Optimization Design Method of Plate Heat Exchanger

2.1 Improve heat transfer efficiency

Plate heat exchanger is a wall-type heat transfer heat exchanger, hot and cold fluid are transferred through the heat exchanger plate, fluid and plates are directly in contact with each other, and the heat transfer mode is conduction and convective heat transfer. The key to improve the heat transfer efficiency is to improve the heat transfer coefficient and logarithmic mean temperature difference。

1). Improving the heat transfer coefficient of the heat exchanger is at the same time to improve the heat and heat on both sides of the plate surface heat transfer coefficient, reduce the thermal resistance of the dirt layer, choose high thermal conductivity of the plate, reduce the thickness of the plate in order to effectively improve the heat transfer.

a. Improve the surface heat transfer coefficient of the plate

Since the ripple of the plate heat exchanger can make turbulence of the fluid at a small flow rate, a higher surface heat transfer coefficient can be obtained. The surface heat transfer coefficient is related to the geometrical structure of the plate ripple and the flow state of the medium。

b. Reduce the thermal resistance of the sludge layer

The key to reduce the thermal resistance of the sludge layer is to prevent the plate from scaling, and when the sheet fouling thickness is 1 mm, the heat transfer coefficient is reduced about 10%. Therefore, we must pay attention to monitor the heat exchanger on both sides of the hot and cold water quality, to prevent the film scaling, and to prevent the debris attached to the plate.

c. Select plates with high thermal conductivity

Plate materials can use austenitic stainless steel, titanium alloy, copper alloy and so on. Stainless steel thermal conductivity is good, and thermal conductivity is of about 14.4W / (m^.k), with high strength, good stamping performance, which is not easy to be oxidized. The price of it is lower than the titanium alloy and copper alloy, most used in heating, but its resistance ability to chloride ion corrosion is poor.

d. Reduce plate thickness

The design thickness of the plate is not related to its corrosion resistance, but related to the pressure bearing capacity of the heat exchanger. Plat thickening can improve the heat pressure capacity.

2). Increase logarithmic mean temperature difference

The plate heat exchanger process has countercurrent, downstream and mixed type. Under the same conditions, the logarithmic mean temperature difference is the largest in the countercurrent, the minimum is the downstream, and the mixed flow is between the two. The method of increasing the logarithmic mean temperature difference is to use countercurrent or near countercurrent a mixed flow，as much as possible to increase the temperature of the hot side fluid and to reduce the temperature of the cold side fluid.

3). Identification of inlet and outlet pipes

The greater the temperature difference of the medium, the stronger the natural convection of the fluid, the more obvious the impact of the formation of the retention zone, so the media inlet and outlet position should be carried out by the hot fluid and the cold fluid in different way, in order to reduce the impact of the belt, Improve heat transfer efficiency.

2.2 Reduce the resistance of the heat exchanger

Improving the average flow velocity of the medium in the inter-plate flow channel, can improve the heat transfer coefficient and reduce the heat exchanger area. But improving the flow rate, will increase the heat exchanger resistance, increase the circulating pump power consumption and equipment costs. When the hot and cold medium flow is relatively large, you can use the following ways to reduce the resistance of the heat exchanger, and to ensure a higher heat transfer coefficient.

1). Use hot mixing plate

On both sides of hot mixing plate are the same corrugated geometric structure, according to the corners of the corrugated angle the plate is divided into hardboard H and soft board L, the angle is generally greater than 90, for the hard board; less than 90 for the soft board. Combine soft and hard board can form HH, HL, LL three flow channels to meet the needs for different conditions.

2). Use asymmetric plate heat exchanger

Asymmetric plate heat exchanger according to the cold and heat fluid heat transfer characteristics and pressure requirements, change the two sides of the board waveform geometry, to form different cross-sectional area of the plate heat exchanger. The heat transfer coefficient of this is small and the pressure drop is greatly reduced.

3). Use multi-process combination

When the hot and cold medium flow is large, you can use multi-process combination of arrangements. In small flow side, using of more processes to improve the flow rate, accesses to get higher heat transfer coefficient. The large flow side uses less flow to reduce the heat exchanger resistance.

4). The heat exchanger bypass pipe

When the hot and cold medium flow is large, you can install the bypass pipe in the large flow side between the heat exchanger inlet and outlet, to reduce the flow into the heat exchanger to get low resistance. For ease to adjust, a regulating valve should be installed on the bypass pipe.

2.3 Rubber gasket material and installation

1). Material selection

Water to water heat exchanger, hot and cold media on the rubber gasket is non-corrosive. The key to select rubber gasket materials is temperature and sealing performance, rubber gasket material can be selected according to the literature.

2). Installation options

The installation of Rubber gasket is commonly adhesive type and buckle type.

2.4 Reasonable selection of plate material

Stainless steel sheet may produce corrosion failure phenomenon such as pitting, crevice corrosion, stress corrosion, intergranular corrosion, uniform corrosion, etc. Stress corrosion rate is higher.

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