a so-called "wide flow channel heat exchanger" technology, the basic idea is to increase the size of the sewage channel until the dirt can pass smoothly, which is contrary to the compact design principles of heat exchanger.

                       Figure 1 shell and tube sewage heat exchanger

                                  Figure 2 plate heat exchanger

    Figure 3 wide flow channel plate and round tube sewage heat exchanger

The so-called wide flow channel heat exchanger has two forms, one is a wide flow channel plate heat exchanger, as shown in Figure 3 left; the other is a wide flow tube tube heat exchanger, as shown in Figure 3 right.

Wide flow channel plate heat exchanger is a fully welded plate heat exchanger, with a very long weld. The main technical defects of flat panel heat exchanger currently are the poor heat transfer effect, poor pressure capacity, high risk of rupture and leakage, serious clogging. Wide flow tube tube heat exchanger is actually a shell and tube heat exchanger with a large size (80 ~ 100 mm) heat transfer tube. Wide flow tube tube heat exchanger is designed to solve the flat pressure bearing capacity. It does not meet the compact, efficient and economical requirements of heat exchangers.

Selection of sewage heat exchanger in addition to heat transfer safety, but also must consider the price. The difficulty of steel consumption and processing is the two factors that determine the cost. While make sewage heat exchanger tender, the heat transfer area and the price per unit area must be considered as the primary assessment indicators. The heat transfer area is the real heat exchanger characteristic parameter, does not changing with the working condition, easy to measure and the acceptance. Bidder to buy the heat exchanger is not considering the heat transfer, in fact, buy heat exchange area. It is not recommended to use "resistance" and "heat transfer coefficient" as the primary assessment parameters, because the resistance and heat transfer coefficient is not the characteristics of the heat exchanger parameters, while they are mainly determined by the external conditions, that is, by the designer or the operator Rather than by the supplier, and the resistance and heat transfer coefficient is not easy to verify the verification. "Resistance" and "heat transfer coefficient" can only be used as auxiliary restrictive parameters.