Relevant HVAC Systems

When designing a building, it is important to consider the many different Heating Ventilation and Air Conditioning systems available to our disposal.  There are a wide range of systems to choose from that each have similar and different characteristics that make them useful.  When considering which system to use it is practical to reflect on what characteristics will make the system work well in the situation at hand.  In the Criteria Matrix and Criteria for System Selection sections, the necessary criteria will be discussed.  This section will give a general outline of each system and how it can be applied in a Demonstration Kitchen for Whole Foods.

Unitary Single Zone:

A Unitary Single Zone system is a direct-expansion (DX) system that supplies a single constant volume of conditioned air to each zone.  Every zone has its own AHU (Air Handling Unit) to cool, heat, and otherwise condition air individually.  Because of this, each zone has separate information that can be interpreted by the system and then adjusted accordingly.

The system works by either bringing in outside air or recycling the air currently in the system and then sending it through the AHU.  The AHU consists of an outside air mixing damper (if air is coming from outside), heating and cooling coils, filters, controls, and fans.  The dampers control the flow of air from the outside to the system and from one part of the system to the other.  The fans/blowers force the air through either the heating or cooling coils which condition the air to the desired temperature.  This air is directed through filters which remove contaminants and then it will be sent from each AHU into the respective ducts. It will then be controlled again by another damper and released into the zone when it is ready to receive the conditioned air.  Finally, this air will be sent back through the vents and ductwork and end up as exhaust or recirculated through its respective AHU system again.  All of this is managed by a control system that reads the current settings and surroundings and then regulates the air as needed.

Since this type of system requires an AHU for each and every zone, it can be costly to consider this for a building that does not require extremely different climates from one room to the next.  Although, it is valid to consider because it uses effective methods to circulate conditioned air throughout the rooms and could potentially remove and recondition air very well for a Demonstration Kitchen for Whole Foods.   It may only be really effective if it were a very large building where multiple AHU’s are necessary to combat the loss in pressure in very long ducts.

Unitary Multi Zone:

A Unitary Multi Zone system is very similar to a Unitary Single Zone System expect for one main aspect.  The Multi Zone system does not have a single AHU for each and every zone but rather has one main AHU and control system that monitors and delivers conditioned air to many zones.  These systems can access and control up to 400 zones in many buildings.

We are considering this system because it has the ability to use a single AHU to deliver conditioned air and remove stale air from the demonstration kitchens.  This is cheaper and more effective because it uses only one AHU and only one set of controls to send the air through the ducts and vents to the zones and return or exhaust the air as needed.  Because we are looking to provide a comfortable climate in a kitchen that will have a lot of heat load and air contaminants, the system must be appropriately sized and out of view.  This system takes advantage of ducts and vents to deliver air so the relative size of the system, which can be very noisy and large at 10 feet by 20 feet by 50 feet deep, will not be effecting the inhabitants of the space.  Also, since it is not a very tall building, multiple AHU’s will not be effective because there will not be a huge drop in pressure that needs to be accounted for.

Above is a table of figures for Air Handling Units provided by Team #1 Group Members: Lemond Brown, Adrian Lu, Chris Snavely

Fan Coil Unit

Fan Coil Units are a very simple system.  They are single unit that lies in the zone of which it is conditioning the air.  It consists of a connection to outside air with a damper and outside air mixer, has a chilled water cooling coil, hot water heating coil, and a hot water and chilled water loop.   The system takes outside air and mixes it, then sends it through either the heating or cooling coils, then it is distributed about the zone.  They are typically wall or ceiling mounted or freestanding on the floor and can be concealed to hide the non-aesthetically pleasing figure.

This system is considered because it can produce heat and cooling to the kitchen at a very low cost when compared to AHU’s (Multi and Single) and VAV and CAV units.  However, it does not have the ability to remove contaminated air particles and odor.  It is also a relatively small system and does not have the ability to condition large rooms with great heating or cooling loads already associated.  The kitchens have will create huge amounts of heating loads which this system will likely not be able to control or handle to effectively keep the zone comfortable.  This system is not pleasing to look at and therefore will get negative scores because it is in plain sight.

VAV

A Variable Air Volume system is different than other systems because it alters the zone temperature by pumping a higher or lower volume of air into the room while keeping it at a constant temperature.  It uses an AHU to condition the air to a desired temperature but each zone has its own VAV box that regulates the volume of air output.  So, the air will be sent through the AHU, through the ducts to the VAV box, and then it will then be delivered to the room as needed.

A VAV system is an efficient system because it can alter the temperature in a zone without forcing the AHU to actually raise or lower the temperature a lot.  This allows for greater energy savings as well as operational cost savings.  This system excels well in buildings with a large area and heavy heating loads generated by people and equipment that need continual cooling, typically all year.  Since VAV systems can put out anywhere from 200 to nearly 8,000 cubic feet per minute (cfm), it could be valuable if used to exhaust and replace a lot of contaminated air and odor from the Demonstration Kitchen.  It could also be used to maintain the large amount of heat generated by the kitchen equipment and inhabitants in an efficient manner and with relatively low cost.

VAV systems are not the best system type for very small buildings with small heating and cooling loads.  In addition, they are not superior in situations where a constant volume is not necessary, such as pressurized rooms, etc.  It is also important to note that VAV systems reheat air at multiple points and is therefore not very efficient in buildings that need a large heating load in winter climates.  For a kitchen that produces large heat loads and would be exhausting a lot of air, these limitations are nearly negligible, and prove that VAV is a worthy candidate.

CAV

A Constant Air Volume system is nearly the direct opposite to that of a VAV system.  CAV systems produce a constant volume of air that is distributed throughout, but the temperature of the air is fluctuated to accommodate the heating or cooling demand.

Because of this, these systems are common within homes and small buildings.  They are also very easy to install and use/maintain, in small buildings they are efficient and cost effective, and they function well without much danger or difficulty.

A typical CAV unit consists of outside air intake fans and ducts which bring outside air into the system to a central source, cooling and heating coils to treat the temperature of the air (although boilers and water chillers can be used in some circumstances), and supply fans to deliver the air through the ducts to the zone or zones.  Return fans and ducts are used to move the air back through the system to reheated and reused or to be expelled as exhaust.  There is one master thermostat that manages the temperature of the air in the system and how often to run the system with the help of many controls and sensors.  Since the system is delivering a constant volume with different temperatures, it consistently shuts off and turns back on.  This is an inefficient process that wastes a lot of energy to the fans and system as a whole.

CAV systems are usually used to heat only one zone, but CAV primary-secondary, CAV multi-zone, and CAV with reheat are available to treat more than one zone at a time in larger buildings.  These systems also require more space and have higher energy consumption than VAV systems because of the high energy use of the fans in CAV systems.

For a much smaller demonstration kitchen, CAV systems could be considered, but are not a likely choice.  Since the buildings and cooling loads would be large, it would not seem extremely cost effective and energy efficient to use this system.

 

 

 

 

Resources:

https://sites.google.com/site/hvacvarietysite/vav

http://www.designbuilder.co.uk/helpv1/Content/_Unitary_single_zone.htm

http://www.designbuilder.co.uk/helpv1/Content/_Unitary_multizone.htm

http://en.wikipedia.org/wiki/Fan_coil_unit

https://sites.google.com/site/ae390hvacsystems/air-handling-unit/numeric-parameters

http://en.wikipedia.org/wiki/Constant_air_volume

http://www.pages.drexel.edu/~jm328/AE390/A6/TheSystems.htm

 

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