1. Refrigeration Plan
A technical and economic comparison should be made based on the cooling requirements (cooling capacity, cooling method, chilled water temperature, etc.), the water supply conditions in the construction area (water temperature, water quality, water quantity, etc.), and the availability of power and heat sources. This requires both good economic indicators and consideration of local availability.
The selection of refrigeration equipment is a key step in determining the cooling plan. The main types of refrigeration equipment are as follows:
1. Compression Refrigeration Unit
Compression refrigeration units include piston (reciprocating), centrifugal, and screw types.
The main component of a piston (reciprocating) compression refrigeration unit is a piston refrigeration compressor, commonly using ammonia (NH3) and Freon-12 and Freon-22 (F-12 and F-22). This type of equipment is driven by an electric motor, is easy to use, and offers a wide variety of products to meet the cooling requirements of various air conditioners. Therefore, it is widely used. However, it requires a large initial investment, high equipment costs, high power consumption, and relatively complex maintenance and management.
The main component of a centrifugal compression refrigeration system is the centrifugal compressor, and the commonly used refrigerants are Freon-11, Freon-113, and Freon-114 (F-11, 113, and 114). This unit shares the same principle as a piston compressor, but operates only in a rotary motion, resulting in smoother operation, less vibration, and fewer components. It is often integrated with the evaporator and condenser, resulting in a compact unit with a small footprint. Centrifugal compressors offer a large cooling capacity and are therefore only used when the air conditioning system requires a large cooling capacity.
The main component of a screw compression refrigeration system is the screw compressor, and the commonly used refrigerants are Freon-12, Freon-22, and ammonia (F-12, Freon-22, and R717). While currently manufactured screw compressors have slightly lower efficiency than piston compressors, they are widely used in various refrigeration systems due to their simple structure, compact size, high suction coefficient, low exhaust temperature, high single-stage compression ratio, insensitivity to wet stroke, low exhaust pulsation, fewer wearing parts, long column repair intervals, and stepless cooling capacity adjustment. This unit generally exhibits excellent technical and economic performance in refrigeration systems with a capacity range of (120-800) x 10⁵ kJ/h.
2. Absorption Refrigeration Units
Absorption refrigeration units utilize thermal energy for refrigeration. Common types include ammonia-water absorption refrigeration units and water-lithium bromide aqueous solution absorption refrigeration units. The former is suitable for low-temperature freezing; the latter is suitable for air conditioning and cooling, producing cold water for air conditioning or process applications above 4°C. In lithium bromide absorption refrigeration, water is the working fluid and lithium bromide aqueous solution is the absorbent. Its advantages are as follows:
It has a high adaptability to steam heat source pressure. When the steam pressure is 0.02-0.1 MPa, a single-effect lithium bromide absorption chiller can be used, while when the steam pressure is 6.0-1.0 MPa, a double-effect lithium bromide chiller can be used.
Secondly, the equipment is simple. Essentially a combination of heat exchangers, it is easy to manufacture. Aside from low-power shielded pumps and vacuum pumps, it has no moving parts, resulting in low vibration and smooth operation. The equipment operates under vacuum, and the working fluid is odorless, non-toxic, and harmless to humans, making it safe to use and simple to operate and manage. It can be easily automated, with the cooling capacity automatically adjustable within a range of 10% to 100%. It is highly adaptable to cooling water temperatures, allowing it to operate normally even at temperatures of 37-38°C. Furthermore, it saves energy. Under the same cooling capacity conditions, a lithium bromide chiller consumes only about 5% of that of a compression chiller, making it ideal for areas with limited power supply but abundant heat sources. However, lithium bromide absorption chillers also have disadvantages and limitations. The main one is that lithium bromide aqueous solution is highly corrosive to metals when exposed to air. Once corroded, this not only affects heat transfer efficiency and reduces service life, but also causes equipment damage. Therefore, the equipment must be strictly sealed and requires careful attention to operation and management.
3. Steam-Jet Refrigeration Unit
Steam-jet refrigeration units also utilize thermal energy for cooling, using water as the refrigerant and steam as the cooling power. The required steam pressure is 0.2-0.8 MPa. Their main advantages include: the unit operates under vacuum and has no rotating parts other than the pump; the refrigerant is water, which is harmless, safe, and easy to use; the unit is simple and easy to manufacture, and can be fabricated in any machinery factory or repair shop; the initial investment is low, and the unit can be installed outdoors, saving on construction costs; and it can utilize waste heat or heating boilers from industrial enterprises, providing heating in winter and steam cooling in summer.
The main disadvantage of steam-jet refrigeration is its low efficiency. Installing a dedicated boiler or using a small heating boiler for steam supply is uneconomical. It can be used when the chilled water temperature is above 5°C and is more economical above 100°C. Its efficiency varies with the chilled water temperature: the lower the chilled water temperature, the greater the steam consumption and the lower the efficiency. Furthermore, the cooling water consumption is substantial. Under the same cooling capacity per unit, a steam jet chiller requires approximately 1.5 times as much cooling water as a compression or thiocyanate absorption chiller, making it unsuitable for use in areas with water shortages.
The heat and moisture treatment process includes heating, cooling, humidification, and dehumidification. The media used for heat and moisture exchange include water, steam, liquid desiccant, and refrigerant. Heat and moisture exchange equipment can be categorized as direct contact or surface type. Direct contact chillers include water spray chambers and steam humidifiers. Surface type chillers include air heaters, water-cooled surface coolers, and direct evaporative surface coolers.
Direct contact heat and moisture exchange equipment is characterized by the medium exchanging heat and moisture directly contacting the air being treated, typically by spraying it into the air. For example, spraying water at varying temperatures in a water spray chamber can achieve various air treatment processes, including heating, cooling, humidification, and dehumidification. Using steam humidifiers to spray steam can achieve isothermal humidification of the air; using spray equipment to spray liquid desiccant can achieve various dehumidification processes.
Surface-type heat and moisture exchange equipment is characterized by the medium that exchanges heat and moisture with the air not being in direct contact with the air; instead, heat and moisture exchange occurs through the metal surface of the equipment. For example, passing hot water or steam through an air heater can achieve isothermal heating of the air; passing chilled water or refrigerant through a surface cooler can achieve isothermal cooling or dehumidification (i.e., drying) of the air.
It should be noted that electric heaters and air handling equipment using solid desiccant do not involve the aforementioned media involved in heat and moisture exchange. Instead, they use electrical energy to heat the air or the physical and chemical effects of the solid desiccant to absorb moisture from the air. Their operating principles differ from those of direct-contact and surface-type heat and moisture exchange equipment.
Common heat and humidity treatment methods are as follows:
1. Heating Method
By passing hot water or steam through an air heater, the air can be heated in a humidified manner. Air heaters are commonly used for primary and secondary heating in air conditioning systems, providing a large amount of heat and being relatively economical.
Electric heaters heat the air by passing an electric current through a resistance wire. They offer advantages such as uniform heating, stable heat generation, high efficiency, compact design, and easy control. Therefore, they are widely used in air conditioning units and small air conditioning systems. In large air conditioning systems and purification systems requiring high temperature accuracy, electric heaters are often used on the air supply branch pipes for controlled local heating. This is known as tertiary heating or precision heating. However, electric heaters consume a lot of electricity and are therefore not suitable for applications requiring high heating capacity.
2. Humidification Method
Use a conventional steam nozzle to humidify the air. A conventional steam nozzle consists of a tube slightly thicker than the steam supply pipe, with several small holes with a diameter of 2-3 mm. Steam, under pressure from the pipe network, is ejected from these small holes and mixed with the air flowing around the steam pipe, humidifying the air. While conventional steam pipes are simple and easy to manufacture, the ejected steam often carries condensed water droplets, affecting the control of humidification. They are typically used in systems with less stringent humidity requirements.
Dry steam humidifiers are used to humidify the air. These humidifiers feature an insulating steam jacket around the nozzle, through which high-pressure steam is passed. This maintains a high temperature on the nozzle wall, preventing condensation inside the pipe. The steam then passes through the humidifier cylinder, a guide box, a guide pipe, and the humidifier cylinder. Finally, the dry steam is ejected through the nozzle to humidify the surrounding air. These humidifiers are typically used in large air conditioning systems.
Electric humidifiers use electricity to generate steam, which is then mixed directly into the air for humidification, rather than transported through pipes. Electric humidifiers can be categorized as either electric heating or electrode-based, depending on their operating principles. Electrode humidifiers are widely used due to their compact structure and easy control of humidification capacity. However, their disadvantages are high power consumption and the electrodes are susceptible to scaling and corrosion. Therefore, they are preferably used in small air conditioning systems. Currently, packaged air conditioning units often use electrode humidifiers, which are integrated with the compressor, evaporator, condenser, and ventilator in a single housing, making them convenient to use. They humidify the air by spraying circulating water. Spray pumps are installed in both the spray chamber and the surface cooling section. In winter, the circulating water in the spray pool humidifies the air. This method handles large air volumes, consumes little power, and easily maintains humidity levels. It is a commonly used humidification method in centralized air conditioning systems.
3. Cooling and Drying Solutions
In the spray chamber, chilled water at a temperature below the air dew point is directly sprayed onto the air to be treated, creating a heat and moisture exchange process, thereby achieving cooling and drying of the air. This is the most commonly used method in air conditioning.
Water spray chambers offer the following advantages for air treatment:
They offer diverse thermal performance, enabling them to reduce enthalpy, temperature, and humidity, as well as increase heat, temperature, and humidity, maintaining a relatively tight relative humidity in all seasons. The water spray chamber can perform different thermal treatments on the air depending on the water spray temperature.
The equipment is relatively easy to manufacture and can be fabricated on-site or manufactured and assembled by a specialized factory. It requires minimal metal and is relatively inexpensive. It can also serve as an air purifier, removing dust, freshening the air, and improving working conditions.
Water spray chambers have the following disadvantages for air treatment:
The equipment requires a large footprint. The minimum length for a two-row horizontal water spray chamber is 1.9 meters, and for a three-row chamber is 2.5 meters. Surface coolers are generally no longer than 0.6 meters, and water spray surface coolers are only 1.4 meters long. The water system is relatively complex. To adapt to changing outdoor weather conditions, the spray water temperature is typically adjusted by varying the spray water temperature, utilizing a portion of the circulating water. Therefore, every air conditioner must be equipped with a water pump. Furthermore, the water system is an open system, with no pressure on the return water. If the return water cannot flow out by gravity or be used for other purposes, a return water tank or return water pump is required. Water comes into direct contact with air, making it susceptible to contamination and dirtiness, requiring regular drainage and replenishment.
Using a cold water surface cooler to treat air, heat and moisture exchange can achieve cooling and drying of the air. This is also a commonly used method in air conditioning.
Compared to a water spray chamber, a cold water surface cooler has similar air treatment capacity and offers the following advantages:
a. The equipment is compact, requiring less machine room space and facilitating easy installation.
b. The water system is relatively simple. Since the water circulation can be closed, a cold water tank, return water tank, and spray pump can be eliminated. Since the water does not come into direct contact with the air, cross-contamination between the air and water is avoided, significantly reducing the amount of water required to replenish the water system.
c. The water supply system consumes low energy. Each air conditioner’s surface cooler requires no water pump. All resistance losses in the surface cooler are borne entirely by the power station’s chilled water supply pump. Although the surface cooler exhibits significant air resistance, the total power consumption for air processing is generally low.
Cold water surface coolers have the following disadvantages when handling air:
Unlike the diverse air treatment processes of a water sprinkling chamber, they can only handle three processes: enthalpy reduction and dehumidification (wet cooling), enthalpy reduction and equalization (dry cooling), and a portion of the heating process (heating). When humidification is required, a separate humidifier is required, making it difficult to achieve stringent relative humidity requirements. This is the primary disadvantage of surface coolers in air processing.
The equipment’s manufacturing process is complex and cannot be fabricated on-site. It must be manufactured and assembled on-site by specialized factories. It also consumes a large amount of metal, generally non-ferrous metals. Its cost is also higher than that of a water sprinkling chamber. The average cost of eight surface coolers, ranging from 10,000 to 160,000 m³/h, is over 30% higher than that of a three-row water sprinkling chamber. No dust removal or odor removal.
To eliminate these shortcomings and increase heat exchange efficiency, a surface cooler with water spray can also be used.
With a surface cooler with water spray, the water spray is used solely for humidification and dust removal. Its treatment function is similar to that of a water spray chamber and is suitable for applications requiring a closed chilled water system with strict relative humidity requirements, where steam humidification is not possible or is unreasonable (for example, when there is no steam source or a 24/7 steam supply). Direct evaporative surface coolers can also be used to treat air, achieving cooling and drying, a method commonly used in small air conditioning units.
Because the direct evaporative method uses the evaporator in the refrigeration system as the air cooler, it offers simple equipment, a small footprint, and minimal initial investment. However, refrigerant piping suffers from significant losses and requires strict sealing. Therefore, the evaporator should not be placed too far from the chiller. Typically, the evaporator of a single chiller supplies a single air supply system, forming an integrated unit. Direct evaporative surface coolers are difficult to adjust and maintain strict relative humidity, and are generally used in systems where relative humidity requirements are not strictly enforced.
2. Air supply scheme
General air supply methods for air conditioning can be divided into centralized and local types. In addition, the purification system requires primary, medium and high efficiency filters. Combining the dual requirements of air conditioning and purification, the purification air conditioning system can be divided into centralized purification air conditioning system and decentralized purification air conditioning system.
1. Centralized purification air conditioning system
The primary and medium efficiency filters of the air and the heat and moisture treatment equipment (fans, heating, humidification and cooling equipment) are concentrated in an air conditioning room and connected to the high efficiency filters arranged at the indoor air outlet by pipes. This system is called a centralized purification air conditioning system.
The air supply methods of the centralized purification air conditioning system can be in the following forms.
(1) Single fan system
a. In order to prevent polluted air from infiltrating into the purification air conditioning system and re-contaminating the air after the medium efficiency filter, the medium efficiency filter should generally be set in the positive pressure section of the system.
b. In order to prevent the air after the high efficiency filter from being contaminated again, the high efficiency filter should be arranged at the end of the system as close as possible to the air outlet of the clean room. In general, it is not advisable to centrally arrange high-efficiency filters in the air conditioning room or on the air supply duct far from the clean room air supply outlet. c. When the return air contains a high dust concentration or large-sized dust, fiber, etc., a medium-efficiency filter can be installed on the return air outlet or return air duct. d. Under the condition of a relatively tight enclosure structure, in order to prevent outdoor polluted air from entering the clean room through the fresh air inlet after the system stops running, in addition to installing a medium-efficiency filter on the return air inlet or return air duct, an electric sealing valve can be installed on the fresh air inlet duct and interlocked with the fan. When the fan starts, the sealing valve opens and closes when it stops running; or a general regulating valve can be installed on the fresh air inlet duct and an additional medium-efficiency filter can be installed. (2) System for setting up a duty fan When the purification air conditioning system is intermittently running, a duty fan can be installed to prevent outdoor polluted air from entering the clean room through the gaps in the enclosure structure or from the fresh air inlet after the system stops running. The air volume of the duty fan is determined by the number of air changes required to maintain the predetermined positive pressure value in the room; if the exhaust system in the clean room is running all day or the air volume is reduced when not in production, the air volume of the duty fan should also take into account the exhaust volume of the exhaust system.
(3) Parallel centralized system
When multiple centralized purification air conditioning systems are arranged in an air conditioning room, several systems can be connected in parallel and only one fresh air heat and humidity treatment system can be set up. This can reduce the cooling load and heating load of each centralized purification room air conditioning system and is more flexible in operation.
When the system is operating normally, valves 1 and 3 are closed, valves 2 and 4 are opened, and fresh air fan 3 and supply air fan 5 are put into operation; when the system is operating on duty, valves 1 and 3 are opened, valves 2 and 4 are closed, fresh air fan 3 is put into operation, and supply air fan 5 is stopped. In addition, when a system does not need to be operated, valves 1 and 2 or valves 3 and 4 can be closed, and the corresponding supply air fan can also be stopped, while the other systems can be used as usual.
(4) Dual fan system
When the system resistance is large, in order to reduce noise, reduce air leakage and facilitate system operation and adjustment, a dual fan centralized purification air conditioning system can be used if it is reasonable after technical and economic comparison.
Since the two fans are used in series, the efficiency of the dual fan system is lower than when each fan is used alone, and the machine room area is also larger.
(5) Two-stage high-efficiency filter system. For laminar clean rooms, in order to extend the service life of the ceiling high-grade filter, reduce its replacement frequency and improve the indoor air cleanliness, a high-efficiency filter can be added at the inlet of the air supply static pressure box or in the air conditioning room. The disadvantage of this approach is that the system resistance increases and the investment also increases accordingly.
(6) Centralized system with partial air direct circulation A centralized system with partial air direct circulation is achieved by using large centrifugal fans. A centralized system with partial air direct circulation is achieved by using small centrifugal fans in parallel. The number of air changes in a laminar cleanroom is very high. If the air-conditioning room is far away from the cleanroom, a large centrifugal fan or several small centrifugal fans with large air volume, high air pressure, high efficiency and low noise can be connected in parallel to realize the direct circulation of part of the air nearby, shortening the length of the large air duct and saving the space occupied by the air duct.
The small centrifugal fan and the high-efficiency filter are combined into a purification air supply unit, and then the purification air supply unit is used to form a laminar cleanroom. The floor space of the machine room is smaller than that of the civil construction type, and the construction period will be greatly shortened.
(7) Centralized system with silencer. The system is composed of assembled clean rooms, local purification equipment (clean workbench, clean shed, laminar flow hood and self-cleaning device, etc.) and general air-conditioning environment. It is called a decentralized purification air conditioning system.
At present, the most widely used form is the combination of centralized purification air conditioning system and decentralized purification air conditioning system. It has the advantages of both centralized and decentralized systems and can better adapt to the requirements of process production and the objective conditions of construction projects.
2. Decentralized purification air conditioning system
The basic forms of decentralized purification air conditioning system are as follows:
(1) Local purification equipment is installed in the environment of centralized air conditioning system.
(2) Local purification equipment is installed in the environment of decentralized air conditioning system.
Selection of air supply scheme
New construction projects should determine whether to adopt centralized purification air conditioning system or decentralized purification air conditioning system as the air supply scheme based on the area, net height, location and noise reduction and vibration reduction requirements of the clean room after comprehensive technical and economic comparison. Generally speaking, clean rooms with large area, high net height, centralized location and strict noise reduction and vibration reduction requirements should adopt centralized system; otherwise, decentralized system can be adopted.
In addition to complying with the above principles, renovation projects can be handled differently according to the specific circumstances of the project. When the original building has a centralized air conditioning system, medium efficiency filters and high efficiency filters can be added to the air conditioning system or high efficiency filters can be added at the air supply outlet, and the fan speed can be increased or the fan can be replaced to increase the fan volume and pressure to form a centralized purification air conditioning system. Alternatively, localized purification equipment can be added indoors to create a decentralized purification air conditioning system.
If the existing building does not have an air conditioning system, a centralized purification air conditioning system can be added. Alternatively, a purification air conditioner or small air conditioner combined with localized purification equipment can be added to create a decentralized purification air conditioning system.
III. Purification Solutions
1. Whole-Room Purification
Whole-room purification is a purification method that uses a centralized purification air conditioning system to create a uniform cleanliness level throughout the entire room. This is one of the earliest developed methods in cleanroom technology and is still used today. This method is suitable for facilities with large, numerous process equipment and a uniform cleanliness level requirement. However, this method requires high investment, complex operation and management, and a long construction period.
2. Localized Purification
Localized purification is a purification method that uses purification air conditioners or localized purification equipment (such as clean benches, vertical laminar flow units, and laminar flow hoods) to create a specific cleanliness level in a localized area within a general air-conditioned environment. This method is suitable for facilities with small production batches or for technological renovations in existing factory buildings. At present, the most widely used purification treatment method is the combination of full-room purification and local purification. This is a purification method that has emerged in the development of clean technology. It can not only ensure a certain degree of cleanliness in the room, but also achieve a high cleanliness environment in the local area, thereby achieving the dual purpose of meeting the production requirements for a high cleanliness environment and saving energy. For example, in an operating section that requires a cleanliness level of 100, when the production batch is small, as long as a local purification equipment such as a clean workbench or laminar flow hood is used in a turbulent clean room with lower cleanliness, a purification treatment method combining full-room purification and local purification can be achieved.
3. Clean Tunnel
The purification treatment method that forms a tunnel-shaped clean environment with two laminar process areas and a turbulent operation activity area in the middle is called a clean tunnel. This is a typical combination of full-room purification and local purification. It is the purification method currently promoted and adopted, and is also called the third generation purification method.
According to the different equipment that constitutes the clean tunnel, the clean tunnel can be divided into the following forms:
(1) Table-type clean tunnel
This type of clean tunnel is to connect the clean workbenches together and eliminate the side walls in the middle to form a tunnel-type production line required for production. When the process requires vertical laminar flow, a vertical laminar flow bench can be selected. When the process requires horizontal laminar flow, a horizontal laminar flow bench is selected. This purification method is easier to ensure high cleanliness in local spaces than whole-room purification, and because the workbenches are interconnected, cross-contamination can be reduced or prevented. In addition, the architectural requirements are relatively simple, requiring only a turbulent cleanroom environment. The disadvantage is that the clean bench has a fixed size, which makes the operating surface lacking sufficient flexibility. The process equipment must adapt to the size of the bench, and adjustment is also relatively cumbersome.
