Ahu Design

7/21/2019 Ahu Design

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Melody BaglioneAssociate Professor of Mechanical Engineering

[email protected]

Introduction

41 Cooper Square uses six air handling units (AHU) to heat, cool, humidify, and ventilate a

indoor spaces. TheBuilding Management System(BMS) utilizes an array of sensors to

identify heating, cooling, and ventilation demands throughout the building. The BMS

analyzes this data with a series of computational algorithms, and subsequently controls the

buildings AHUs to maintain comfortable indoor conditions in an efficient manner.

Figure 1. Exterior view of an AHU from the roof of 41 Cooper Square.
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Air handling units are large heat exchangers, in which a flow of air is heated or cooled usin

water-filled heating and cooling coils. The air handling units draw air from outside the

building using large centrifugal fans, and pass this flow through various smaller heat

exchangers, filters, and humidifiers to supply air at the temperature and relative humidity

specified by the BMS. Additionally, carbon-dioxide levels are monitored throughout the

building in order to ensure that air-handling units are providing a sufficient flow of fresh air

to keep indoor spaces safely ventilated. Figure 2 depicts one type of air-handling unitinstalled at 41 Cooper Square.

Figure 2. Schematic of 100% Outside Air AHU with Dehumidification Coil.

Figure 2 shows an AHU that treats a flow consisting of only outside air. Some other units i

41 Cooper Square are fitted with humidifiers or recirculation systems. This schematic,

however, provides a good overview of the basic components of an AHU. First, the flow

passes through a damper, which can be opened or closed to allow air to be taken into the

unit. The air is subsequently filtered and passed through three radiator coils that heat, cool

and dehumidify the flow. It should be noted that these coils are filled with primary hot or

chilled water from theboiler orchiller,respectively. The treated air, at the desired

temperature and relative humidity, is subsequently passed through a centrifugal fan before

it is directed to indoor spaces at the desired flow rate using supply dampers.
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BackgroundPsychrometrics:

The study of the thermodynamic properties of humid air (a water vapor and air mixture) is

known as psychrometrics. Psychrometrics allow engineers to define and quantify the state

and energy content of a water vapor and dry atmospheric air mixture using seven distinct

properties[1],listed below:

1. Dry Bulb Temperature: The Dry Bulb Temperature is the temperature of the air and

water vapor mixture as measured by a simple thermometer. (Measured in Celsius or

Fahrenheit)

2. Wet Bulb (or Saturation) Temperature: When discussing a mixture of water vapor and

air, the Wet Bulb Temperature is the temperature that a volume of air would have if

cooled adiabatically to saturation by the evaporation of water, all latent heat being supplied

by the volume of air. This property is usually measured using a wet bulb thermometer or

psychrometer (Measured in Celsius or Fahrenheit).

3. Relative Humidity: A quantity used to describe the ratio of water vapor to air in a humid

air sample. Thermodynamically, this quantity is defined as the ratio of the partial pressure

of water vapor in the air-water mixture to the saturated vapor pressure of water at the same

conditions (pressure and temperature of the mixture). (Usually stated as a percentage)

4. Dew Point: The temperature at which the water vapor in humid air begins to condense

When air is at 100% relative humidity, it is at dew point and water vapor will begin tocondense if it is cooled any further. (Measured in Celsius or Fahrenheit)

5. Humidity Ratio: The Humidity Ratio is defined as the mass ratio of liquid water to dry

air in a gas and vapor mixture. This quantity is usually expressed in pounds of moisture pe

pound of dry air.

6. Specific enthalpy: Enthalpy is a thermodynamic quantity equivalent to the total heat

content of a substance, equal to the internal energy of the mixture plus the product of

pressure and volume: h = u + pv. Specific enthalpy is the enthalpy of the humid per unitmass of dry air, and is usually expressed in Btu/lb of dry air or KJ/kg of dry air.

7. Specific Volume: The volume of an air and water vapor mixtusre that contains one uni

mass of dry air. Usually expressed in cubic meters per kilogram of dry air, or cubic feet per

pound of dry air.

These seven properties are graphically represented on psychometric chart, also known as

Mollier Diagram, shown below in English units. The colored lines represent constant values

of the corresponding property shown in the legend. Note, however, that the Dew Point line
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only exists where it is depicted in blue, coinciding with the line of constant 100% Relative

Humidity (RH). This blue curve is also known as the saturation line.

Figure 3. Psychometric Chart. [2]

Using this chart, any two properties of humid air can be used to determine the other five

thermodynamic properties listed above. A psychometric chart allows HVAC engineers to

completely define the state of a water vapor and dry air mixture on a single diagram. Usin

this chart, it is possible to determine which processes are necessary to treat outside air to

desired temperature and humidity content. For example, the following two figures show the

manner in which hot, humid summer air is treated to comfortable set points.


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Figure 4. Cooling and dehumidification process shown on psychometric chart.


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Figure 5. Schematic of cooling and dehumidification process in AHU.

The outside air starts at a dry bulb temperature of 80 F and 80% relative humidity,

depicted as Point 1 in the figures above. After passing through various filters, the flow is

cooled by the cooling coils to the dew point temperature. As it cools further, water

condenses out of the mixture, effectively dehumidifying the air until it reaches point 2. At

this point, the flow is passed through a reheating coil that brings the temperature of the flow

up to the nominal AHU supply set point of 55 F, while decreasing the humidity level to60%[3].Note, the first heating coil is not utilized in this process. This heating coil is

primarily used to heat outside air during the winter months.


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AHU Design

41 Cooper Square uses six air handling units to provide treated air to classroom, office,

auditorium and laboratory spaces. There are two main design differences between the six

AHUs in 41 Cooper Square. First, AHUs can either be fitted with a humidifier or reheating

coil. Second, the AHUs can either utilize 100% outside air or be fitted with a recirculation

system. The application and principle of operation of these different designs are briefly

covered in the following two sections[4].

Humidifier vs. Reheat Coil

The Rose Auditorium and most of the classrooms and offices in 41 Cooper Square are

fitted with aradiant heating and cooling system.When in cooling operation, it is possible fo

the surface of the copper tubing in the radiant panels to be below the dew point

temperature of the indoor air, and consequently cause condensation. This condensate can

drip down from the panels and potentially damage electronic equipment. Consequently, it i

important to dehumidify the air that enters spaces fitted with radiant cooling system. To

accomplish this, AHUs 2, 3, and 6 are fitted with reheating coils. The reheating coils allow

humid air to be dehumidified as illustrated in the Background section. Below is a BMS

screenshot of AH-3, which utilizes a reheat coil to provide treated air to classrooms and

offices in the cellar and on the ground floor.

Figure 6. BMS Screenshot of AH-3, which uses 100% outside air and a reheat coil.
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Laboratory spaces in 41 Cooper Square are not fitted with radiant cooling systems.

Consequently, air for these spaces is supplied by AHUs fitted with humidifiers that serve to

increase the moisture content in the flow of air. These humidifier units simply spray droplet

of liquid water into the dry air. The droplets absorb heat from the air to evaporate and form

a vapor-gas mixture. Humidifiers are usually fitted downstream from heating coils so that

dry winter air that has been heated can be brought up to a comfortable humidity level. The

figures below depict this heating and humidification process on a psychometric chart andair handler schematic, respectively.

Figure 7. Heating and humidification of cold, dry air shown on psychometric chart.


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Figure 8. Schematic of heating and humidification process in AHU

The psychometric chart and schematic show how cold outside air is drawn in at 40 F and

20 % Relative Humidity, depicted as point 1. From point 1 to point 2, the dry bulb

temperature increases and the relative humidity decreases as the flow passes through the

heating coil. At point 2, the flow is at the nominal supply temperature of 55 F, but is very

dry (RH 10%). Thus, the flow is passed through the humidifier, effectively increasing the

relative humidity to a comfortable 60%.The screenshot below shows a BMS diagram of AH

1, which is fitted with a humidifier to serve laboratory spaces in Lower Level 2.


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Figure 9. BMS Screenshot of AH-1, which uses 100% outside air and a humidifier.

100% Outside Air vs. Recirculation System

Building codes require that all air which is circulated through laboratory spaces must be

immediately exhausted outside the building in order to prevent potential contamination of

the air supply system by harmful substances present in these laboratories. Using an air

handling unit which treats 100% outside air to ventilate these spaces would be extremely

wasteful and inefficient, as the energy invested into cooling or heating the air would only be

utilized momentarily before being exhausted back outside. Consequently, 41 Cooper

Square utilizes an innovative re-circulation system that allows treated air from non-

laboratory spaces air to be recycled, and used to ventilate laboratory spaces before being

exhausted.

41 Cooper Square has a large central atrium around which most classrooms and offices

are located. These classroom and office spaces are ventilated using AHUs that treat 100%

fresh air. This fresh treated air enters the classroom and office spaces and is naturally

exhausted to the large central atrium. At the top of the atrium, large air handling units re-

use this treated air to meet ventilation demands in the buildings laboratories. The

schematic below represents an AHU system with recirculation.


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Figure 10. Schematic of Air-Handler with Recirculation from Atrium.

The schematic above illustrates the manner in which the recirculation system can be used

during the winter months to efficiently ventilate the building with warm air. The recirculation

system draws warm indoor air from the top of the atrium using a large centrifugal fan. Som

of this flow is exhausted outside the building, while the other portion is directed towards theair handling unit. At this point, the warm return flow is mixed with fresh, but cold, outside ai

and treated to the appropriate temperature and humidity set points. By mixing the warm

return air with cold outside air, the AHUs heating load and energy consumption are

significantly reduced. Theoretically, it would be most efficient to treat only the return flow of

air in a continuous loop. However, as air is passed through the building, it is contaminated

with carbon dioxide from the occupants. Thus, it is always necessary to mix a certain

amount of fresh air with the return flow in order to keep the building properly ventilated.

The flow rates of the outside air, return, exhaust and supply flows is modulated usingvariable air dampers. TheBMScontrols the position of these dampers (the degree to which

they are opened or closed) by monitoring the flow rate, temperature, humidity, and carbon

dioxide levels of the four flows with a large array of sensors. By analyzing the collected

data, the BMS is able to calculate a ratio of return air and outside air that can be treated

efficiently to meet the buildings ventilation demands, and subsequently output appropriate

commands to the AHUs dampers and other components. Below is a BMS screenshot of

AH-4, which ventilates laboratory spaces on levels 3 through 9 of 41 Cooper Square.
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Figure 11. BMS screenshot of AHU #4, which controls most of the lab spaces in 41

Cooper Square.

The BMS screenshot above shows the location of various sensors in the air handling unit.

It should also be noted that this air handling unit is connected in parallel with AH-5, which

also recirculates air from the atrium. A similar system is used in the Rose Auditorium,

where significant energy savings can be achieved in treating the large space with arecirculation system.

AHUs with re-circulation systems have a greater upfront cost than those that use only

100% outside air. This increase in cost is largely due to the installation of the ducts and

hardware necessary to collect and recirculate the return flow. 41 Cooper Squares central

atrium, however, allows the return flow to be conveniently collected at single point using

natural ventilation. Thus, the building is able to efficiently treat small spaces with high

occupancy rates, like classrooms and offices, with 100% outside air, and cheaply re-use

this air to treat spaces with high ventilation demands, like the auditorium and laboratories.

Air Delivery: VAV Dampers and Reheat Coils

Once air is treated by the AHUs, it is sent through a channel of ducts to rooms throughout

the building. Each room in 41 Cooper Square is fitted Variable Air Volume (VAV) dampers.

These dampers control the flow of air into each room. Using these VAV dampers, the BMS

is able to supply each room with the minimum amount of air required to maintain

comfortable indoor conditions. For example, if an office is unoccupied, the VAV will only be


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opened slightly so that 40 CFM of air is allowed into the space, the minimum airflow

required by New York Building codes. Reducing the airflow when unoccupied reduces the

buildings overall energy consumption. Below is a BMS screenshot of an unoccupied office

space illustrating the manner in which the VAV damper is used to control airflow into the

room.

Figure 12. BMS Screenshot of HVAC systems in an office, regulated by AHU #6.

Some VAV dampers in the building, particularly those found in large spaces like classroom

and laboratories, are fitted with reheat coils that treat the flow of air prior to entering the

room. These reheat coils allow the supply air temperature to be modulated according to th

needs of individual spaces. Below is a BMS screenshot that shows a reheat coil fitted to th

VAV damper in laboratory 407. In addition, the BMS laboratory 407 control panel illustrates

the extensive fume hood exhaust systems that are installed in laboratory spaces to ensure

that dangerous fumes can be safely ventilated out of the building.


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Figure 13. BMS screenshot of HVAC systems in a lab space, highlighing the VAV

damper that regulates airflow supplied into the room by the AHU.

Control:

TheBMS controls the operation of the AHUs to efficiently deliver air at the specified

temperature and humidity. In addition, the BMS controls the flow of supply air to the variou

spaces to meet ventilation requirements. A brief overview of the various control sequences

utilized by the BMS to operate these various air handling systems is provided the following

sections.

Control of AHU with 100% Outside Air:

The AHUs that treat 100% outside air consist of a single flow of treated air. Below is a BMS

screenshot of AH-6 that highlights the various sensors and actuators used to monitor and

control this type of AHU. First, the BMS opens the outside air (OA) damper to allow the uni

to intake fresh outside air to be treated. This outside air is passed through various filters. In

order to ensure that the filters are not clogged and functioning properly, the differential
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pressure (the difference between the air pressure before and after the filter) is measured

using a differential pressure (DP) sensor. If the BMS detects a pressure drop exceeding a

safe set point, an alarm is activated to notify technicians[3].

Figure 14. Components of an AHU as viewed by the BMS. This particiular air handler is

AHU #6, which controls the airflow through most of the classrooms and offices in 41

Cooper Square.

After filtration, the flow passes through various heating and cooling coils. The exact numbe

and arrangement of these coils depends upon the type of room served by the AHU, as

outlined in the Background section. However, after each heating or cooling coil, the

temperature of the airflow is monitored by a resistance temperature detectors(RTD)

temperature sensor. Using this temperature data, the BMS regulates the flow rate of

primary water through the coils. For example, if the BMS detects that the airflow is too cold

upon exiting the heating coil, the electronic valve actuator opens the valve allowing a

greater primary hot water flow rate to enter the coil[3].
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Treated air at the desired temperature and relative humidity is subsequently passed

through a large centrifugal fan. The AHUs fan motor is controlled using a Variable

Frequency Drive (VFD), which allows the BMS to modulate the operating speed and powe

consumption of the fan to meet ventilation demands. The BMS controls the fans

operational speed as a function of the static pressure (SP) in the supply duct. When the

supply SP is detected as falling below 2 inches of water column the fan speed is

increased[3].Conversely, when this SP set point in exceeded, the fan motor is rampeddown to conserve energy. After exiting the fan, the temperature and humidity level of the

flow are fed back to control the heating, cooling, and humidifying elements in the AHUs.

Control of AHUs with Re-Circulation:

There are additional sensors and actuators necessary to control the mixture of outside air

and return air that is input to an air handling unit with re-circulation. Below is a BMS

screenshot of AH-4, which depicts the various sensors and actuators used to control the re

circulation system.

Figure 15. Components of AHU#4, viewed from the BMS.


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As described in the Background section, the return airflow for AH-4 is collected at the top o

the atrium. The temperature, relative humidity, and carbon dioxide content of this return

flow are monitored using sensors in the return duct. The return flow is subsequently passed

through a centrifugal fan. The fans speed is controlled by a VFD as a function of the static

pressure at the discharge point.

After passing through the fan, the flow is either exhausted outside the building or re-

directed towards the air-handling unit. The flow rate of air that is exhausted and re-

circulated is controlled using variable air dampers, which are opened or closed by the BMS

The outside air damper in this type of unit is also variable, and the BMS is consequently

able to control the exact ratio of fresh and return air that is treated by the AHU. This ratio is

mainly controlled as a function of the carbon dioxide content in the return airflow.

The flow rate of air to the various spaces is controlled by the required ventilation in the

various building spaces. Ventilation rates to indoor spaces are commonly quantified in

terms of the number of times all the total volume of air in the space has been exhaustedand replaced with treated air. For example, occupied laboratory spaces are programmed to

receive between 10 and 12 air changes per hour. In order to conserve energy, this set poin

is lowered to 4 air changes per hour when the laboratory is unoccupied[5].

AHUs with re-circulation systems are also outfitted with a free cooling feature, which allows

the unit to detect when conditions allow for maximum use of the re-circulation system to

reduce energy consumption. As outlined in the Sequence of Operations, the AHU detects

that the outside supply air (OSA) enthalpy is greater than the return air enthalpy, and close

the OSA damper to provide only the minimum amount of fresh air to meet ventilationdemands. Thus, the AHU is able to conserve energy by utilizing return air that is already

cooled below atmospheric conditions to lower the units cooling load[3]

[1]Y. engel and M. Boles. Thermodynamics: an engineering approach, 7th ed. New York

McGraw-Hill, 2011[2] "Psychrometric Chart Use (inner frame)." Homepages Web Server -

UITS - University of Connecticut. N.p., n.d. Web. 20 Feb.

2012. http://www.sp.uconn.edu/~mdarre/NE127/NewFiles/psychrometric_inset.html[3] G.Sampton. Sequence of Operation The New Academic Building of Cooper Union.

Morphosis Architects, Los Angeles, CA, Rep. 15959, June 2005. p. 4-6.

[4]GMP Set - The New Academic Building of Cooper Union Modular Outdoor Air

Handling Units. Syska Hennessy Group, New York, NY, 2007.

[5]Laboratory Ventilation Codes and Standards, Rev. 4, Siemens Building Technologies

Inc., Munich, Germany, 2002, pp. 19.
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