Air Side loop
The first loop is the air side loop, and the first component of this loop is the conditioned space. The first two comfort requirements mentioned were dry bulb temperature and humidity. In order to maintain the dry buld temperature in the conditioned space, heat (referred to as sensible heat) must be added or removed at the same rate as it leaves or enters the space. In order to take care of the humidity level within the space, moisture (sometimes mentioned as latent heat) must be added or removed at an equivalent rate because it leaves or enters the space.Most HVAC systems used today deliver conditioned (heated, cooled, humidified or dehumidified) air to the conditioned space to feature or remove sensible heat and moisture. This conditioned air is called supply air. The air that carries the warmth and moisture out of the space is named return air.
Imagine the conditioned supply air as a sponge. In the cooling mode, as it enters the space, this “sponge” (supply air) absorbs sensible heat and moisture.
The amount of sensible heat and moisture absorbed depends on the temperature and humidity, as well as the quantity, of the supply air. Assuming a fixed quantity of air, if the supply air is colder, it can remove more sensible heat from a space. If the availability air is drier, it can remove more moisture from the space.
In order to determine how much supply air is needed for a given space, and how cold and dry it must be, it is necessary to determine the rate at which sensible heat and moisture (latent heat) enter, or are generated within, the conditioned space.
Sources of warmth and moisture, which are commonly called cooling loads:
1) Conduction heat gain from outdoors through the roof, exterior walls, and glass windows or skylights.
2)Solar radiation heat gain through glass windows or skylights.
3)Conduction heat gain through ceiling, interior partition walls, and the floor.
4) Internal heat and moisture generated by people, lights, appliances, and equipment in the space.
5) Heat gain from air infiltrating into the space from outdoors.
In addition to those depicted on other common sources of heat and moisture include:
1) Heat gain from outdoor air deliberately brought into the building for ventilation purposes.
2) Heat generated by the fans and motors in the system.
SUPPLY FAN AND FILTER
The next component of the airside loop may be a supply fan that delivers the availability air (SA) to the space. This same supply fan is usually wont to also draw the return air out of the space. Alternatively, some systems use a second fan, called a return fan, to draw air from the space and move it back to the equipment that contains the availability fan.
Another one for the comfort requirement is to provide an adequate amount of fresh, outdoor air to the space. For example, the required amount of outdoor air(OA) for ventilation is brought into the building and mixed with the recirculated portion of the return air(RA). The remaining return air, that which has been replaced by outdoor air, is exhausted as exhaust air(EA) from the building, often by an exhaust(or relief) fan.
For example, outdoor air is at 95°F(35°C) dry bulb mixes with recirculated return air at 75°F(23.9°C) dry bulb. This mixture contains 25 percent outdoor air and 75 percent recirculated return air, so the resulting temperature of the mixed air(MA) is 80°F(26.7°C) dry bulb.
Another comfort requirement is to make sure that the air within the conditioned space is clean. Bringing in an adequate amount of fresh outdoor air, and exhausting a number of the air from the space , can help meet this requirement. However, the air must also be filtered. In a typical HVAC system, the mixed air passes through a filter to get rid of many of the airborne contaminants.
Another one for the comfort requirement is to provide an adequate amount of fresh, outdoor air to the space. For example, the required amount of outdoor air(OA) for ventilation is brought into the building and mixed with the recirculated portion of the return air(RA). The remaining return air, that which has been replaced by outdoor air, is exhausted as exhaust air(EA) from the building, often by an exhaust(or relief) fan.
For example, outdoor air is at 95°F(35°C) dry bulb mixes with recirculated return air at 75°F(23.9°C) dry bulb. This mixture contains 25 percent outdoor air and 75 percent recirculated return air, so the resulting temperature of the mixed air(MA) is 80°F(26.7°C) dry bulb.
Another comfort requirement is to make sure that the air within the conditioned space is clean. Bringing in an adequate amount of fresh outdoor air, and exhausting a number of the air from the space , can help meet this requirement. However, the air must also be filtered. In a typical HVAC system, the mixed air passes through a filter to get rid of many of the airborne contaminants.
COOLING COIL
As mentioned earlier, during the cooling mode, the availability air must be cold enough to soak up excess sensible heat from the space and dry enough to soak up excess moisture (latent heat). A heat exchanger, commonly known as a cooling coil, is often used to cool and dehumidify the supply air before it is delivered to space.
For example, the cooling coil cools and dehumidifies the entering mixed air from 80°F(26.7°C) dry bulb to a supply air temperature of 55°F(12.8°C) dry bulb.
A typical cooling coil includes rows of tubes passing through sheets of formed fins. A cold fluid, either water or liquid refrigerant, enters one header at the top of the coil then flows through the tubes, cooling both the tubes and therefore the fins.
For example, the cooling coil cools and dehumidifies the entering mixed air from 80°F(26.7°C) dry bulb to a supply air temperature of 55°F(12.8°C) dry bulb.
A typical cooling coil includes rows of tubes passing through sheets of formed fins. A cold fluid, either water or liquid refrigerant, enters one header at the top of the coil then flows through the tubes, cooling both the tubes and therefore the fins.
CHILLED WATER COOLING COIL
As the warn, humid mixed air passes through the coil, it comes into contact with the cold tubes and fins. Sensible heat is transferred from the air to the fluid inside the tubes, causing the air to be cooled.
Additionally, if the outside surface temperature of the tubes and find is below the dew point temperature of the entering air, moisture contained in the air will condense on the tubes and fins. This condensed liquid then flows down the fin surfaces into a drain pan located underneath the coil, and is piped away. The air (supply air) leaving the coil is colder and drier than when it entered.
Many HVAC systems also use the airside loop for heating and humidification. Often, a heating coil or humidifier is found near the cooling coil within the same airside loop. Alternatively, a heating coil or humidifier may be part of second, separate airside loop. Assuming a hard and fast quantity of air, if the availability air is warmer, it can add more sensible heat to the space. If the supply air is more humid, it can add more moisture to the space. For simplicity, however, this era focuses totally on comfort cooling.
Additionally, if the outside surface temperature of the tubes and find is below the dew point temperature of the entering air, moisture contained in the air will condense on the tubes and fins. This condensed liquid then flows down the fin surfaces into a drain pan located underneath the coil, and is piped away. The air (supply air) leaving the coil is colder and drier than when it entered.
Many HVAC systems also use the airside loop for heating and humidification. Often, a heating coil or humidifier is found near the cooling coil within the same airside loop. Alternatively, a heating coil or humidifier may be part of second, separate airside loop. Assuming a hard and fast quantity of air, if the availability air is warmer, it can add more sensible heat to the space. If the supply air is more humid, it can add more moisture to the space. For simplicity, however, this era focuses totally on comfort cooling.
CONSTANT VOLUME SYSTEM
Finally, the availability air is delivered to the conditioned space. The temperature and humidity of the availability air, however, are only applicable for one point in time. The cooling requirement (load) within the conditioned space varies changing cooling loads within the conditioned space by varying either the temperature or the amount of the air delivered to the space..
A constant volume system provides a continuing quantity of supply air and varies the availability air temperature in response to the changing cooling load within the space. A thermostat compares the dry bulb temperature within the conditioned space to a setpoint. It then modulates cooling capacity until the space temperature matches the setpoint.
A constant volume system provides a continuing quantity of supply air and varies the availability air temperature in response to the changing cooling load within the space. A thermostat compares the dry bulb temperature within the conditioned space to a setpoint. It then modulates cooling capacity until the space temperature matches the setpoint.
VARIABLE AIR VOLUME SYSTEM
A variable air volume (VAV) system varies the number of constant temperature supply air in response to the changing cooling load within the space.
In this system, a VAV terminal unit is added to the airside loop. Each conditioned space, or group of comparable spaces (called a zone), features a separate VAV terminal unit that varies the number of supply air delivered thereto space or zone. The VAV terminal unit contains an airflow modulation device, typically a rotating blade damper.
A thermostat compares the dry bulb temperature within the conditioned space to a group point. It then modulates the number of supply air delivered to the space by changing the position of the airflow modulation device within the VAV terminal unit. The capacity of the availability fan is modulated to deliver only the number of supply air needed, and cooling capacity is modulated to take care of a continuing supply air temperature.
In this system, a VAV terminal unit is added to the airside loop. Each conditioned space, or group of comparable spaces (called a zone), features a separate VAV terminal unit that varies the number of supply air delivered thereto space or zone. The VAV terminal unit contains an airflow modulation device, typically a rotating blade damper.
A thermostat compares the dry bulb temperature within the conditioned space to a group point. It then modulates the number of supply air delivered to the space by changing the position of the airflow modulation device within the VAV terminal unit. The capacity of the availability fan is modulated to deliver only the number of supply air needed, and cooling capacity is modulated to take care of a continuing supply air temperature.
FCU
A simple example of the airside loop is a Fancoil unit. Return air from the space is drawn into the unit at rock bottom and should be mixed with outdoor air that enters through a separate damper within the rear of the unit. This mixed air passes through a filter, a supply fan, and a cooling coil before being discharged from the very best of the unit, directly into the conditioned space.
CENTRAL AIR HANDLER
Another example for the airside loop is a central air handling system. A central air handler is usually installed outside of the conditioned space, possibly on the roof or during a dedicated mechanical room.
Return air from the space is drawn into the unit through the return air dampers and mixes with outdoor air that enters through another set of dampers. This mixed air passes through the filters, the availability fan, and therefore the cooling coil before being discharged from the air handler.
Unlike the instance fan coil unit that was installed within the conditioned space, the central air handler needs a way for delivering the availability air to the conditioned space(s).
Return air from the space is drawn into the unit through the return air dampers and mixes with outdoor air that enters through another set of dampers. This mixed air passes through the filters, the availability fan, and therefore the cooling coil before being discharged from the air handler.
Unlike the instance fan coil unit that was installed within the conditioned space, the central air handler needs a way for delivering the availability air to the conditioned space(s).
SUPPLY AIR DISTRIBUTION SYSTEM
A supply air distribution system, typically constructed of sheet ducts, fittings, and diffusers, is employed to direct the availability air from the central air handler to at least one or more conditioned spaces. The example airside loop includes a central air handler and ductwork to deliver supply air to multiple VAV terminal units.
For each VAV terminal unit, the supply air travels through a section of flexible duct to remotely located diffusers. Diffusers are wont to distribute the availability air effectively to the conditioned space. Proper air diffusion is a crucial comfort consideration, especially in VAV systems, to avoid dumping cold supply air on the occupants of the space.
For each VAV terminal unit, the supply air travels through a section of flexible duct to remotely located diffusers. Diffusers are wont to distribute the availability air effectively to the conditioned space. Proper air diffusion is a crucial comfort consideration, especially in VAV systems, to avoid dumping cold supply air on the occupants of the space.
CEILING PLENUM RETURN
Air returns from the conditioned space to the central handler through an open ceiling plenum. The plenum is that the space between the ceiling and therefore the roof, or floor,above.
Alternatively, a separate return air passage system might be wont to direct the return air back to the air handler.
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