High Heat Exchanging Coefficient Brazed Heat Exchangers

1. Basic Concepts of Heat Transfer.

Heat always moves from high-temperature objects to low-temperature objects, and the greater the temperature difference between them, the faster the heat transfer speed. When the temperature difference is zero, heat transfer stops.
There are three basic ways of heat transfer: radiation, conduction and convection.
Radiated heat is transmitted outward in the form of electromagnetic waves.
Thermal conduction is the heat exchange that occurs when objects come into direct contact. In order to improve the heat transfer effect of the heat exchanger, materials with good thermal conductivity such as copper, aluminum, etc. are often used. Stainless steel also has good heat transfer performance.
Convection is a process in which heat is transferred by relative motion of molecules after the fluid itself is heated.
In the heat transfer process of a heat exchanger, several heat transfer methods often occur at the same time.
Because the heat transfer amount of convection heat transfer is large and easy to control, it is the main method of heat transfer in heat exchangers.

The basic formulas for heat conduction and convection heat transfer are: Q=KA(Tg-Td)

In the formula, Q is the heat transfer, the unit is KW
K is the heat transfer coefficient, the unit is W/m2°C
A is the heat transfer area of the heat exchanger
Tg is the temperature on the high temperature side
Td is the temperature on the low temperature side

The temperature difference between high and low temperatures actually changes, so in practice the logarithmic average temperature difference Δt is used to calculate it.

2. Type of Heat Exchanger.

There are many classification methods, but the most important classification method is classification by structural form:

1. Shell and tube heat exchanger: This is the oldest heat exchanger, but it is also the most reliable and practical heat exchanger.

As shown in the figure below, this is a U-shaped tube dry evaporator often used in refrigeration systems. The refrigerant flows inside the tube and water (secondary refrigerant) flows outside the tube.

The liquid refrigerant enters from the lower part of the left end cover, vaporizes after passing through the U-shaped tube, and turns into refrigerant gas and comes out from the upper part of the left end cover.

In order to increase the flow rate of water (refrigerant) and make the refrigerant better contact with the outer wall of the tube, baffles are installed in the shell of the shell-and-tube heat exchanger.

2. Casing-and-tube heat exchanger: Two types of tubes with different diameters are used, which are stacked and wound into a serpentine or circular coil. Refrigerant flows outside the tube and water flows inside the tube. It's low cost, but works erratically.

3. Plate heat exchanger: It is an efficient and compact heat exchanger that has been developed and widely used in recent decades. It is composed of a series of thin metals that are parallel to each other and have corrugated surfaces. Under the same metal consumption, the heat transfer area of plate heat exchangers is much larger than that of shell and tube heat exchangers. Because the fluid flow in the corrugated channels between the heat exchange plates can cause severe disturbances, the heat transfer coefficient is large. For liquid-liquid plate heat exchangers, the heat transfer coefficient can be as high as 2500-7000W/(㎡·℃), which is 2-4 times higher than the shell and tube type. When hot and cold fluids exchange heat in countercurrent, very close temperatures can be obtained. Plate heat exchangers are widely used in industrial sectors such as thermal power, food, medicine, wine making, beverages, synthetic fibers, metallurgy and chemicals, and with improvements in plate shape and structure, its application areas are further expanding.

There are two types of plate heat exchangers: assembled type and integral brazed type. The assembled type is composed of several pressed corrugated metal heat transfer plates, with corner holes at the four corners, as shown in Figure 6-33. Adjacent plates are separated by special sealing gaskets, so that cold and hot fluids flow in from one corner hole respectively, flow between the plates at intervals along the flow channels set by the gaskets and corrugations, and then flow from the other side. A diagonal corner hole flows out, as shown in Figure 6-34. The assembled plate heat exchanger has the advantage of being easy to disassemble and clean, but its pressure resistance is limited. Figure 6-35 shows an integrally brazed plate heat exchanger. The plates of this type of plate heat exchanger are the same as those of the assembled type. The ends of the plates are integrally brazed. It has a high pressure-bearing capacity, but it is inconvenient to clean. You should pay attention when using it. Ensure the fluid is clean. Generally, the heat exchange capacity of a single integral brazed heat exchanger is smaller than that of an assembled one.

Heat transfer plates are the core components of plate heat exchangers. Each heat transfer plate overlaps in a certain order to form a flow channel between the plates. Cold and hot fluids flow in their respective flow channels on both sides of the plate and pass through the plate. The chips are heat exchanged, as shown in Figure 6-34. The surface of the plate is corrugated, and the fluid flow direction is perpendicular to the corrugations, or at a certain inclination angle.

Different forms of plates directly affect the heat transfer coefficient, flow resistance and pressure resistance. The material of the plates is usually stainless steel. At present, heat exchange plates mostly adopt a herringbone shape, as shown in Figure 6-35. It has been reported that assembled plate heat exchangers are used as condensers and evaporators in lithium bromide absorption chillers, while integral brazed heat exchangers have been widely used in small water source heat pump units as evaporators.

1. The change of a substance from liquid to vapor is called evaporation, and the related components are called evaporators; the change from vapor to liquid is called condensation.
The associated component is called the condenser.
A lot of heat is required in these changes.

We all have experienced that when alcohol evaporates on our skin, we feel cold. This is evaporation, which is refrigeration.

For example, 1 kilogram of water is heated from 50°C to 90°C. There is no change in state. The required heat is:
Q=1(kg)x1(kcal/kg.°C) x(90-50°C)=40kcal= 40x1.163=46.52 watts
But turning 1 kilogram of water at 100°C into water vapor at 100°C requires 838.8 kcal = 838.8x1.163 = 9116.5 calories.
The difference is 20.97 times.
This heat is called "latent heat"
For example, the latent heat of vaporization of liquid R22 is 48.51 kcal/kg, and the specific heat of gas R22 is 0.157 kcal/kg°C.
Therefore, the heat absorbed during the evaporation of 1 kg of R22 from 2°C to 7°C is:
Q=48.51 kcal/kg+0.157 kcal/kg°Cx(7-2)°C=48.51+0.785=49.3 kcal/kg
It is obvious that the latent heat in the middle is the main one, accounting for 98%.
In order to make the refrigeration system small, latent heat must be used

2. Refrigeration cycle.

The liquid refrigerant is reduced in pressure in the evaporator and continuously evaporates into steam. At the same time, it continuously absorbs heat from the surrounding medium, causing the temperature of the medium to drop.

These refrigerant vapors are sucked into the compressor and compressed, increasing the pressure and temperature, that is, converting the work into heat. Then it is discharged into the condenser to dissipate heat and condense into refrigerant liquid.

These refrigerant liquids enter the throttle valve, throttle and reduce pressure, and evaporate after entering the evaporator...

As long as the compressor continues to work and perform work on the refrigerant, the refrigerant will continue to evaporate in the evaporator and continuously absorb heat from the surrounding medium, thereby achieving the purpose of refrigeration.

3. How does the brazed plate heat exchanger work in the refrigeration system?

Brazing Operation Manual

Brazed Heat Exchanger Installation Guide 1

Brazed Heat Exchanger Installation Guide 2