Characteristics and principles of air-cooled and water-cooled units

2024-09-05

I. Introduction to Air-cooled Chillers
Air-cooled chiller: One type of chiller. It cools normal temperature water to a certain temperature through the compressor of the chiller to enhance the cooling of molds or machines. Used as a stand-alone unit, the heat dissipation device is a built-in fan. There are mainly three interconnected systems: refrigerant circulation system, water circulation system, and electrical automatic control system.
Refrigerant circulation system: The liquid refrigerant in the evaporator absorbs the heat in the water and begins to evaporate. Eventually, a certain temperature difference is formed between the refrigerant and the water. The liquid refrigerant completely evaporates and turns into a gaseous state. Then it is sucked in and compressed by the compressor (pressure and temperature increase). The gaseous refrigerant absorbs heat through the condenser and condenses into a liquid. After throttling through an expansion valve (or capillary tube), it becomes a low-temperature and low-pressure refrigerant and enters the evaporator to complete the refrigerant circulation process.
Water circulation system: The water pump is responsible for pumping water from the water tank to the equipment that the user needs to cool. After the chilled water takes away the heat, the temperature rises and then returns to the chilled water tank.
Electrical automatic control system: Includes power supply part and automatic control part. The power supply part supplies power to the compressor, fan, water pump, etc. through contactors. The automatic control part includes a thermostat, pressure protection, time delay device, relay, overload protection, etc. which are combined to achieve functions such as automatic start and stop and protection according to the water temperature.
II. Characteristics of air-cooled chillers
Air-cooled water chillers can be used in small systems. Most large units are water-cooled. The initial investment in water-cooled main units is less than that of air-cooled ones.
Air-cooled water chillers adopt air cooling method, eliminating the cooling tower, cooling water pump and pipeline system essential for the cooling water system. This avoids condenser scaling and water pipe blockage caused by areas with poor water quality and also saves water resources. It is the most economical and simple type of machine for maintenance in cold water air conditioning equipment products.
The one-time investment in air-cooled water chillers is slightly higher than that in water-cooled water chillers. The maintenance cost is about lower than that of water-cooled or boiler systems. The noise and volume of air-cooled water chillers are larger than those of water-cooled ones and can only be installed outdoors. Most water-cooled units are installed in basements.
Product characteristics of air-cooled chillers:

The unit compressor selects scroll fully enclosed compressor and electrical control components;
Equipped with various safety protection devices, the performance is stable and the operating status is clear at a glance;
The models have single compressor or multi-compressor combined refrigeration systems;
The whole machine has a box structure, without the need for cooling towers and cooling water pumps. Installation and maintenance are simple and convenient;
III. Basic components of chiller refrigeration system:
Condenser: In the refrigeration process, the condenser plays the role of outputting heat energy and condensing the refrigerant. After the high-pressure superheated steam discharged from the refrigeration compressor enters the condenser, it transfers all the heat absorbed during its working process, including the heat absorbed from the evaporator, the refrigeration compressor and the pipeline, to the surrounding medium (water or air) to be taken away; the refrigerant high-pressure superheated steam recondenses into a liquid. (According to different cooling media and cooling methods, condensers can be divided into three categories: water-cooled condensers, air-cooled condensers, and evaporative condensers.)
Receiver: The receiver is installed after the condenser and is directly connected to the drain pipe of the condenser. The refrigerant liquid of the condenser should flow into the receiver unobstructed. In this way, the cooling area of the condenser can be fully utilized. On the other hand, when the heat load of the evaporator changes, the demand for refrigerant liquid also changes. At that time, the receiver plays the role of regulating and storing the refrigerant. For small chiller refrigeration system installations, receivers are often not installed, but the condenser is used to regulate and store the refrigerant.
Drying filter: In the chiller refrigeration cycle, it is necessary to prevent the entry of moisture and contaminants (oil stains, iron filings, copper filings). The main source of moisture is the trace moisture contained in the newly added refrigerant and lubricating oil, or the moisture brought in when air enters during the overhaul of the system. If the moisture in the system is not completely removed, when the refrigerant passes through the throttle valve (thermal expansion valve or capillary tube), sometimes the moisture will freeze into ice due to the drop in pressure and temperature, blocking the channel and affecting the normal operation of the refrigeration device. Therefore, a drying filter must be installed in the chiller refrigeration system.
Evaporator: The evaporator is a heat exchange device that absorbs the heat of the cooled medium by the evaporation (actually boiling) of the refrigerant liquid. Its function in the refrigeration system is to absorb heat (or output cooling capacity). In order to ensure that the evaporation process can be carried out stably and continuously, the evaporated gas must be continuously pumped away by the refrigeration compressor to maintain a certain evaporation pressure.
Thermal expansion valve: The thermal expansion valve in the chiller refrigeration system is not only a flow regulating valve but also a throttle valve in refrigeration equipment. It is installed between the drying filter and the evaporator in the refrigeration equipment. Its temperature sensing bulb is wrapped at the outlet of the evaporator. Its main role is to throttle and reduce the pressure of the high-pressure normal temperature refrigerant liquid when it flows through the thermal expansion valve, turning it into low-temperature and low-pressure refrigerant wet vapor (mostly liquid and a small part is steam) and entering the evaporator. It vaporizes and absorbs heat in the evaporator to achieve the purpose of refrigeration and cooling.
Refrigerant: Most industrial chillers used in modern industry use R22 or R12 as refrigerants. The refrigerant is the flowing working medium in the refrigeration system. Its main role is to carry heat and achieve heat absorption and release when the state changes.
IV. Differences between water-cooled chillers and air-cooled chillers
Industrial chillers are divided into water-cooled industrial chillers and air-cooled industrial chillers. The difference between them mainly lies in the different condensers.
The condenser of a water-cooled chiller mainly takes away heat through circulating cooling water. Therefore, the condenser of a water-cooled chiller is also often called a water cannon. Different from water-cooled ones, air-cooled industrial chillers use fans for heat dissipation. Usually, finned condensers are used. The fins actually refer to aluminum fins. That is, external aluminum fins are installed at the parts that need heat exchange to achieve efficient heat dissipation, and then the hot air is drawn out by a powerful fan.
So when the workshop temperature is high, in addition to being affected by itself (if the temperature of the condenser is too high, it will directly lead to a high-pressure alarm of the industrial chiller and a decrease in refrigeration capacity), the air-cooled chiller will also have a direct impact on the workshop temperature. The advantages of water-cooled chillers are obvious. Water-cooled industrial chillers need cooling towers and circulating cooling water to cool down. Therefore, to use water-cooled chillers, cooling towers, cooling water circulating pumps, and pipelines need to be installed, which is more cumbersome. Compared with air-cooled industrial chillers, the convenience of movement is inferior. But in most cases, factories are already equipped with cooling water circulation systems.
V. Refrigeration principle
Air-cooled chillers and water-cooled chillers are used for outdoor installation and operation. The air-cooled machine is directly circulated mechanically by air. The condensing coil of the machine directly expels heat and cools by atmospheric air. Cooling machines are similar, except that they spray water mist on the condenser coils to help cool the condenser, making the cooling machine more efficient than traditional air cooling. No remote cooling towers are usually required for these types of air or refrigerators with packaged cooling.
Refrigeration principle of water-cooled indirect cooling in the electroplating industry:
The water in the bottom basin of the cooling tower is pumped to the condenser of the chiller by a water pump to cool the condenser. When it flows back to the cooling tower and sprays down, the water is cooled by the fan on the top of the cooling tower, and then flows back to the bottom basin of the cooling tower. It operates in this cycle.
While the condenser dissipates heat, the refrigerant inside liquefies and then flows into the evaporator in the water tank for evaporation. When evaporating, it absorbs heat, thereby cooling the water in the water tank. The cooled water is pumped to the heat exchanger (separated in the middle, with water on one side and sulfuric acid on the other) through a water pump. Through the heat transfer process, the sulfuric acid is cooled. This is a cycle. The advantage of this product is that it is easy to install and has a relatively longer service life than direct freezing. It is not easily corroded by acid and alkali.
Refrigeration principle of water-cooled direct cooling:
The water in the bottom basin of the cooling tower is pumped to the condenser of the chiller by a water pump to cool the condenser. When it flows back to the cooling tower and sprays down, the water is cooled by the fan on the top of the cooling tower and then flows back to the bottom basin of the cooling tower. It operates in this cycle. While the condenser dissipates heat, the refrigerant inside liquefies and then flows into the evaporator for evaporation. When evaporating, it absorbs heat, thereby cooling the sulfuric acid (tank liquid) in the evaporator. The cooled sulfuric acid is pumped to the electroplating (oxidation) tank. This is a cycle.
Chillers with direct freezing and indirect freezing have the same usage effect, but it is obvious that direct freezing chillers are not as safe as indirect freezing chillers, and the former equipment is easily corroded and has a relatively shorter service life. Direct freezing equipment has a slight price advantage, but compared in terms of service life, indirect freezing chillers are longer than direct freezing chillers. From this, the value of indirect freezing chillers can be calculated. This is also one of the development trends of environmentally friendly and energy-saving chillers.
VI. Four characteristics of chiller units
The COP efficiency of air-cooled chillers is lower, around 2.6 - 3.3. The building area used is generally not more than 10,000 square meters. But it can cool and heat. It is more cost-effective for general shopping malls, small commercial buildings, and office buildings.
The COP of water-cooled chillers is generally relatively high, around 4 - 5.5. It is suitable for large buildings and some process cooling.
In principle, only the condensers are different. Air-cooled chillers cool the refrigerant in the condenser through fans. Water-cooled chillers remove the heat in the condenser through cold water and dissipate it to the atmosphere through cooling towers.
Water-cooled chillers can only cool. If heating is required, other heating methods such as electric heating, steam heating, or hot water boilers are needed.