[Mechanical Equipment Technology Edition] Daily Question 201708029 – Refrigeration Design

An evaporative condenser is a heat exchange device used for cooling cold storage, and it is composed of components such as a fan, condensing coil, heat exchange fins, and a housing. An evaporative condenser, also known as an evaporative cooler or cooling (condensing) unit, is a device that makes use of the fact that some of the water sprayed outside the coil evaporates, thereby absorbing the heat from the high-temperature gaseous refrigerant inside the coil; this process causes the refrigerant inside the coil to gradually change from a gas state to a liquid state. Working principle: The evaporative condenser is the main heat exchange device in a refrigeration system. Its working principle is as follows: The superheated, high-pressure refrigerant gas discharged by the compressor in the refrigeration system passes through the condensing coils in the evaporative condenser, where the high-temperature gaseous refrigerant exchanges heat with the spray water and air outside the coils. That is, the gaseous refrigerant enters the exhaust pipe through the upper opening and is gradually condensed into liquid refrigerant as it moves downward. The high wind force generated by the accompanying exhaust fan ensures that the spray water covers the surface of the coil evenly; with the help of this wind force, the heat exchange efficiency is greatly improved. The spray water, as its temperature rises, partially turns into gas; the latent heat of vaporization of water is used to carry away a large amount of heat by the wind. The water droplets in the hot gas are captured by an efficient dehydrator, and together with the rest of the water that has absorbed heat, they are dispersed into the heat exchange layer of the PVC spray nozzles, where they are cooled by the passing air. Once their temperature drops, they return to the water tank, where they continue to circulate via the circulation pump. The water evaporated into the air is automatically replenished by the water level regulator. Heat exchange tubes come in various forms; the most commonly used ones include carbon steel tubes coated with zinc (which can be either round or elliptical in shape), aluminum alloy tubes, and stainless steel tubes (including 314/316 round tubes and corrugated tubes). Among these, carbon steel tubes coated with zinc are the earliest type of evaporative condenser to have been used, and they account for the majority of applications in China. They are primarily used for the condensation and cooling of high-pressure gases, with a pressure range of 0–30 MPa. Aluminum alloy evaporative condensers represent a new generation of energy-efficient products, suitable for the condensation of refrigerants used in ice makers, with a pressure range of 0–5 MPa. Stainless steel tube evaporative condensers are mainly used in chemical plants for the cooling and condensation of corrosive gases. The casing of refrigeration evaporative condensers is generally made of galvanized steel sheet coated with paint; however, due to its poor corrosion resistance, coated aluminum-zinc steel sheets have gradually been used instead. The latest material in use on the market is aluminum-magnesium-zinc coated steel sheet, which possesses the advantages of stainless steel – it does not rust, has an attractive appearance, and is easy to process. Operation principle: The spray water is pumped by a water pump from the water collection tank to the spray pipes at the top of the evaporative condenser, where it is sprayed through nozzles onto the outer surface of the condensation tubes, forming a thin layer of water. Part of the water in this layer absorbs heat and evaporates into water vapor, while the remaining water falls back into the water collection tank to be reused by the water pump. The axial flow fan forces air to be drawn in from the top and the lower part of the side walls, flowing through the coil; meanwhile, the saturated hot and humid air is discharged into the surrounding atmosphere. The water droplets contained in this hot and humid air are trapped by a water collector, thereby effectively preventing their loss into the atmosphere. The water vapor lost to the atmosphere is replenished with cooling water in the system, which is controlled by a float valve. 1. The condensing coil is made from ultra-long, specially designed steel tubes, which are subjected to strict testing under a pressure of 2.5 MPa in water, and the entire coil is heat-dip galvanized at a high temperature of nearly 430°C. 2. The cabinet is made of high-quality galvanized sheets, either imported or domestically produced; electrostatic spraying can be used for coating, and the water collection tray can be made of stainless steel. 3. Special high-efficiency, low-noise axial flow fans are used for the blowers; these specialized fans have a long range of delivery, which helps to prevent hot air from circulating back. 4. The water pump features low power, high flow rate, and low head; it is designed for outdoor use, and a descaling device can be installed as per the customer’s requirements. 5. The water distribution system, high-efficiency water collectors, and cooling fillers utilize nozzles with high flow rates and anti-clogging properties, enabling precise calculation for a continuous and even water coverage; this prevents the formation of dry spots on the pipe walls. These components are made from high-quality PVC material, which is resistant to aging and offers low air resistance. The groove-shaped water collectors ensure that the water loss rate remains below 0.001%. The cooling fillers have a honeycomb-type cross-flow structure, making them suitable for use in circulating water cooling systems. 6. Low-flow cooling circulating water evaporates within the condensing coil, and gaseous ammonia condenses into a liquid state inside this coil; hence, it is called an evaporation-type condenser. Intense evaporation makes evaporative condensers more prone to scaling than other types of condensers, and there is no ideal method for descaling them. An evaporative condenser with severe scaling loses its advantage of saving electricity. Only high-quality cooling water is suitable; careful consideration should be given when selecting it! Advantage 1: The operating cost of the system is low. Having a condensation temperature within 8.3°C of the wet-bulb design temperature is highly practical and economical. As a result, the compressor consumes at least 10% less power compared to other cooling tower/condenser systems, and 30% less power than air-cooled condenser systems. The power consumption of the fan is similar to that of fans in cooling tower/condenser systems, and it is approximately one-third of the power consumption of fans in air-cooled condensers of similar specifications. Due to the lower head and reduced water flow, the power required for the water pump is approximately 25% of that needed in conventional cooling tower/condenser systems. 2. Reduced initial investment: The evaporative condenser integrates the cooling tower, condenser, circulation tank, circulation pump, and piping together, thereby reducing the need for separate devices such as a cooling tower and circulation pumps as well as piping. This also lowers the costs associated with handling and installing individual components in the cooling tower/condenser system. Since evaporative condensers make efficient use of the evaporative cooling heat exchange mechanism, they can effectively reduce the heat exchange area, the number of fans, and the power consumption of the fan motors. 3. Space savings: The evaporative condenser saves valuable space by combining the condenser coils with the cooling tower, and it does not require large water pumps and pipelines as is the case with cooling tower/condenser systems. An evaporative condenser requires only about 50% of the windward area of a similarly specified air-cooled condenser.

It mainly depends on the temperature and flow rate of the cooling medium, the condensation area, and the type of condenser, among other factors