What Is the Function of a Compressor in an HVAC System?

What Is the Function of a Compressor in an HVAC System?

The compressor is the heart of the cooling function of heating, ventilation and air conditioning (HVAC) systems. The air conditioner component of a building's HVAC system essentially is a type of refrigerator that cools and dehumidifies the air in the building, ensuring thermal comfort for the occupants. Most HVAC compressors look like a black box with lines running to and from it.

Compressor Function
Air conditioners are devices that transfer heat from an enclosed space to the outside air. The motor-driven compressor in an air conditioning system powers the whole heat-transfer cycle. Air conditioners rely on two facts of nature. One is that heat flows from a high-temperature area to a lower temperature area. The other is that gases always flow from a high-pressure area to a low-pressure area.
Air Conditioning Cycle
The heat-transfer cycle starts as the compressor squeezes the refrigerant. This squeezing action raises its temperature well above that of the surrounding atmosphere. The squeezing action also pressurizes the refrigerant just to its liquefying point so it can flow through the system. The hot, pressurized, liquified refrigerant flows to a condenser coil where it gives up its excess heat to the cooler atmosphere. Typically, a fan blows air through the condenser coil to facilitate transfer of the excess heat.
Expansion Phase
After shedding its excess heat to the atmosphere, the refrigerant flows to an evaporator coil where it expands into a gas at the reduced pressure in the evaporator, which is located in the space to be cooled. This expansion requires heat, which is drawn from the air in the enclosed space that's being cooled. This warms the refrigerant. The cold evaporator also draws humidity from the air. A fan blowing across the evaporator coil facilitates this transfer of heat and humidity. As the compressor pushes hot pressurized refrigerant toward the condenser, it pulls the warm vaporized refrigerant from the evaporator to start the cycle all over again.

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How is humidity controlled with an AC system?

How is humidity controlled with an AC system?
Humidity is becoming more of a concern to building operators and owners. High indoor humidity leads to mold and mildew growth inside the building. The are several methods of controlling indoor humidity. The simplest (and most expensive) method is to connect a humidistat to an electric heater. When the humidity inside the building rises above the humidistat set point, the heater is turned on. The additional heat causes the air conditioning system to run longer and remove more moisture.
A more efficient method of controlling humidity is to use the waste heat from the refrigeration cycle itself. Instead of rejecting the waste heat outdoors, the heat is directed inside when humidity control is required. One form of heat reclaim is called hot-gas reheat or “refrigerant desuperheating” where refrigerant is passed through a heat exchanger located downstream of the cooling coil. The hot high pressure vapor leaving the compressor passes through this heat exchanger prior to entering the condenser coil. This in turn heats the indoor air and again causes the AC system to run longer to meet the thermostat set point. Although more energy is used, this is much more efficient than turning on an electric heater. Another form of heat reclaim is called sub-cool reheat. This strategy takes the warm liquid refrigerant from the condenser and passes it through a heat exchanger located downstream of the cooling coil. Less heat is available using this method because the majority of the heat has already been rejected at the condenser. Since more energy is used to pump liquid (as opposed to a gas) through the heat exchanger it would appear that this method is less efficient than the hot-gas method, however, the liquid in the heat exchanger is sub-cooled in the cold supply air stream which increases the capacity of the air conditioner. Since more capacity is available, the AC units is able to meet the thermostat more quickly.
Heat pipe heat exchangers or run-around coils perform a similar function when humidity control is required. Two heat exchanger are placed in the air stream, one upstream of the cooling coil and the other downstream of the cooling coil. These heat exchangers are connected together with piping. A heat transfer fluid, whether it be water or refrigerant, is either pumped or gravity fed from one heat exchanger to the other. The heat exchanger down stream of the cooling coil (re-heat coil) cools the liquid medium inside the heat exchanger and heats the air passing over the heat exchanger. The cold liquid inside the heat exchanger is moved to the heat exchanger upstream of the cooling coil (pre-cool coil) where it pre-cools the air passing over the heat exchanger and warms the liquid passing through the heat exchanger. The affect of a heat pipe or run-around coil is to reduce the sensible heat capacity of the AC system. The latent capacity of the AC system increases if direct-expansion equipment is used or remains relatively constant if chilled water equipment is used. Since the sensible capacity of the AC system has been reduced, the system must run longer to meet the thermostat set point thereby removing more moisture.

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What type of AC systems are available?

What type of AC systems are available?

Cooling Only Split-System
A split system is a combination of an indoor air handling unit and an outdoor condensing unit. The indoor air handling unit contains a supply air fan and an air-to-refrigerant heat exchanger (or cooling coil), and the expansion device. The outdoor condensing unit consists of a compressor and a condenser coil. Split-systems are typically found in residential or small commercial buildings. These systems have the highest energy efficiency rating (EER) of all the available AC systems. Manufacturers are required to take the EER rating a step further and provide a seasonal energy efficiency rating (SEER) for use by consumers. SEER ratings vary widely and range from 10 to 20. The higher the SEER rating, the more efficient the AC system operates. If heating is required, an alternate method of heating the interior of the building must be used, usually in the form of electric or gas heating.
Cooling Only Packaged-System
A packaged system is a single unit combining all the components described in the split system. Since the unit is a package, it must be placed outside the building and indoor air is “ducted” from the building to the packaged system and back through an air distribution system. These units typically have SEER rating from 10 to 18. If heating is required, an alternate method of heating the interior of the building must be used, usually in the form of electric or gas heating.
Heat Pump
Heat pumps are similar to cooling only systems with one exception. A special valve in the refrigeration piping allow the refrigeration cycle to be operated in reverse. A cooling only system cools the indoor air and rejects heat to the outdoors. A heat pump can also cool the indoor air, but when the valve is reversed, the indoor air is heated. A supplementary electric resistance heater may also be used to assist the heat pump at lower outdoor temperatures. In colder climates, heat pumps require a defrost period. During defrost times the electric heater is the only means of heating the interior of the building. These units are manufactured as either split or packaged systems.
Chilled Water System
In a chilled water system, liquid water is pumped throughout the building to “chilled water coils”. Since the liquid water needs to be at a cold temperature, a “cooling plant” is required. The plant is typically referred to as a chiller plant. Vapor compression equipment in the plant, similar to that described in “How does my AC work”, cool water to a cold temperature and pump the cold water to air-to-water heat exchangers where needed.
Window Air Conditioners
As the name implies, a window air conditioner is typically installed in a window or custom opening in a wall. The Window AC can only cool small areas and are not intended to provide cooling to multiple rooms or zones. These air conditioners are manufactured as cool only or can provide both cooling and heating. An optional damper in the unit can provide fresh outdoor air if necessary.
Packaged Terminal Heat Pump
Packaged terminal heat pumps (PTHP) are are similar to a window-mounted air conditioner. These units are typically installed in a sleeve passing through the outdoor wall of an apartment, hotel, school classroom, etc. PTHPs are completely self contained and require only an electrical connection in addition to the opening in the building shell. They use the outdoor air as the heat source in winter and as a heat sink in summer. They also can provide ventilation air. Flexibility and lower installed cost are the primary advantages of the PTHP. Disadvantages include in-room maintenance, higher operating cost, relatively short life, imprecise "on-off" temperature control, and they can be rather noisy.

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What does the Expansion Valve do in an Air Conditioning System

What does the Expansion Valve do in an Air Conditioning System
The modern air conditioner is quite a marvel of engineering. The complex working of its components is the reason that only professionals can effectively diagnose and repair a malfunctioning air conditioner.
One of the key components allowing an air conditioner to work is a part called the expansion valve. Without this valve, an AC would not be able to draw sufficient heat from the inside of a home.

How the expansion valve helps cool down your home
In order to explain the purpose of an expansion valve requires first explaining the basics of how refrigerant works in an air conditioner. Refrigerant is a heat transference fluid that shifts between liquid and gaseous state, absorbing and releasing heat as it does so. When the refrigerant moves through the outdoor condenser coil, it releases heat to the outside. Cooled down, it then moves toward the indoor evaporator where it absorbs heat.
However, at the point after leaving the condenser, the refrigerant—which is in a high-pressure liquid form—is still too hot to effectively go carry out the process of evaporation that will draw heat out of the indoor air. This is where the expansion valve does its job: the valve is designed to remove pressure from the refrigerant and rapidly cool it down into a cold gas. The valve doesn’t remove heat, only pressure—but removing the pressure has the effect of lowering the temperature of the refrigerant. After it leaves the valve, the refrigerant will be at its the coldest. The refrigerant is now ready to move through the indoor evaporator coil, where it will meet warm indoor air and through evaporation lower the temperature of the air.
Live any component inside an air conditioning system, the expansion valve can malfunction, leading to an AC that won’t provide any cooling. Always call on professionals to look your air conditioner when it encounters trouble: a number of different malfunctions might lurk behind the problem, and it takes training to locate the actual cause. If a broken expansion valve is at fault, the technician will repair or replace it.

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What does a condenser in an ac unit do?

The condenser does just what the name says: in the condenser the hot gaseous refrigerant coming from the compressor is cooled which causes it to condense into a liquid. Compressing a gas makes it much hotter, enough hotter than the outside air on a hot day that it can transfer its heat to that hot outside air  which makes the hot air even hotter, and cools the high pressure gas enough to make it condense into a fairly warm liquid.

The pressurized liquid refrigerant flows to the evaporator, where it is sprayed through a small nozzle into the evaporator (which is kept at low pressure because the refrigerant pump is constantly sucking on this part of the AC unit.)

When the warm high pressure refrigerant liquid sprays into the low pressure evaporator, it - you guessed it - evaporates. This lowers it temperature a lot, to near or below 0 degrees C. The air you wish to cool is blown over the cold evaporator, and the refrigerant gas gets somewhat warmer.

The warm low pressure refrigerant gas is sucked up and compressed into the condenser. And once it has transferred that heat to the outside air, it condenses, and the cycle repeats.

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Classification of Valves

Classification of Valves

Fluid control valves are classified in many ways. Following is the classification of valves.

Multi Turn and Quarter Turn Valves

Multi turn valves need many turns of actuator to bring closure member (disc) from fully open position to fully closed position. Examples of multi turn valves are gate valves, globe valves, needle valves, and diaphragm and pinch valves.
While quarter turn valves need only quarter ,0 to 900 degree, motion of closure member (disc) to bring it from full open position to full closed position.  Examples of quarter turn valves are butterfly valves, ball vales and plug valves etc. Quarter turn vales are quickopening valves.

Self actuated Valves

Self actuated valves don’t need actuator to operate them. These valves operate based on some property of fluid line like pressure or flow. Self actuated valves don’t need operator’s interference to control the fluid. Examples of self actuated valves are check valves and pressure relief valves or safety valves.

Linear Motion and Rotary Motion Valves

In linear motion valves, disc moves in linear or straight path to open, close or throttle the flow in valve. While in Rotary motion valves, disc rotates to open, close or throttle the flow. Examples of linear motion valves are gate valves, needle valves, globe valves, diaphragm valves and pinch valves. Examples of Rotary motion valves are ball valves, butterfly valves etc.


General Types of valves

In general following are the types of valves.
• Gate valve
• Globe Valve
• Pinch Valve
• Needle valve
• Diaphragm valve
• Ball valve
• Butterfly Valve
• Check valve
• Pressure relief Valve (safety or relief valve)

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Types of Valves

Types of Valves

Various types of Valves have been developed to cater to the diversified requirements of fluid control in various applications. These types of valves match the systems, fluids, and environments to accomplish their intended function.

Each type of valve has been designed to meet specific needs. Some valves are capable of throttling flow, other valve types can only stop flow, others work well in corrosive systems, and others valves handle high-pressure fluids. Each valve type has certain inherent advantagesand  disadvantages. Understanding these differences and how they affect the valve’s application or operation is necessary for the successful operation of a facility.

Although all valves have the same basic components and function to control flow in some fashion, the method of controlling the flow can vary dramatically.
Each method controlling flow has characteristics that make it the best choice for a given application of function. One valve may be best fit for one application but may not be fit for other at the same time.

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