Dry Cooler for Organic Rankine Cycle (ORC) System

Dry Cooler plays a key role in Organic Rankine Cycle (ORC) system. Its core function is to act as a condenser to condense the low-pressure gaseous organic workmass into liquid state after expansion work to complete the closed loop of the cycle. Since the ORC system relies on low boiling point organic media (e.g., R245fa, R134a, pentane, etc.) to recover low-grade heat (e.g., industrial waste heat, solar energy, geothermal energy, etc.), the design of the dry cooler needs to be matched in depth with the characteristics of the media, the conditions of the heat source, and the environmental factors in order to ensure the efficiency and stability of the system.

The basic cycle of the ORC system includes evaporation → expansion → condensation → compression of four links, the role of the dry cooler focuses on the “condensation” stage:

When the high-pressure gaseous organic work by the expander (turbine) after the work, the pressure and temperature is reduced to low-pressure gaseous work;

dry cooler through the air forced convection (or natural convection) to absorb the heat of the workpiece, so that the gaseous workpiece; dry cooler through the air forced convection (or natural convection) to absorb the heat of the workpiece, the heat of the gaseous workpiece. Dry cooler through the air forced convection (or natural convection) to absorb the heat of the workpiece, so that the gaseous workpiece condensed into liquid;

liquid workpiece by the workpiece pump pressurized and re-fed into the evaporator, to complete the cycle.

Core objective: to control the condensation temperature of the mass in a reasonable range (usually 5-15℃ higher than the ambient air temperature), to ensure that the mass is fully liquefied, and at the same time to minimize the condensation pressure loss, so as to avoid affecting the efficiency of the expander and the suction conditions of the mass pump.

The low-grade heat source characteristic of ORC system (usually the heat source temperature is 50-200℃) and the physical properties of organic substances put forward different design requirements for the dry cooler from the conventional industrial cooling:

1. Adaptation of heat transfer characteristics of organic substances

Compared with water or traditional refrigerants, organic substances have low boiling point, low thermal conductivity, high viscosity (when in the liquid state), etc., and need to be targeted to optimize the heat transfer structure:

fin+tube Fin + tube design: by increasing the surface area of fins (finning ratio is usually 10-30) to make up for the defects of low thermal conductivity of organic materials (e.g., the thermal conductivity of R245fa is only 1/20 of water).

Optimization of phase change heat transfer: the mass undergoes a phase change from gas to liquid in the condensation process, which requires the design of a reasonable flow channel structure (e.g., countercurrent arrangement) to strengthen the heat transfer coefficient in the phase change zone (usually the heat transfer coefficient of the phase change is 3-5 times that of the single-phase flow).

Pressure loss control: ORC system mass flow rate is low (due to the small density of the mass), the dry cooler flow resistance needs to be strictly controlled (usually ≤ 5kPa), to avoid additional consumption of the mass pump power.

2. Adapt to a wide range of ambient temperature fluctuations

The efficiency (net output power) of the ORC system is directly related to the “evaporating temperature - condensing temperature difference”: the larger the temperature difference, the higher the system efficiency. The condensing temperature of a dry cooler is significantly affected by the ambient air temperature and needs to be adaptable to a wide temperature range:

High temperature environments (e.g. above 35°C in summer): the heat transfer capacity needs to be improved to avoid a sudden drop in system efficiency due to high condensing temperatures (e.g. above 40°C) (e.g. for every 5°C rise in condensing temperature, ORC efficiency may drop by 3%-5%). Frequency control of multiple fans (e.g. EC fans) is often used to dynamically adjust the airflow with the ambient temperature (increasing fan speed when the temperature rises).

Low temperature environment (e.g. below -10℃ in winter): need to prevent solidification of the work material (e.g. the solidification point of pentane is - 130℃, the risk is low; however, the solidification point of some work materials such as R123 is - 30℃, so need to be vigilant). The design can use “minimum air flow protection” or electric tracer (localized heating) to ensure that the substance remains liquid after condensation.

3. Anti-fouling and long-lasting stability

ORC systems are often used in industrial environments (e.g., cement plants, waste heat recovery in steel plants) or outdoor scenarios (e.g., solar thermal power generation), where the air may contain dust, particulate matter or corrosive components, and the dry cooler needs to be able to withstand fouling:

Fin design: Serrated or corrugated fins are used (not easy to accumulate dust compared to flat fins), and the spacing of fins is adjusted according to the ambient dust concentration ( The fin spacing is adjusted according to the ambient dust concentration (usually 3-5mm, but the spacing is increased to 6-8mm when there is a lot of dust).

Material selection: Base tube is made of corrosion-resistant alloy (e.g. 316 stainless steel), fins are made of galvanized aluminum or anticorrosive coating (e.g. epoxy coating) to avoid corrosion of the process material or the environment (especially when the process material contains traces of acidic components).

Cleaning convenience: reserved flushing interface, support for regular high-pressure water or compressed air cleaning, to maintain the efficiency of heat transfer (ash accumulation can reduce the efficiency of heat transfer by 10% -20%).