Room Air Conditioners and Dehumidifiers Basics

By | January 3, 2016

Room Air Conditioners and Dehumidifiers Basics
A room air conditioner is an assembly designed as a unit for mounting in a window or through a wall. Its purpose is to provide cool, or warm, conditioned air to the room.
Some are designed to be used with ductwork, while others are not to be connected to a ductwork. Each of these units is equipped with a refrigeration system. They provide dehumidification, a means for biting and cleaning the air, and in some units a means for ventilating and/or exhausting the air.
The basic function of a room air conditioner is to give comfort by cooling, dehumidifying, filtering or cleaning, and circulating the room air.
Ventilation may be provided by introducing outdoor air into the room and/or exhausting room air to the outside. Comfort may be provided by controlling the temperature of the room through the use of a thermostat.
Room units provide heat by use of resistance elements, by use of a heat pump cycle, or by a combination of both these these methods.
In operation, warm air passes over the cooling coil and gives up heat to the refrigerant inside the coil. This conditioned air is then circulated inside the room by either a fan or a blower.
The heat given up by the room air to the coil evaporates the liquid refrigerant inside the tubes. The evaporated vapor then carries the heat to the compressor, which compresses the vapor and increases its temperature to a point higher than the temperature of the outdoor air.
In the condenser, the hot high-pressure vapor is condensed, giving up the heat from the room air to the outdoor air. The high-pressure liquid refrigerant then passes through the flow control device, which causes a reduction in its pressure and temperature. This cool liquid refrigerant then enters the evaporator coil and the cycle is repeated.
Both the heating and cooling capacities of room units are measured and rated in terms of Btu/hr. There is a wide range of sizes available from approximately 4000 to 36,000 Btu/hr. These units are equipped with electrical cords that may be plugged into standard or special electrical outlets. The majority of room units are designed to operate on 115, 230, or 208 V single-phase current.
The maximum amperage of the 115-V models is generally limited to 12 A because this is the maximum allowable current for a single-outlet, 15-A circuit permitted by the National Electrical Code. Heat pump models are also available and are designed to operate on 208/230-V or 110/129 applications.
Heat pump room units are generally designed for reverse-cycle operation when heating is required. However, some units use electric resistance heating elements for either supplementing the heat pump cycle or for providing the total heating capacity at temperatures below a given point. They are also available with these elements in a regular cooling unit to provide the total heating requirements.

Room Air Conditioners and Dehumidifiers Basics Internal View

There is a range of compressor capacities available from approximately 4000 to 48,000 Btu to be used in room air conditioning units. The design data are available from the various compressor manufacturers with rating conditions for both standard and high-efficiency compressors.
A standard rated room air conditioner compressor has an evaporating temperature of 45°F and a high efficiency compressor has an evaporating temperature of 49°F. Manufacturers of these compressors offer complete performance curves at the various evaporating and condensing temperatures to aid in selecting the proper compressor for the application being considered.
Coils used for these purposes are generally of the tube-and-plate-fin variety or of the tube-and-spine-fin variety. Performance information on these types of coils is generally available from the manufacturers and suppliers. The design characteristics to be considered when selecting these coils are Btu/hr/ft2, dry bulb and moisture content of the entering air, air-side friction loss, internal refrigerant pressure drop, coil surface temperature, air volume, and velocity.

Basically there are three types of flow control devices used on room air conditioning units: (1) the thermostatic expansion valve, which maintains a constant superheat from a point near the evaporator outlet to a point on the suction line; (2) the automatic expansion valve, which maintains a constant suction pressure; and (3) the capillary tube, also known as a restrictor tube. This is the most popular type of flow control used on window air conditioning units. It has a low cost and a high reliability factor, even though it is not the most desirable refrigerant control over a wide range of ambient temperatures.

There are two types of fan motors generally used on room air conditioners: (1) the shaded pole, which is a low-efficiency motor, and (2) the permanent split capacitor motor, which has a higher efficiency rating and requires the use of a run capacitor. The air movers, or blowers, are of two different types: (1) the forward-curved blower wheel and (2) the axial, or radial, flow fan blade. In most applications the blower wheel is used to move small to moderate volumes of air through a high-resistance system, while the fan blades are used to move moderate to high volumes of air in a low-resistance system.

The applied combination of the motor and the air mover is such an important part of the system operation that manufacturers pay particular attention in their selection. The service engineer should always follow the manufacturers’ recommendations when replacing either of these components.

The installation features for room air conditioners vary widely because they can be mounted in a variety of ways. The proper mounting for the given installation should be selected so that the user and the local codes and ordinances can be satisfied. The more common mounting methods are:
1. Inside flush mounting: The inside face of the unit is approximately flush with the inside wall of the building.
2. Balance mounting: The unit is installed approximately half inside and half outside the building.
3. Outside flush mounting: The outer face of the unit is either flush or slightly beyond the outside wall of the building.
4. Special mounting: In casement windows, horizontal windows, office windows with swing units (or swinging windows) to permit window washing, and transoms over doorways.
5. Through the wall mounts or sleeves: These devices are used for installing the window unit chassis, the complete unit, or consoles in the walls of an apartment building, motel, hotel, and residences.
Over the years, these units have become more compact because of consumer demand for minimum loss of window light and minimum projection both inside and outside the wall. This requirement has resulted in the design of a mount that is simple to apply and has an attractive appearance.
There are several types of expandable mounts now available for fast, dependable window unit installation in both single- and double-hung windows, and for windows of the horizontal sliding type. The standard installation kit includes all the parts needed for structural mounting such as gaskets, panels, and seals for weather-tight installation.
The instructions and procedures provide for easy installation and should be followed carefully to assure a safe and satisfactory completion. The NEC requires that adequate wiring and the proper sized fuses be used for the service outlet. The necessary information is generally provided on the instruction sheets shipped with the unit or they may be stamped on the unit near the service cord or in the serial plate data. It is important that the manufacturers’ recommendations be followed for the size and type of fuse.
All window air conditioners are provided with a grounding-type plug cap on the service cord when shipped from the manufacturer. Receptacles with the necessary grounding contact designed to fit the unit service cord plug cap should be used when installing the unit. One type of room unit is the integral chassis design, with the outer cabinet permanently fastened to the chassis. Most of the electrical components are accessible by partly dismantling the control area without removing the unit from the installation.
A second type is the slide-out chassis design. This design permits the outer cabinet to remain in place while the chassis is removed for service.

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