What Factors Affect the Heat Dissipation Efficiency of Dry Cooler?

What Factors Affect the Heat Dissipation Efficiency of Dry Cooler?

Influence of Design Parameters on Heat Dissipation Efficiency

1. Heat Transfer Area and Fin Structure

Fin Density and Height: The denser and higher the fins are, the larger the contact area between the air and the coil, and the higher the heat dissipation efficiency. But too dense fins may increase the air flow resistance (such as wind speed reduction), need to be optimized through fluid dynamics calculations (typical fin spacing of 2 ~ 3mm).

Coil arrangement: Fork row (staggered arrangement) has stronger air disturbance than parallel row (parallel arrangement), and the heat transfer coefficient can be increased by 10%~15%, but the resistance is also increased accordingly.

2. Fan performance and air volume

Fan type and speed: axial fans are suitable for large flow and low air pressure scenarios (e.g., ship's cabin), while centrifugal fans are suitable for high air pressure environments; fan speed has a direct impact on the air volume (air volume ↑→heat dissipation efficiency ↑), but it needs to be matched with the power of the motor (too high a speed may lead to a surge in energy consumption).

Wind pressure and flow field uniformity: the distance between the fan outlet and the air inlet of the dry cooler is too close, which will lead to short-circuiting of the airflow, and it is necessary to ensure that the air flows uniformly through the fins through the design of the deflector plate (the uneven flow field can make the local heat dissipation efficiency drop by 30%).

3. Tubes and Surface Treatment

Thermal Conductivity: Copper tubes (thermal conductivity 400W/m・K) are more efficient than aluminum tubes (200W/m・K), but aluminum tubes are lightweight and low-cost, and the difference in thermal conductivity is often compensated for in marine applications by increasing the area of the fins.

Anti-corrosion coating impact: part of the anti-corrosion coating (such as zinc-aluminum coating), although the thickness of only 50 ~ 100μm, but may reduce the thermal conductivity of 5% ~ 8%, the need to balance between anti-corrosion and heat dissipation (such as the use of thin nano-coating).

Running conditions and the impact of cooling medium

1. Coolant parameters

Inlet temperature and flow rate: the higher the inlet temperature of the coolant, the greater the temperature difference with the air, the higher the theoretical efficiency of heat dissipation; but the flow rate is insufficient (such as pipeline clogging), the coolant in the coil to prolong the residence time, the exit temperature increases, resulting in a decline in the efficiency of the heat dissipation (it is recommended that the flow rate fluctuation control within ±10%).

Physical properties of the coolant: the concentration of the aqueous solution of ethylene glycol affects the specific heat capacity and viscosity (e.g., the specific heat capacity of 50% concentration is about 3.3kJ/kg・K), and too high a concentration will increase the viscosity and deteriorate the fluidity, so it is recommended that it be deployed according to the designed concentration (usually 30%~50%).

2. Generator set load

The higher the load, the higher the heat generation power of the generator set, which requires the dry cooler to enhance the heat dissipation power (e.g., increase the speed of the fan or connect the modules in parallel). If the load exceeds the upper limit of the dry cooler design (e.g., the rated heat dissipation power is 1000kW, but the actual heat dissipation power is 1200kW), it will lead to a sudden drop in the heat dissipation efficiency and over-temperature of the equipment.

Direct effect of environmental factors

1. Air temperature and humidity

Ambient temperature: the higher the air temperature, the smaller the temperature difference with the coolant, the lower the heat dissipation efficiency (e.g., the inlet temperature of the coolant is 80℃, and when the ambient temperature rises from 25℃ to 40℃, the heat dissipation efficiency may drop by 20%).

Influence of humidity: high humidity environment air thermal conductivity is slightly higher (25 ℃, 90% humidity thermal conductivity of about 0.026W/m・K), on the heat dissipation has a weak role in promoting, but the humid air is easy to lead to condensation on the surface of the fins may accelerate the adhesion of salt particles (ocean-going vessels need special attention).

2. Air velocity and pollutants

Head wind velocity: Ideal head wind velocity is 2~3m/s, if the wind velocity is too low, the air is not sufficiently exchanged with the fins, and if it is too high, the resistance increases and the energy consumption increases (for every 1m/s increase in the wind velocity, the power of the fan may increase by 50%).

Pollutant adhesion: Salt spray, sand, dust, oil and so on in the marine environment adheres to the fin surface, forming an insulating layer (such as 0.5mm salt scale can make the heat dissipation efficiency decreased by 10%), need to be cleaned regularly.

Maintenance and failure on the impact of thermal efficiency

1. Scale and blockage

fin scale: salt particles, dust accumulation leads to air flow cross-sectional area reduction, wind speed reduction (such as scale so that the cross-sectional area of the air ducts to reduce the 20%, the wind speed may fall by 15%), while the thermal resistance increases (fin surface thermal resistance ↑ ¡ú thermal efficiency ↓).

Coil internal scaling: the coolant may generate calcium carbonate scale after long-term use (especially when the water hardness is too high), the thermal resistance of the inner wall of the coil increases (e.g., 1mm scale can make the heat conduction efficiency drop by 40%), which needs to be removed by chemical cleaning.

2. equipment abnormality and loss

fan failure: impeller wear, belt slack or motor speed will lead to wind volume decline (such as fan efficiency decreased by 30%, cooling efficiency synchronized decline), need to regularly check the dynamic balance and transmission system.

Leakage of coolant: Insufficient flow rate due to low liquid level or change of concentration (e.g. glycol concentration increases due to evaporation of water) will affect the heat dissipation efficiency (immediate replenishment is required when the leakage exceeds 5%).