Ventilation Lecture 4 PH Alleen Lezen

5/13/2013

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Lecture 4: Displacement ventilation

IEE/09/631/SI2.558225 28.10.2011

Educational Package Ventilation

Displacement Ventilation Introduction

Displacement Ventilation Characteristics

Displacement Ventilation vs. Mixing Ventilation

Ventilation Effectiveness

Design ConsiderationsThermal Comfort

Humidity Control

Acoustics

Designing with AHUs and RTUs

Diffuser Type, Layout and Location

DV Supply Air Methods

Case Study

Summary

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IntroductionAirflow in ventilated spaces generally can be classified by two different types:

mixing (or dilution) ventilation displacement ventilation.

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Mixing ventilation systems generally supply air in a manner such that the entire room volume is fully mixed.

Displacement ventilation systems introduce air into the space at low velocities which causes minimal induction and mixing.

Source: E.H. PRICE (2007) Displacement Ventilation DESIGN GUIDE

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Displacement ventilation takes advantage of the difference in air temperature and density between an upper contaminated zone and a lower clean zone.Cool air is supplied at low velocity into the lower zone.Convection from heat sources creates vertical air motion into the upper zone where high level return outlets extract the air

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Introduction

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Fig.3

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While each system has its comparative benefits and drawbacks, displacement

ventilation is slowly gaining in popularity due to its less energy intensive nature.

5Source: Abbas, T. (1999) Displacement ventilation and static cooling devices (COP 17/99)

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Flexibility as loads change within the space, a displacement system will be able to

compensate. For example, if the space was designed to have a fairly even load distribution and now has the loads concentrated to one side, the system is able to compensate as the buoyant forces drive supply system and will draw the air towards the loads.

IAQ because fresh supply air is pooling at the floor level, personal thermal plumes

draw fresh air up the body. All of the warm and polluted air is extracted at the high return. When properly designed, there should always be a greater amount of fresh air in the breathing zone when compared to a conventional dilution system.

Energy Savings the lower pressure drop associated with displacement ventilation outlets, may

allow a reduction in fan energy with the selection of a smaller fan components. economizer operating hours can be increased to take advantage of free cooling

because supply air temperatures are higher than with overhead air distribution systems.

chiller efficiency may be increased when the system is not dehumidifying, as there is a lower supply air temperature and higher return air temperature.

6Source: E.H. PRICE (2007) Displacement Ventilation DESIGN GUIDE

Introduction

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Application Restrictions/additional comments Examples

IndustrialShould not be used where pollutants are heavier than air unless they are directly removed by local extraction.

Production/assemblyhalls, stores, factories.

Offices

Likely to need additional cooling devices.Works best when ceiling heights are greater than 2.5m casual loads are moderate occupants are mainly sedentary buildings are airtight

Open plan or cellular.

RecreationCan achieve very low noise levels compared to mixing systems.Underseat air supplies should be used with particular attention to local draught conditions.

Theatres, cinemas,auditoria, sports halls, fitness rooms and restaurants.

LaboratoriesShould not be used where pollutants are heavier than air unless they are directly removed by local extraction.

All types.

General Public Areas Significant solar gain must be precluded.

Conference rooms,airport concourses,atria, shops.

7Source: Abbas, T. (1999) Displacement ventilation and static cooling devices (COP 17/99)

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Thermal Plume

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Stratification Height

Heat sources such as people, computers, lights, etc. create a rising convection flow known as a thermal plum

The upward moving airflow contained in thermal plumes that form above heat sources

The downward moving airflow resulting from cool surface

The supply airflow into the room from a low side-wall diffuser

Stratification Height

An important objective indesigning and operating adisplacement ventilationsystem is to maintain Ystnear to the breathing zone(1,7m standing work position and1,1 m seated work position)

DV Characteristics

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Source: E.H. PRICE (2007) Displacement Ventilation DESIGN GUIDESource: Hazim B. Awbi, Ventilation of Buildings, Second Edition, 2003

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Room Airflow Pattern Quite different than in a

mixing system. The convection flows are

created by heat sources within the room cause the formation of horizontal air layers.

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The warmest air layers are near the ceiling The coolest air layers are near the floo

Airflow PenetrationObstruction

Irregular Room Geometry

When designing the system to deal with the cooling demand of the space, the penetration depth of a displacement diffuser can be 26 30 feet from the face of the diffuser. For rooms exceeding 30 feet in length or width, diffusers on several walls would normally be required.

DV CharacteristicsD

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Fig.5.a

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Typical temperature profile for a room with displacement ventilation.

The stratification is affected by several factors: supply air volume room cooling load location and type of heat

source space height.

The greater the volume of air supplied into a room, the lower the temperature difference between floor and ceiling


DV Characteristics

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Fig.5b

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Cool Air Supply Isothermal Air Supply Heating Air Supply

In order to avoid draft it is essential for the displacement diffuser to uniformly deliver the supply air across the entire diffuser face at low velocity. This requires an internal equalization baffle in combination with a low free area face.

DV CharacteristicsD

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Fig.6 a,b,c

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If heat sources are located in the lower part of the room, the temperature gradient is greater in the lower part of the room and lessens in the upper part.

When heat sources are located in the upper part of the room the greatest stratification occurs near the ceiling


DV Characteristics

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DV CharacteristicsD

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Displacement ventilation with wall-mounted diffuser

Room with DV and wall

mounted diffuser

Source: Hazim B. Awbi, Ventilation Systems Design and performance, 2008

Wall mounted low velocity diffuser and smoke visualization

of the stratifiedflow from the diffuser

Design chart for an air distribution system with wall

mounted diffusersfor displacement ventilation

Fig. 7 a,b

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DV Characteristics

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Displacement ventilation with floor-mounted diffusersFlow in a room with

displacement ventilation and a floor mounted

diffuser


High momentum displacement ventilation

Impinging jet ventilation system

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DV CharacteristicsD

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High momentum displacement ventilationImpinging jet ventilation

MAIN ADVANTAGES over a standard displacement system are:

the air can be supplied over a much wider temperature range than can be

tolerated in a displacement system. Tests have shown that a supply

temperature down to 15 C can be tolerated as the jet quickly entrains room air

at higher temperature as it leaves the diffuser;

the ability to use the impinging jet system for heating by supplying air at

higher temperature than room air, which is not possible in a standard DS;

the impinging jet spreads quickly as it descends on the floor creating a thin

layer that can travel a long distance on the floor even with the presence of

heat sources. In a standard displacement system, the supply air does not

normally travel long distances as it is quickly consumed by heat sources in its

path.


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DV Characteristics

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High momentum displacement ventilationConfluent jets ventilation


Flow pattern

Typical confluent jets installations

Fig.7 e, f

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Performance

System Configuration Cooling Capacity

Displacement ventilation system (DVS) 20 35 W/m

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DVS with induction of room air into supply and lower primary

temperature35 60 W/m2

DVS with chilled ceiling panels 70 100 W/m2

DVS with passive chilled beams Allows additional cooling of 200 250 W/m length of beam

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DV CharacteristicsD

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Source: Abbas, T. (1999) Displacement ventilation and static cooling devices (COP 17/99)

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With mixing ventilation, contaminants are diluted with supply air and are distributed evenly throughout the

space

Displacement ventilation improves occupant air quality by reducing

the contaminants in the lower portion of the room. The general

upward motion of air causes contaminants to concentrate within

the upper zone

Contaminant distribution in a space is influenced by several factors such as supply air method contaminant source

type location within the

space heat sources space height.

DV vs MV

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Fig. 8 a,b

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Mixing ventilation is preferable in the following cases: Where the main pollutants are colder/denser than the

ambient air (e.g. dust) Where overheating is the main problem and not air quality Where ceiling height is below 2.3m Where the air flow is often disturbed by movement in the

room

Displacement ventilation has the following advantages: Less cooling needed for a given temperature in an occupied

zone Longer periods with free cooling Better air quality in the occupied zone (though there can be

more airborne respirable dust in the breathing zone)

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DV vs MVD

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Source: Schild, G. P. (2004) Displacement Ventilation, AIVC, Ventilation Information Paper VIP 05

The main weak points of displacement ventilation are: Risk of cold sensation or draught near the floor Wall mounted diffusers take up a lot of space and must not

be blocked Wall mounted diffusers do not permit a lot of flexibility

when it comes to future changes in the layout of the building

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Appropriate ventilation systems fordifferent flow rates and heat loadsThe T lines are temperature

difference between room and supply airwhen ventilation alone is used for cooling

DV vs MV

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Source: Schild, G. P. (2004) Displacement Ventilation, AIVC, Ventilation Information Paper VIP 05

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DV vs MVD

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Comparison between different air distribution systems

Five different air distribution systems


Fig.10 a, b, c, d, e

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Room mean age of airNominal time constant

Pollutant mean concentration in the room Pollutant concentration in the exhaust

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Source: E.H. PRICE (2007) Displacement Ventilation DESIGN GUIDESource: Chapter 25 of 1997 ASHRAE Fundamentals Handbook

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Pollutant mean concentration at breathing level

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Supply diffuserThe type of supply diffuser used will have a direct impact in the ventilation effectiveness.

The overall ventilation effectiveness of overhead diffuser systems may vary due to diffuser type (0.7 < < 1.0 with average = 0.9) and mode of operation

Well-designed displacement ventilation air distribution systems have a ventilation effectiveness that are at least = 1.2 and have the potential for greater ventilation effectiveness when used in combination with dedicated outdoor air systems and radiant heating/cooling systems.

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Ventilation Effectiveness for Different Types of Air Distribution Systems [Source: Krantz]


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Thermal Comfort

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ASHRAEStandard55definesthermalcomfortasaconditionofthemindwhichexpressessatisfactionwiththethermalenvironment.Thisdefinitionisbasedonthefactthateachpersondefineswhatisthermallycomfortablebasedupontheirownphysiologicalandpsychologicalstates.

Controlling stratification in the occupied zone is critical to maintaining occupant comfort. ASHRAE Standard 55 requires the temperature difference between the head and foot level of a standing person not to exceed 5F.

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Design Considerations

Source: E.H. PRICE (2007) Displacement Ventilation DESIGN GUIDESource: Architectural Energy CorporationDisplacement Ventilation Design Guide: K-12 Schools, Final Report, 2006

ASHRAE Thermal Comfort RangeSource: ASHRAE Standard 55-2004

Controlling humidity is the most common question when discussing the concepts of

underfloor air distribution, or displacement air distribution. means different things to different people as their personal

perspectives are different, e.g.: In the office environment, humidity control means limiting the upper

humidity level to the guidelines of ASRHAE Standard 55 in order to provide good thermal comfort.

Museums often need humidity levels to be maintained in a narrow range to slow or prevent decay in artwork and historical displays.

Building Shells Are Sources of HumidityAll buildings leak air through the building shell. This leakage is a direct transfer of moisture into or out of the interior zones and needs to be accounted for in the building moisture loads.

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Humidity Control

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Humidity ControlDesign suggestions

Pretreat Ventilation AirIn a humid climate, the biggest source of moisture is typically the ventilation air from the outside => Pretreated the air for humidity control will control the entire building humidity load without any additional moisture removal

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SeriesFanTerminal

SideStreamBypassHumidityControl

1st approach for humidity control

2nd approach for humidity control

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Direct Expansion Roof Top UnitsDX packaged roof top units may be used to condition raised floor cavities and displacement ventilation. However, care must be exercised to select the proper sized equipment and controls to maintain moisture removal.

Control Tip When a DX system is oversized, the compressors will remove

the cooling load with very little cycle time; in this case, the compressors shut down and the moisture on

the coil will re-evaporate and be added to the air; additionally, the ventilation air is still required and will also

transport moisture into the zone; the net effect is a humid occupied zone.

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Dedicated Dehumidification and Energy RecoveryDehumidification

ASHRAE has several recommendations for dehumidification of a building: Dry the ventilation air first as the bulk of the moisture load in buildings is due to the ventilation air. Lower the design dew point and raise the interior set point dry bulb temperature. When the occupants of a building are in a dry climate, RH < 45%, they will have the same perceived comfort level at 78F as they would at 74F and 50% RH. Interestingly, most people find the dryer and warmer combination more comfortable. Downsize the cooling equipment and use a dehumidifier. If the cooling system is not required to remove latent loads, it can typically have a smaller cooling capacity. This will raise the overall efficiency of the HVAC system and allow for more localized cooling in high sensible loadings such as call centers. This is a great approach for the use of fan air columns in a raised floor application. Remember to analyze the dehumidification cycle at the peak moisture removal load as well as the peak temperature point.

AcousticsThere are typically at least 5 primary sources of sound generation in a displacement ventilation application:

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Fanpoweredterminals1

Controlvalves2

Diffusers3

Airhandlingequipment4

Structuralbornesound.5

The 1st most commonly considered

sound

The 2nd most commonly considered

sound

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Acoustics

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Sample Performance Data for a DF3displacement outlet (example of diffuser noise)

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The NC values for the diffusers are calculated usingASHRAE Standard 70.Largely due to their low pressure drop, displacementdiffusers do not typically have NC values above NC 30.

AcousticsD

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ASHRAE Standard 70: All diffusers are discharging air into a typicalspace that will experience a sound absorption of 10dB in all bands.The tabulated NC values may be corrected for the type of space inyour design by using this formula:

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Catalog NC = Room NC 10 + SEF 10*log 10 N

The design goal Nb of outlets in the space

Example:Private Office space (Design NC = 30).10 ft x 10 ft with 2.5 CFM/SF (295 CFM)The SEF = 5.Number of supply diffusers is 1 (250 CFM)and the number of return diffusers is 1.Catalog NC = 30 10 + 5 10*log 10 (1+1)Catalog NC = 22DF1 Diffuser (48x24x13) at 295 CFMgenerate an NC values < 15 NC.The return grill would be selected to havean NC values of 19 or less as well.

Space Effect Factor [ASHRAE Fundamentals]


Acoustics-Diffusers

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Designing with Air Handling Units AHUs in a displacement ventilation systems must

be able to supply an off-coil supply airtemperature of ~65F [18C] in order to limitdiscomfort.

When climate permits, the use of an economizer isrecommended.

Where dehumidification is required, side steam by-pass or heat recovery wheels can be used.

Variable speed drives on a VAV system will help tosave energy under partial load conditions and willhelp to promote stratification in the space.

Demand control ventilation can be incorporatedinto the displacement system to help reduce theenergy demand of the system in low load casesand still provide the proper space ventilation.

33Source: E.H. PRICE (2007) Displacement Ventilation DESIGN GUIDE

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Fig.11

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Loading Within the Space

A traditional mixing systemconditions the whole space to bean even temperature. The systemthen has be designed to coolthe entire volume of the space

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With displacement ventilation only the occupied zone needs to be conditioned to meet comfort conditions.


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Overhead Air Distribution

Occupied Zone

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Solar, Conduction, and Overhead Lighting

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A- Solar energy gain in the space is both radiant and convective, the amount heating however, depends on the design of the window treatment. Without treatment, the majority of this load falls on the floor.

B- Shades at windows will reduce the amount of energy transferred to the space as the shades will absorb and reflect the energy. Some of the energy will become a convective energy gain outside of the occupied zone



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External/Lighting Loads Fig.15 a,b

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C- In the case of conduction and lighting loads, only a portion of the load remains in the occupied zone. Wall conduction, for example, shown in the figure, will contribute a predicable amount of the heat to the occupied zone

D- Only the radiant component of overhead lighting loads are considered because the convective loads from the lighting remains above the occupied zone by convectingdirectly to the upper zone

Solar, Conduction, and Overhead LightingLoading Within the Space


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External/Lighting Loads Fig.16 a,b

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People and Equipment LoadsPeople and equipment transfer heat to their surroundings by four heat transport methods:

Conduction Convection Radiation Evaporation

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Sensible heat gain to the space

Latent heat gain

ASHRAE 2005 Fundamentals Chapter 30, Nonresidential Cooling, and Heating Load Calculations, give general heat load generated by people and equipment in various states of activities for both sensible and latent components.

Loading Within the SpaceD

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Thermal Plume

Source: E.H. PRICE (2007) Displacement Ventilation DESIGN GUIDEFig.17

Sensible Heat The sensible heat gain to the occupied zone is only a portion of the total sensible load emitted from the occupants. When using displacement ventilation for cooling, only this portion is considered when sizing the air volume and supply air temperature.

The rate at which occupant heat is generated is dependent on several factors:

Clothing levels Metabolic rate Environmental conditions Activity level, etc.

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Latent Heat All of the latent load generated by people and equipment need to be accounted for in the air volume calculation.

Evaporation from occupants, humid air generated by certain equipment, and warm moist air exhaled by occupants all contribute to the space latent load.

Control of the latent portion of the heat generated in the space is critical to controlling the relative humidity of the space.


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Diffuser Type

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One-Way Diffuser

Recessed Diffuser

Wall Mounted

Three-Way Diffuser

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Fig.19 a,b,c,d

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Displacement Linear Enclosure

Semi Circular Diffuser

Circular Diffuser

Corner Diffuser

Diffuser Type

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IndustrialDiffuser

RoundFloorGrille

LinearFloorGrille

Diffuser TypeD

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Fig.21 a,b,c

Some general recommendations for supply diffusers:

Rectangular or semi-circular units are often located on walls opposite to the exterior windows and walls.

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For large rooms wider than 9 m, consider mounting the diffuser on two opposite walls.

For large open spaces, round or rectangular diffusers can be placed in the mid of the space.

For rooms longer than 5 m, consider several diffusers along the wall.

Diffuser Layout and Location

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Long Rooms

Wide Rooms

Large Open Rooms

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Some general recommendations for supply diffusers:

Place diffusers no closer than 2 feet from occupants as shown in the figure:

Avoid placing large obstacles near the diffuser face. Place more diffusers in areas which have a higher cooling load.

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Diffuser Layout and LocationD

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Fig.23

The most common method to supply air to a displacement diffuser is via a ducted connection. Diffusers can be connected from the top, bottom, and sides depending on the function and design of the diffuser.

Balancing dampers required for diffusers should be mounted at least 3 duct diameters away from the inlet connection on the diffuser, in a ducted configuration

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Ducted Connection

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When utilizing a pressurized plenum with a displacement diffuser, ensure that the plenum is properly sealed .

Advantages of using a pressurized plenum are reduced ductwork easier balancing and quicker installation.

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Pressurized Plenum

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Fig.25

Small Office ExampleSmall office in renovated office building.

North facing room Wide =3 m Long =3,65 m Height = 2,7 m 2 occupants Occupation:

from 8:00 to 12:00 and from 13:00 to 17:00 Computer with LCD monitor T8 florescent lighting Control temperature of 22C

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Case Study

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Boardroom ExamplePrivate boardroom in a new office building

In the center of the building Wide = 7,3 m Long = 4,3 m Height = 3 m 8 occupants Computer with LCD monitor T8 florescent lighting Control temperature of 22F Large white board at the west end of the room Cabinets along the south and east end of the

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Case Study D

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Fig.26b

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References

Abbas, T. (1999) Displacement ventilation and static cooling devices (COP 17/99).

Bracknell: BSRIA

Schild, G. P. (2004) Displacement Ventilation, AIVC, Ventilation Information Paper

VIP 05

E.H. PRICE (2007) Displacement Ventilation DESIGN GUIDE

Chapter 25 of 1997 ASHRAE Fundamentals Handbook

Architectural Energy CorporationDisplacement Ventilation Design Guide: K-12

Schools, Final Report, 2006

Hazim B. Awbi, Ventilation Systems Design and performance, 2008

Hazim B. Awbi, Ventilation of Buildings, Second Edition, 2003

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Ventilation Lecture 4 PH Alleen Lezen