Finned Tube Heat Exchangers for Air Heating with Steam

Finned Tube Heat Exchangers for Air Heating with Steam

A finned tube heat exchanger is a device that enhances heat transfer efficiency by adding fins (ribs) to the surface of the heat exchanger tubes, and is especially suitable for scenarios where there is a large difference in the heat transfer coefficients of the fluids on both sides (e.g., air heated by steam). The principle of operation is as follows:

Hot side (steam): High-temperature steam flows in the tube, releasing latent heat through condensation and transferring heat to the tube wall.

Cold side (air): Low-temperature air flows horizontally outside the tube, absorbing heat through the fins and the tube wall, and the temperature rises.

Role of fins:

Increase the heat transfer area on the air side to make up for the low heat transfer coefficient on the air side (the thermal resistance on the air side usually accounts for more than 90% of the total thermal resistance).

Accelerate air turbulence, reduce the thickness of the boundary layer, and further enhance the heat transfer efficiency.

Key Design Parameters and Selection Points

Steam Parameters

Pressure and temperature: Steam pressure determines the pressure strength of the base tube (e.g., low-pressure steam ≤ 0.7MPa, medium-pressure steam 0.7~3.5MPa), and the temperature affects the heat resistance of the fin material (e.g., aluminum fins are usually applicable to ≤ 200℃).

Condensation rate: Trap discharge needs to be matched to avoid condensate retention leading to reduced heat transfer efficiency or risk of water hammer.

Air Side Design

Flow rate: The air flow rate is generally controlled at 2~8m/s (too low a flow rate is prone to dust accumulation; too high a flow rate increases the resistance loss).

Circulation area: Calculate the spacing of tube rows according to the demand of air volume (horizontal spacing S₁=2~3 times the tube diameter, vertical spacing S₂=1.5~2 times the tube diameter).

Fin spacing and height:

Conventional scenario: fin spacing 5~10mm, height 10~25mm (dense fins are optional for low dust content).

High dust environment (e.g. grain drying): fin spacing ≥15mm to prevent dust accumulation and clogging.

Material selection

Base tube: Carbon steel tube (low cost) for low corrosive steam side, stainless steel tube for high humidity or corrosive steam (e.g. sulfurous steam).

Fin:

Aluminum fins: lightweight, good thermal conductivity, suitable for dry air, low cost but easy to corrode.

Copper fins: best thermal conductivity, corrosion resistance, mostly used for low-temperature steam (e.g., air conditioning systems).

Steel fins: high strength, high temperature resistance, suitable for high pressure steam or air containing particles.

Resistance Calculation

Air-side pressure drop: Calculated by empirical formulas (e.g. Kays & London formula), to be controlled within the allowable range of the fan head (usually ≤500Pa).

Pressure drop on the steam side: The resistance to steam flow is small, and the main concern is the smoothness of condensate discharge.

Application Scenarios and Typical Cases

Industrial field

Plant heating: using boiler steam to heat air, through the finned tube heat exchanger made into a heater, to provide hot air for the workshop.

Drying process: For example, in food processing, steam heats the air and then sends it to the drying kiln to dry grains, medicinal herbs, etc. (need to be matched with dust removal device).

Chemical reaction preheating: before the entrance of the reactor, steam is used to heat the air to the specified temperature (e.g., the preheating section of catalytic combustion device).

Civil and HVAC

Centralized heating system: Steam boiler heats indoor air through finned tube radiators (mostly found in large buildings in the north).

Fresh air preheating for air conditioning: In winter, fresh outdoor air is preheated through steam heat exchangers before being fed into the air conditioning unit to reduce energy consumption.

Agriculture and Storage

Grain drying: Steam heats the air and then passes into the drying tower to reduce the moisture content of grains (e.g., corn and wheat drying).

Greenhouse greenhouse warming: steam heat is transferred to the air through finned tube heat exchangers to maintain the temperature inside the greenhouse.