Natural Convection Evaporators
Natural convection evaporators are often used in applications where low speed air desirable or damage dehydration product possible. The speed of air circulation over the heat-exchange surfaces of these cooling coil is a function of the temperature difference that exists between the evaporator and the refrigerated space. The greater the temperature difference will produce higher rates of air circulation, because the difference between the density of warm and cold air increases. The speed of air circulation also depends on the form, size, configuration and location of the evaporator. Small coils that are one or two rows deep and covers most of the ceiling field of refrigeration space to have produced the best natural circulation. As the depth of the coil increases resistance leads to a free circulation of air is increased. Consequently, die average temperature of space, as the heat rises transfer rate decreases.
Cold air is denser than warm air. Therefore, the natural circulation evaporators, usually mounted high above the floor die refrigerated Cabinet.
Enough space is left between the evaporator and the top of the Cabinet to allow air circulation over the top of the die coil. As a rule, one of ceiling evaporator is often used in refrigerators less than eight feet wide. As the width of the Cabinet increases beyond this limit, two or more evaporators are often used instead of building one big block. This simplifies the production, delivery and assembling of the evaporator. Partitions are used to ensure good air circulation in small cabinets with natural convection in rolls. They are intended to direct the flow of air across the surface of the coil and the entire refrigeration space. Design baffle determines how much of the natural convection currents are created and maintained in the Cabinet.
Screens tend to increase the length of the Cabinet. Width of warm and cold air passages (flue) are based on the Cabinet's width. As a rule, the width of the cold air flue is approximately one-sixth of the cabinets width and the width of the warm air of smoke is about one-seventh of the enclosures. Warm air flue little more due to the large volume of specific warm air. The wide opening allows mass flow rate of hot air and cold air streams to equalize.
To maintain optimum air circulation in the evaporator, the distance separating the coil from the ceiling at least three inches and approaches, as a rule, the width of warm air in the chimney. Vertical side of a partition in direct die a warm stream of air at the top of the evaporator. It should extend approximately one inch taller and three to four inches below the die horizontal axis of the coil. The distance between the side edge of the baffle and evaporator, as a rule, from four to seven inches. Horizontal partitions (coil deck slopes of one to two inches in a foot on the deck to give a direction of a stream of cold air and allow to drain from the coil, and in a trough. Reel deck must be isolated to prevent moisture condense on them underground and spread oneself die product. The distance between coil deck and bottom of the evaporator, as a rule, from two to four inches, die measured at the middle of the deck FORCED CONVECTION EVAPORATORS.
At the beginning of refrigerators used embossed plate evaporators formed into a square and installed in the top corner of the box. These natural convection coils have been replaced with a forced convection of evaporators in today's commercial and residential premises. Forced convection evaporators are known as air coolers, fan coils, chillers and air blower coils. They are built as naked finned tubes or coils, encased in a rugged aluminum enclosure and is equipped with one or more fans. Partitions are used forced convection evaporators die for direct air flow around the Cabinet.
Dry surface coolers are usually mounted on the ceiling of the Cabinet. They discharge air cooled horizontally away from the door of the study. The fan is located in die die center of warm air duct which is formed in the evaporators housing. Speed of the air leaving the die evaporator fan is a function die coils application. Low speed of air coolers have discharge, air velocity below 200 ft/min (61 m/min). These coils are used in applications where the Cabinet humidity should be maintained at a higher value. Low density in diese applications low to prevent frost that forms on the surface of reducing the air flow below the desired level. Low speed coils are used in floral display cases, meat cutting rooms and similar humidity and air speed applications.
The average speed of air coolers are dumping more than low-speed coils and, as a rule, less than 400 m/min (122 m/min). The higher the speed extracts more water in food. Thus, these evaporators are used in vegetables wrapped meat and dairy products, refrigerators, where dehydration does not occur easily.
The standard speed of air coolers have reset velocities greater than the speed of environment coils and less about 600 m/min (183 m/min). These evaporators are used in refrigerators, where packaged products are stored and evaporation of moisture, does not matter. Evaporators with discharge of more than 600 m/min (183 m/min) are used for shock freezing applications and industrial applications.
Large commercial premises have floor mounting styles that work with a dry or wet coils. Wet coils constantly sprayed with brine or antifreeze solution. The wetted external surfaces of the heat exchanger increase the rate of heat transfer to die refrigerant, allowing the temperature to leave coil temperature, which is very close to die saturation temperature of the refrigerant. Due to their heavier construction and mass die liquid spreads, these evaporators fastened to the floor of the Cabinet and the injection of air and vertically. Separators are inserted in the discharge air stream to minimize the transportation of liquid droplets carried away in the stream of air.