Lesson#3 Air Side Loop-2
- In the previous lesson we talked about:
·The goal of HVAC system.
· Comfort requirements.
· Dissecting HVAC system to loops.
- In this lesson we will dissecting air side loop to 3 components as the following:
· Conditioned space.
· Supply fan and filter.
· Cooling Coil.
- After dissecting the air side loop to these components we will talk about each component in details
1.Conditioned space:
1.1 Dry bulb temperature& humidity:
In order to maintain the dry bulb temperature in the conditioned space Sensible heat must be added or removed at the same rate as it leaves or entered the space.
To maintain humidity level in the conditioned space latent heat must be added or removed at the same rate as it leaves or enter the space.
Most HVAC systems deliver conditioned (heated, cooled, humidified or dehumidified) air to the conditioned space to add or remove sensible, latent Heat to or from the conditioned space.
In this lesson we will talk about cooling mode only, we will talk about heating mode later
The conditioned air is called supply air, the air that carries the sensible and latent heat out of the space is called return air.
<a href="https://1.bp.blogspot.com/_JsHFF95a2-U/ScpE3P686cI/AAAAAAAAABM/wKTa78Ap9F0/s1600-h/Air+side.bmp" target="_blank" rel="nofollow noopener noreferrer">Air+side.bmp (image)</a>
In the cooling mode the supply air absorbs sensible and latent heat from the conditioned space.
The amount of sensible and latent heat absorbed depend on temperature and humidity as will as the quantity of the supply air
Assuming a fixed quantity of supply air , if the supply air is cooler it can remove more sensible heat from the conditioned space ,if the supply air is drier it can remove more latent heat from the space.
1.2 Overview of cooling load components:
In order to determine how much supply air is needed for a given space, how cold and dry it must be it must to determine the rate at which sensible And latent heat enters the conditioned space.
The figure below shows the typical sources of sensible and latent heat which are commonly called room cooling loads:
·Conduction heat gain from outdoors through glass windows and skylights.
·Solar radiation heat gain through glass windows and skylights.
·Combined conduction and solar radiation heat gain to external walls and roofs.
·Conduction heat gain through the ceiling, internal Partitions and floors.
·Internal heat gain generated by people, lights, appliances and equipments in the space.
·Heat gain from infiltrating into the space from outdoors.
<a href="https://3.bp.blogspot.com/_JsHFF95a2-U/ScpFVnZ42XI/AAAAAAAAABU/Eiwe3MtjqpA/s1600-h/Cooling+load+components.JPG" target="_blank" rel="nofollow noopener noreferrer">Cooling+load+components.JPG (image)</a>
In addition to those sources displayed on the figure there is other common sources of sensible and latent heat sources:
·Heat gain from outdoor air deliberately brought into the building for ventilation purposes.
·Heat generated by fans and motors in the system.
·Conduction heat gain through ducts
For further information on the various components of building cooling loads refers to later lessons.
2.Supply fan and filter:
The next component of the airside loop is a supply fan that delivers the supply Air (SA) to the space. In the example in the Figure below, air is supplied to theConditioned space to maintain a desired temperature of 75ºF (23.9ºC) and relative humidity 50% in the space (we neglected the cooling coil in this figure; we will explain it in the next component of the air side loop).
<a href="https://4.bp.blogspot.com/_JsHFF95a2-U/ScpF4aqauLI/AAAAAAAAABc/YxXc9yNKZNA/s1600-h/Supply+fan+and+filter.JPG" target="_blank" rel="nofollow noopener noreferrer">Supply+fan+and+filter.JPG (image)</a>This same supply fan is often used 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 supply fan.
Another one of the comfort requirements is to provide an adequate amount of Fresh, outdoor air to the space. In this 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.
In this example, outdoor air at 95ºF (35ºC) dry bulb mixes with recirculated return air at 75ºF (23.9ºC) dry bulb. This mixture contains 25 percent outdoor air and 75 percent recirculated air, so the resulting temperature of the mixed air (MA) is 80ºF (26.7ºC) dry bulb.
Another comfort requirement is to ensure that the air in the conditioned space is clean, the mixed air passes through a filter to remove many of the airborne contaminants.
3.Cooling coil:
the supply air must be cold enough to absorb excess sensible heat from the space and dry enough to absorb 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 the space.
In this example in the figure below, the cooling coil cools and dehumidifies the entering mixed air from 80ºF (26.7ºC) dry bulb to a supply-air temperature of 55ºF (12.8ºC) dry bulb.
<a href="https://4.bp.blogspot.com/_JsHFF95a2-U/ScpGeAivuTI/AAAAAAAAABk/o_3VpNj_ye4/s1600-h/Cooling+coil.JPG" target="_blank" rel="nofollow noopener noreferrer">Cooling+coil.JPG (image)</a>
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 end of the coil and then flows through the tubes, cooling both the tubes and the fins.
Another Figure shows an example of a cooling coil that has chilled water flowing Through it. As the warm, 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.
<a href="https://2.bp.blogspot.com/_JsHFF95a2-U/ScpGtJAhXHI/AAAAAAAAABs/pfRZm-orqH8/s1600-h/cooling+coil+2.JPG" target="_blank" rel="nofollow noopener noreferrer">cooling+coil+2.JPG (image)</a>
Additionally, if the outside surface temperature of the tubes and fins 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 under the coil, and is piped away. The air (supply air) leaving the coil is colder and drier than when it entered.
** Now this lesson is finished write your questions about this lesson in the comments below in the next lesson we will complete the air side loop subject see you soon
Source:<a href="https://hvac-world.blogspot.com" target="_blank" rel="nofollow noopener noreferrer">hvac-world</a>
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