Heat Transfer - Chenyi Heat Exchanger 2 007-12-16 21:49:441. Classification of Heat Exchangers

Classified by use: heater, cooler, reboiler, condenser, condenser, etc.

According to the principle of heat transfer and the method of realizing heat exchange, there are two kinds of heat exchangers: interwall heat exchanger and hybrid heat exchanger.

Types of Interwall Heat Exchangers

There are three types of wall heat exchangers: tube type, plate type and fin type.

1. Tube heat exchanger: snake-tube heat exchanger (immersion snake-tube heat exchanger, spray heat exchanger)

Casing heat exchanger and tube heat exchanger

Tubular heat exchanger is the most widely used heat exchanger in chemical industry.

Main advantages: the unit volume has large heat transfer area, good heat transfer effect, simple structure, large operating flexibility, can be made of a variety of materials, applicability is strong, especially in high temperature, high pressure and large-scale devices using tube-and-shell heat exchangers.

When the temperature of two fluids exceeds 50 C, thermal compensation measures should be taken because thermal stress may cause equipment deformation, pipe bending or even rupture.

According to the different thermal compensation methods, the main forms of tubular heat exchangers are as follows:

Fixed tubesheet (fixed tubesheet heat exchanger with heat compensation ring)

The tube sheet and shell at both ends of the heat exchanger are integrated, which makes it difficult to clean the shell side. Therefore, it is required that the fluid in the shell side be clean and scale-free, and the temperature difference between the two fluids is less than 70 C.

U-tube heat exchanger

Each tube of U-tube heat exchanger is bent into U-shape. The inlet and outlet of fluid are installed on both sides of the same end. The head is divided into two chambers with a partition board. Each tube is flexible.

It is suitable for high temperature and high pressure occasions. It is difficult to clean the pipe. Therefore, it is required that the fluid in the pipe be clean and not scaling.

Floating head heat exchanger

One end of the tubesheet is not connected with the outer shell, and the end is called a floating head.

Easy to clean and repair, the application is more common, but the structure is more complex and the cost is higher.

There are a series of standards for the above-mentioned tubular heat exchangers, which can be selected.

Specification models usually indicate type, shell diameter, heat transfer area, pressure to withstand and number of tube paths.

For example: FA600-130-16-2 heat exchanger

FA: Floating head type A, heat exchange tube is 19 *2mm.

Orthogonal triangle arrangement. The nominal diameter of the shell is 600 mm, the nominal heat transfer area is 130 m2, the nominal pressure is 16 kgf/cm2, and the length of the tube is 2.

2. Plate heat exchanger

(1) Jacket heat exchanger

(2) Spiral plate heat exchanger

Advantages: High total heat transfer coefficient, not easy to scale and plug, low temperature heat source can be used, compact structure.

Disadvantage: Operating pressure and temperature should not be too high, not suitable for maintenance.

(3) Flat plate heat exchanger

Advantages: high heat transfer coefficient, compact structure and detachable structure.

Disadvantages: small processing capacity, low operating pressure, less than 1.5 Mpa, operating temperature can not be too high.

3. Fin Heat Exchanger

(1) Finned-tube heat exchanger

(2) Plate-fin heat exchanger

3. Enhancement of Heat Transfer Process in Heat Exchangers

From the heat transfer rate equation Q=KS Delta TM

Increasing the heat transfer coefficient K, the heat transfer area S or the average temperature difference can increase the heat transfer rate Q. Therefore, the enhancement of heat transfer process in heat exchangers should also be considered from these three aspects:

Increasing Heat Transfer Area: Mainly Increasing Heat Transfer Area per Unit Volume

(2) Increasing the average temperature difference: generally using counter-current operation

(3) Increasing the total heat transfer coefficient is an important consideration in strengthening heat transfer.

To increase the total heat transfer coefficient, it is necessary to reduce the convective heat transfer coefficient, fouling heat resistance and wall heat resistance on both sides of the tube wall. The main way is to reduce the larger thermal resistance.

General: The wall thermal resistance will not become the main thermal resistance.

Fouling thermal resistance: scaling should be prevented and scaling layer should be removed in time. For scaling-prone fluids, the flow rate can be increased and flow in the pipe.

Convective heat transfer resistance: The heat transfer coefficient can be improved by increasing the flow rate and disturbance to the fluid.

IV. Design and Selection of Tube Heat Exchangers

The core of the design and selection of tubular heat exchanger is to calculate the heat transfer area of the heat exchanger. To calculate the heat transfer area, it is necessary to know that K and Delta tm, K values are related to the structure, size and flow channel of the equipment.

TM is related to the final temperature of the two fluids. Therefore, many problems need to be considered in the design and selection of heat exchangers.

Problems to be considered in the design or selection of tubular heat exchangers

Choice of Fluid Flow Channel

Selection of Fluid Flow Velocity

DETERMINATION OF TEMPERATURE AT TWO END OF FLUID

In general design, the temperature difference between the two ends of cooling water can be 5-10 C.

Specification and Arrangement of Pipes

Adoption of China's Series Standards for Tube-and-Tube Heat Exchangers

The length of standard pipes is 6 meters for the pipes of 25 *2.5 mm and 19 *2 mm, so the reasonable length of pipes should be 1.5 meters, 2 meters, 3 meters and 6 meters. Most of them are 3 meters and 6 meters long.

L/D is 4-6.
Determination of Pipe Size and Shell Size

In order to increase the flow velocity in the pipe, multi-pass can be used, but at the same time, the flow resistance increases and the average temperature difference decreases. When the correction coefficient of temperature difference is less than 0.8, multi-shell path can be adopted.





Baffle baffle

Aim To improve the convective heat transfer coefficient in shell side, the most commonly used one is circular flange baffle.

Determination of shell diameter

Material selection

Calculation of Resistance (Pressure Drop) of Fluid Passing through Heat Exchanger

2. Selection and design calculation steps of tubular heat exchangers

Estimation of heat transfer area and primary selection of heat exchanger type

(1) Determine the flow passage of two fluids in heat exchanger

Calculate heat transfer according to heat transfer task

Determine the temperature of the fluid at both ends of the heat exchanger, calculate the qualitative temperature, and determine the physical properties of the fluid.

According to the temperature difference between two fluids, the type of heat exchanger is determined.

Calculate the average temperature difference and determine the shell length or adjustment according to the principle that the correction coefficient of temperature difference is not less than 0.8.