Heat Transfer Principles and Selection Principles of Finned Tubes

Fin tubes, i.e. extended surface tubes. As the name suggests, finned tubes are processed with many fins on the original tube surface (either the outer surface or the inner surface), so that the original surface is expanded and a unique heat transfer element is formed.

Why use finned tubes? What role does processing fins on the original surface play? To answer this question, we need to start with some basic principles of the heat transfer process.

First, we need to understand a definition of heat transfer: the heat exchange between a solid surface and the fluid in contact with it is called convection heat transfer. The convection heat transfer we are most familiar with is the heat exchange between the outer surface of the radiator and the air. Life experience tells us: the larger the radiator area, the higher the surface temperature (that is, the greater the temperature difference between the surface temperature and the air), and the longer the heating time, the greater the heat transfer and the warmer the room. This shows that the convection heat transfer is proportional to the heat transfer area, the temperature difference, and the time. In order to compare the strength of convective heat transfer under different circumstances, we need to define a physical quantity: called “heat transfer coefficient”. The heat transfer coefficient refers to the convection heat transfer per unit area, unit temperature difference (temperature difference between the wall and the fluid), and unit time. Its unit is “J”/(s.㎡.℃) or W/(㎡.℃). The convection heat transfer coefficient is commonly represented by the symbol “h”.

The size of the heat transfer coefficient mainly depends on the following factors:

1. Types and physical properties of fluids: For example, water and air are completely different, and their heat transfer coefficients are very different;

2. Whether the fluid undergoes phase change during the heat exchange process, that is, whether it boils or condenses. If a phase change occurs, the heat transfer coefficient will be greatly increased;

3. It is also related to the flow rate of the fluid and the shape of the solid surface.

Fin tubes are classified according to their working principles

At present, the most common and widely used type at home and abroad is the dividing wall type. The design and calculation of other types of heat exchangers are often based on the dividing wall type heat exchanger. As a general equipment, heat exchangers are classified according to their working principles and can be divided into partition type, hybrid type and regenerative type.

When finned tube is used in evaporator of low temperature refrigeration system working under low temperature condition (below 0℃), there is a common problem of frost on its surface. The effect of frost formation on the performance of heat exchanger is manifested in the reduction of heat transfer coefficient and the increase of air resistance, and the reasonable structure of heat exchanger should reduce the influence of both. When frost begins to form, the surface roughness of the evaporator increases, causing the heat transfer area to increase, while the gas flow rate also increases, so the heat transfer coefficient K increases in the initial frost formation, but with the continuous thickening of the frost layer, the thermal resistance of heat transfer increases, and the final heat transfer coefficient K decreases.

When the air flow through the evaporator, due to the water vapor in the air continuously deposited on the surface of the finned tube, the relative humidity of the air decreases, and the amount of frost on the surface of the finned coil gradually decreases along the direction of the air flow, so the frost of the first few rows of finned tubes in the evaporator is more serious, and the frost of the last few rows of tubes is relatively light.

If the structure of variable pitch fins is adopted, the fin spacing downstream along the wind direction is smaller and smaller, it can maintain its high heat transfer efficiency under frost conditions, and extend its frost flushing time. After the evaporator adopts the variable pitch fin tube structure, when the air is longitudely grazing the staggered fin tube, the staggered distribution of fin tubes makes the upstream fin tube have a flow effect on the downstream fin tube, which strengthens the heat transfer capacity of the front half of the fin tube, and the distribution of the rear fin tube Narrows the flow channel and increases the flow rate, so that the heat transfer of the rear half of the fin tube is also strengthened.

Structure and characteristics of finned tube radiator

The finned tube is made of the whole rolled aluminum tube, no contact thermal resistance, high strength, heat resistance and mechanical vibration, good thermal expansion performance, and has considerable expansion of the heat exchange surface, with this finned tube heat exchanger, the effect is ahead of the series or wound plate heat exchanger.

What are the advantages of hot-dip galvanized finned tube radiators over ordinary finned tube radiators

Hot dip galvanized fin tube radiator is on a steel pipe, the spiral fin is welded by high frequency resistance, the steel strip is evenly wound on the outer surface of the base tube, and then the fin tube as a whole is hot dip galvanized. The galvanized layer can form a special metallurgical structure that can withstand mechanical damage during transportation and use; Every part of the plating can be zinc plated, even in dents, sharp corners and hidden places can be fully protected. It is relatively stronger in corrosion resistance and protection than ordinary fin radiators.

What are the advantages of stainless steel finned tube radiators over carbon steel

The two thermal conductivity properties are not very different, but their main differences are:

Stainless steel: beautiful appearance, super corrosion resistance, expensive, compared with carbon steel material price is several times as much

Carbon steel: cheap but easily oxidized in the air

As to whether the material is stainless steel or carbon steel when designing the heat exchanger, it needs to be selected according to the applicable place.