What is a heat exchanger? A summary of its role and features

1. The role of heat exchangers

As mentioned at the beginning, a heat exchanger is a device that transfers heat (exchanges heat) from a warm fluid to a cold fluid, and the fluid used can be water or ambient air.

When installed in a chiller for cooling purposes, the cold chiller water removes heat from the target liquid inside the heat exchanger.

There are many types of heat exchangers depending on the target fluid, cooling medium, structure, etc., so it is important to carefully consider the installation environment, size, maintainability, etc. when selecting one.

2. [By type] Advantages and disadvantages of major exchangers

Here we will explain the types and features of the most commonly used heat exchangers.

■ ①．Air-cooling type heat exchanger

The structure of an Air-cooling type heat exchanger is mainly composed of a tube bundle (fins and tubes, etc.) and a fan. The liquid to be cooled is passed through the tube bundle, and the fan blows air through the tube bundle, cooling (dissipating heat) the liquid due to the temperature difference between the liquid and the air.

Since they do not require any infrastructure other than the surrounding air and power supply source and can be installed anywhere, they are sometimes installed outdoors.

Advantages of air-cooled heat exchangers

• ・Few restrictions on installation location
• ・Low running and maintenance costs, and environment friendly

Disadvantages of air-cooled heat exchangers

• ・If the temperature difference between the ambient air and the liquid is small, the cooling efficiency decreases and it is not possible to cool the liquid below the temperature of the ambient air.
• The device itself is large

■ ②．Fan coil unit

Cool air is generated by blowing air toward a coil through which cooling water is circulated. It is also possible to switch circulating water in the coil to hot water to generate hot air.

Advantages of fan coil units

• ・Installation location is relatively flexible, with ceiling-mounted and floor-standing models available

Disadvantages of fan coil units

• ・Requires equipment to send cooling water

■ ③．Brazed plate heat exchanger

The structure allows high and low temperature liquids to flow alternately between stacked heat transfer plates, allowing for efficient heat exchange and making the most of even the slightest temperature difference.

Brazing is mainly done with copper, but in areas where clean specification are required, such as the pharmaceutical and semiconductor industries, heat exchangers with stainless steel or nickel brazing are used.

Advantages of brazed plate heat exchangers

• High pressure performance
• ・Good heat exchange efficiency
• - The smallest size among general heat exchangers
• ・Low cost

Disadvantages of brazed plate heat exchangers

• ・Cannot be disassembled, making maintenance difficult
• ・Clogs may occur in the flow path

■ ④．Gasket plate heat exchanger

The basic structure is similar to that of a brazed plate, in which two liquids are passed between stacked heat transfer plates to exchange heat, but the major feature of this type is that the plates are separated by rubber gaskets and the entire unit can be disassembled.

Both sides are sandwiched between fixed plates and held in place with bolts, and it is possible to increase cooling capacity by replacing gaskets and plates or adding plates.

Advantages of gasketed plate heat exchangers

• ・Can be disassembled for easy maintenance
• ・ capacity can be increased by adding more plates
• ・High heat exchange efficiency
• ・Relatively small

Disadvantages of gasketed plate heat exchangers

• ・Larger than the brazing type
• ・Maintenance space is required at the installation location

■ ⑤．Shell and tube heat exchanger

It consists of a body (shell) containing heat transfer tubes, each of which exchanges heat by passing a liquid through it.

The structure can be designed to minimize pressure loss and can also handle high viscosity fluids, so it is also used in hydraulic equipment and chemical plants.

Advantages of Shell and Tube Heat Exchangers

• ・Can handle a wide range of conditions, including low and high temperatures, low and high pressures, heating and cooling, evaporation and condensation
• Some models can be disassembled
• ・Relatively easy maintenance

Disadvantages of Shell and Tube Heat Exchangers

• -Larger size compared to plate type

■ ⑥．Immersion type heat exchanger

Heat exchange is performed by submerging a tubular heat exchanger directly into the tank of the liquid to be cooled. No circulation path for the liquid to be cooled is required, making it easy to install.

Advantages of throw-in type heat exchangers

• ・Easy to implement
• ・Low cost
• ・Can be designed to fit tank sizes

Disadvantages of throw-in type heat exchangers

• ・A mixer is required due to poor conduction efficiency
• ・It is difficult to predict the amount of heat exchanged
• ・Condensation may occur and mix with the liquid being cooled.

■ ⑦. Jacket tank

It has a double structure in which the inner tank is wrapped in an outer tank (jacket part), and temperature-adjusted water is circulated through the outer tank (jacket part) to cool, heat, and keep the inner tank warm.

They are often made of stainless steel and are easy to clean, so they are also used in food and pharmaceutical manufacturing processes.

Advantages of jacketed tanks

• ・Easy maintenance

Disadvantages of jacketed tanks

• ・Due to poor conduction efficiency, a stirrer is required if necessary.
• ・It is difficult to predict the amount of heat exchanged

3. List of heat exchanger features

We have compiled a list of the features of the representative heat exchangers we have introduced. Choose the one that best suits your intended use and installation environment.

4. Key points for increasing the heat exchange capacity of a heat exchanger

capacity of a heat exchanger is greatly affected by three factors: the heat transfer area, the overall heat transfer coefficient, and the logarithmic mean temperature difference, as shown in the formula below.

The greater the amount of heat exchanged, the higher capacity of the heat exchanger.

[Equation for heat exchange]
Q=A×U×ΔT
Q: heat exchange amount (kW), A: heat transfer area (m²), U: overall heat transfer coefficient (kW/m²/K), ΔT: logarithmic mean temperature difference (K)

Here are some tips to increase the heat exchange capacity.

■ Point 1. Increase the heat transfer area

The heat transfer area is the surface area of the component that transfers heat, such as the tube bundle through which the cooling water flows. The larger the heat transfer area, the greater the amount of heat exchanged, so it is more effective to increase the number of tube bundles and plates in the heat exchanger.

However, the size of the heat exchanger increases in proportion to the amount of heat to be exchanged, which creates limitations depending on factors such as the installation location.

■ Point 2. Increase the overall heat transfer coefficient (heat transfer efficiency)

The overall heat transfer coefficient (heat transfer efficiency) is the ease with which heat is transferred throughout the entire device. The way heat is transferred also depends on the material and condition of the heat exchanger, so keep the following points in mind to increase heat transfer efficiency.

• ・Use a heat exchanger made of a material with high thermal conductivity
• - Make the heat exchanger wall thinner
• ・Remove dirt from the heat transfer surface
• - Creating unevenness on the heat transfer surface
• ・Increase the flow rate

■ Point 3. Increase the logarithmic mean temperature difference

The logarithmic mean temperature difference is the logarithmic mean of the temperature difference between the high-temperature fluid and the low-temperature fluid inside the heat exchanger. The greater the temperature difference between the two fluids that are exchanging heat, the greater the amount of heat exchanged. Therefore, in the case of cooling, the lower the temperature of the cooling medium, the greater the amount of heat exchanged.

In addition, the amount of heat exchange can be increased by flowing low-temperature fluid and high-temperature fluid in opposite directions (countercurrent).