Underfloor Air Distribution

What is it Underfloor Air Distribution?

Underfloor air distribution (UFAD) is a method of bringing conditioned air into a space through an underfloor plenum that is below a raised floor system.

Per a Trane engineering newsletter on UFAD systems from 2001 [1], the potential benefits of a UFAD system include improved thermal comfort due to local direct control of supply airflow and reduced ventilation rates because of higher zone air-distribution effectiveness due to stratification. Space stratification, or the temperature differential in a conditioned space that changes with height, and thermal performance of underfloor plenums are key drivers in load calculations for UFAD systems.

UFAD systems typically deliver conditioned air to the space through diffusers mounted in the raised floor. This swirling supply air mixes with air from within the space, creating a mixed-air region in the lower portion of the space. Then, temperature stratification occurs in the upper portion of the space (see figure below).

Displacement ventilation (DV) systems are similar to UFAD systems; however, it typically involves delivering slow-moving air into the space from diffusers placed low in sidewalls [2]. In this system, heat sources (such as people or equipment) induce the local airflow from the floor toward the ceiling. The UFAD diffuser airflows are typically larger than for DV systems, which reduce the ceiling height necessary for acceptable temperature stratification. Additionally, displacement ventilation does not include any effect of heat transfer into the underfloor plenum on the cooling airflow.

Though in real life while many conventional air distribution systems experience some stratification, in TRACE 700 overhead distribution systems assume well mixed spaces where UFAD and DV systems have stratification. TRACE 700 has been able to run UFAD and displacement ventilation calculations since version 6.2 in December 2008.

How does stratification and underfloor plenum heat transfer affect space conditions?

Space stratification is a simple concept where several layers of air at a range of heights have different temperatures. Stratification is important to consider when low velocity air distribution or distribution at the floor level is being considered.

In the case of the UFAD system, temperature stratification in the conditioned space will result in higher temperatures at the ceiling (higher than the room thermostat set point) which will change the heat transfer dynamics in a room and between floors.

To compare between cooling airflow quantities used by a UFAD system and an overhead system, there is a defined acceptable comfort condition for standing occupants in a stratified room. These requirements were taken from the ASHRAE Journal in October 2007 on calculations for UFAD by Bauman [3].

Average occupied zone temperature is calculated as the average of measured temperature profile from the ankle level (4 inches) to head level (67 inches), is equal to the desired thermostat set point.

The occupied zone temperature difference that is calculated from the head to ankle does not exceed the maximum limit of 5 degrees F as specified by ANSI/ASHRAE Standard 55-2010.

UFAD systems provide supply air at higher temperatures when compared to overhead systems based upon the maximum difference specified by ASHRAE 55-2010. As a result of the higher supply air temperature, the potential exists to increase airside economizer operation hours in some climates.

The figure below shows a schematic diagram of an example room air temperature profile identifying the important factors of stratification. The temperature differentials come from the EnergyPlus room air model method UCSD UFAD Interior Underfloor Air Distribution which is also used as the basis for the TRACE 700 room air temperature profiles.

T67 = room air temperature at a height of 67 inches (i.e., head height)

T4 = room air temperature at a height of 4 inches (i.e., ankle height)

Ts = underfloor supply air delivered by diffuser

Tset = thermostat set point at 48 inches above floor

Toz,avg = average room air temperature in occupied portion of space

Toz = temperature difference between the head and the ankle of a standing person in the occupied zone

Furthermore, the radiant contribution of the room loads can have a major effect on temperatures in the room and plenums. Including the radiative heat transfer from the ceiling to the floor and the conduction through the raised floor into the supply plenum, there will be a 35% to 45% heat gain into this supply underfloor plenum and a net heat loss from the return plenum of 10% to 15% [3].

As cool supply air passes through the underfloor plenum, it is exposed to heat gain from both the floor slab below and the raised floor above. This heat gain can be large and result in a temperature gain to the supply air inside the plenum. This will then lead to a possible increase in supply airflow to counter the heat gain.

UFAD Other Considerations

Supply air leakage from the underfloor plenum into the room can often be in the range of 10% to 30% of the design airflow depending on whether or not the underfloor is pressurized [3]. This leakage is accounted for in the design calculations by including the estimated maximum air leakage rate into the predicted airflows. Any leakage above 20% may influence the amount of stratification in the room as well as the ability for the room to control the thermostat set point. However, the leakage will change the load in the space and offset the required amount of increased supply airflow to the diffusers.

How is space stratification calculated in TRACE 700 with underfloor plenum heat transfer?

The UFAD algorithm in TRACE 700 is derived from research by the Center for the Built Environment (CBE) at the University of California as well as some methodology in the EnergyPlus program [2].

The key aspect to determining space stratification is the temperatures from ankle height (4 inches) to head height (67 inches). These temperatures are used to determine a dimensionless temperature, Phi, which is a ratio of the set height temperature to the supply and return air temperatures.

= (T - Ts)/(Tr - Ts)

T = temperature at given height above floor

Ts = supply temperature

Tr = return air temperature

This Phi value is predicted for underfloor air distribution systems as a function of Gamma a non-dimensional parameter [4, 5, 6]. According to Lin and Linden [7], the primary parameters for controlling stratification can be defined in Gamma as the buoyancy flux of the heat source and the momentum flux of the cooling jets. These fluxes are derived through plume theory for the heat source and a fountain model for the diffuser flow. These are non-dimensional values. A large Gamma reduces stratification where a small Gamma produces the opposite effect. The theoretical and experimentally tested model has been implemented in EnergyPlus by using a slightly different definition of Gamma.

Gamma is used in two different calculations, for interior zones as well as perimeter zones as defined by Liu [8]. According Perimeter zones account for the line plumes generated by heat gains from exterior windows and walls. However, TRACE 700 will only use the interior zone relationship in the stratification calculation.

The interior zone relationship is as follows:

= Non-dimensional parameter representing the ratio of buoyancy to inertia forces in the zone

Q = room airflow [m3/s]

Ad = Diffuser effective area

Cos = discharge angle for diffuser flow [m2]

n = number of diffusers

m = number of plumes (i.e., occupants or workstations)

W = room cooling load [kW]

The perimeter zone relationship is as follows:

= Non-dimensional parameter representing the ratio of buoyancy to inertia forces in the zone

Q = total perimeter zone airflow [m3/s]

Ad = Diffuser effective area [m2]

Cos = cosine of discharge angle for diffuser flow

n = number of diffusers;

WL = zone extraction rate per unit length of zone [kW/m], the zone cooling load (supply and return plenum cooling loads are not included) divided by the length of the external wall of the perimeter zone considered

In addition, the temperatures can drastically change based on the type of diffuser used to distribute the air. The choice of a diffuser will give its effective area and a discharged angle factor which will be used in the Gamma equation listed above. The diffuser effective area and angle factor were determined through laboratory testing and manufacturers catalogues.

Diffuser Type

Supply Fan or System Type

Zone Type

Diffuser Effective Area, ft2(m2)

Angle Specific to Diffuser Type, []

Angle Factor Specific to Diffuser Type, cos

Swirl

Constant Volume

Interior

17.05 (0.011)

28

0.883

Variable Area

Variable Volume

Interior and perimeter

54.25 (0.035)

45

0.707

Linear Bar Grill

Constant Volume

Perimeter

38.75 (0.025)

15

0.966

Horizontal Swirl

Displacement Vent

Interior and perimeter

9.3 (0.0060)

73

0.292

Each diffuser has a Gamma-Phi relationship that determines the stratification within the room. This relationship was tested experimentally through CBE [2] and forms the basis for the UFAD stratification algorithm in TRACE 700. However, when selecting a variable air volume UFAD system in TRACE 700, the diffuser type is Variable Area. When selecting a constant volume UFAD system, the diffuser type is Swirl. All displacement ventilation systems assume a diffuser type of horizontal swirl.

Zone

Diffuser Type

Gamma

Phi 4in

Phi 67in

Interior

Swirl

< 4.3

4.3 < < 46

> 46

0.4212

0.2684 0.3089

0.875

0.944

Interior

Variable Area

0.745

0.956

Perimeter

Linear Bar Grill

< 11

11 < < 29.56

> 29.56

0.47

0.2199+0.0224

0.882

0.882

Perimeter

Variable Area

0.631

0.874

Furthermore, after determining the Phi value based on Gamma, the temperatures at head height and ankle height can be calculated but any downstream air leakage associated with the UFAD system will affect how these temperatures are calculated. These temperatures will then be used in solving for the average occupied zone temperature.

The average occupied zone temperature is important as it is the acceptable comfort condition for standing occupants. Depending on the stratification in the space, the average Toz is typically lower than the single point thermostat temperature at 48 inches for overhead distribution systems. From an application standpoint, according to Bauman [3], to maintain equivalent comfort conditions (well-mixed to a stratified environment) it is recommended to raise the thermostat set point at 48 inches by one degree F for a Toz at three degrees F and by 0.5 degrees F for a Toz at two degrees.

In TRACE 700, the UFAD stratification calculations are iterative due to the constantly changing fraction of total space sensible loads being transmitted to the underfloor. The conduction calculation derived from Bauman, H. Jin, T. Webster [9] is very complex; however, it is more or less a steady-state energy balance iteration that is performed until the conduction load through the raised floor and average underfloor supply air temperature come into balance.

FSLO = oz (1 - FSLU)

FSLO = Fraction of Total Space Sensible Loads to Occupied Layer

FSLU = Fraction of Total Space Sensible Loads to Underfloor

Once the space sensible load to the underfloor is determined based on conduction through the underfloor over the overall total space sensible load, the fraction is used in conjunction with the dimensionless temperature of the average occupied zone to determine a fraction of total space sensible loads to the occupied layer.

FSUS = (1 FSLO FSLU)

FSLO = Fraction of Total Space Sensible Loads to Occupied Layer

FSLU = Fraction of Total Space Sensible Loads to Underfloor

FSUS = Fraction of Total Space Sensible Loads to Upper Stratified Layer

Every time the fraction of total space sensible loads to the occupied layer changes, the required airflow to the space changes, which also changes Gamma and Phi. The change in the required space airflow will also affect the convective and radiant exchange in the underfloor supply plenum. The conductive loss will then increase or decrease through the raised floor which will impact the space sensible load fraction to the underfloor.

During both the Design and System Simulations the program uses an algorithm to check whether hourly conditions will actually produce temperature stratification for a particular room. For example, if the supply air is warmer than the room air, the driving force for the convective plumes will not be present. Similarly, if the room is not occupied and has low internal heat gains, the driving force that causes the heat to rise towards the ceiling exhaust will not affect the flow pattern. Chilled ceiling panels or passive chilled beams that supplement the cooling capacity of displacement ventilation or UFAD systems will create downdrafts in the occupied portion of the space, thus reducing the amount of convective fraction that goes to the upper stratified layer. For any hour that these conditions occur the program assumes that stratification does not occur and the associated room air is fully mixed.

Furthermore, stratification will not occur in TRACE 700 under these conditions:

1) If UFAD or DV system is NOT selected

2) Supply air temperature is warmer than the room air temperature

3) Room has low internal heat gains, i.e., (InternalSpaceSensibleNoStratification - ConductionLossToUnderfloorPlenum)/RoomFloorArea < 0.1 Btuh/sqft)

4) Chilled ceiling panels or passive chilled beams are operating at more than 5% of their aux cooling capacity

5) Kill stratification during heating or deadband mode (otherwise this will increase heating load by moving cooling space load to the upper return air layer)

What are the key input parameters for UFAD systems in TRACE 700?

There are several parameters in TRACE 700 that are important to the methodology behind the UFAD system load calculations.

Supply air path / duct location

For Supply air path / duct location, there are four options to choose from. Additionally, these options will also be affected by supply duct / other losses if entered.

No duct, Supply via Underfloor Plenum

Ducted through Underfloor Plenum

Supply via Sidewall Displacement Ventilation

Supply via Underfloor Displacement Ventilation

If No duct, Supply via Underfloor Plenum or Supply via Underfloor Displacement Ventilation is selected, a complex heat exchange can occur between the slab, raised floor panels, side walls (if they exist), and the supply air as it flows through the underfloor air plenum. If the room is warmer than the underfloor plenum, the supply air traveling in that plenum will pick up heat from the room. If the floor slab has absorbed any heat or is exposed to outdoor conditions or an unconditioned space, then the supply air temperature will be impacted based on conduction heat gain/loss into the underfloor plenum.

If Ducted through Underfloor Plenum or Supply via Sidewall Displacement Ventilation is selected, then no heat exchanged is assumed between the underfloor plenum and the supply air. The Ducted through Underfloor Plenum is also used in configurations where directed airflows are used to move the air through an underfloor plenum with very minimal interaction with the floor slab and raised floor.

As discussed earlier, there can be significant leakage in UFAD systems due to the floor panel configuration as well as poor construction/sealing. In general, downstream leakage is assumed to leak into the occupied layer above the current spaces underfloor plenum while the upstream leakage is assumed to bleed into the corridor return air path, thus the plenum temperature will not be affected but the return air condition to the AHU coils will be impacted.

The upstream leakage fraction is the upstream leakage flow divided by the supply fan design airflow and it is a constant for all part load ratios. The downstream leakage fraction is a constant fraction of the mixing box airflow, which may vary during system operation for a VAV. The sum of the two fractions at design will not equal the total leakage fraction.

The auxiliary cooling coil losses to plenum %, the auxiliary cooling coil devices which are either recessed or suspended from the ceiling (such as passive, active chilled beams and chilled ceiling panels) will lose a portion of their capacity in the form of radiation and/or convection to the ceiling plenum. This loss will be proportional to the hourly cooling load.

For UFAD systems, in the Advanced options of Create Systems, If the Supply Duct / Air Path Location = Return Air, the supply ducts are assumed to be located in the ceiling plenum and so both the upstream and downstream duct leakage is assumed to leak into the associated return plenum, either raising or lowering the plenum temperature accordingly; however, if no ceiling plenum exists for a particular room, then the leakage is assumed to bleed into the corridor return air path and so will still effect the return air condition seen by the AHU coils. If the return air path is Room Direct, all leakage occurs directly to the room instead of the ceiling plenum.

If the Supply Duct / Air Path Location = Other, then the leakage is assumed to bleed to ambient and the supply air lost and its air conditioning effect wasted.

For all other Supply Duct / Air Path Location locations, i.e., supply air delivered via an underfloor and/or displacement ventilation, the site of the downstream vs upstream leakage differs. In order to distinguish where the leaks occur, the downstream leakage is assumed to leak into the occupied layer above the current rooms underfloor plenum while the upstream leakage is assumed to bleed into the corridor return air path and so will not affect the plenum temperature but will still affect the return air condition seen by the AHU coils.

To better explain upstream/downstream leakage, lets take a look at a basic example of a system with 10% upstream leakage and 10% downstream leakage for a standard VAV system. If the supply fan airflow is 10,000 CFM, then the upstream leakage is 1,000 CFM (10% of 10,000 CFM) and 9,000 CFM reaches the terminal mixing boxes. The downstream leakage is therefore 900 CFM (10% of 9000 CFM) and 8,100 CFM reaches the room via the supply diffusers. This means that a total of 1,900 CFM or 19% of the 10,000 CFM supply fan airflow has leaked from the ducts.

Space Sensible Gains to Occupied Layer

If cool supply/ventilation air is introduced from the floor or a wall near the floor at certain velocities, a plume is formed around the people or miscellaneous equipment load. A portion of sensible heat will then travel upward causing thermal stratification to occur. This value represents the percent of space sensible cooling load that remains in the occupied lower layer of a stratified room during UFAD or displacement ventilation operation and can be input into the Space Sensible Gains to Occupied Layer field. TRACE 700 will automatically predict a value for pressurized underfloors only. However, if necessary, these values can be overridden. In general, since this research is still in a state of flux and the user is allowed to override the predicted value should better research values findings become available. Below is a table of defaults assumed by TRACE 700.

This field only shifts the sensible portion of the internal heat gains and has no effect on space latent heat gains. Heat gains due to the envelope loads are directly affected by the temperatures of the stratified air regions and are accounted for automatically by TRACE 700.

Defaults

Supply Duct / Air Path Location

Space sensible gains to occupied layer, %

Design Supply Air Dry Bulb

No duct, Supply via Underfloor Plenum

63 F

Ducted through Underfloor Plenum

50

63 F

Supply via Sidewall Displacement Ventilation

See Appendix

*63 F

Supply via Underfloor Displacement Ventilation

65 F

*Note that DV with induction mixes cold primary air with room air to achieve a mixture of 65F so Dsadbc defaults to 55F in that case.

Since a portion of the space sensible loads are shifted to the upper stratified layer, there is a reduction of the space sensible load used to calculate the cooling supply airflow rates. Cooling airflow may not decrease depending on whether or not a higher cooling supply air dry bulb is typically used with this configuration. Perimeter zones differ from interior zones in type and magnitude of heated thermal plumes. In perimeter zones, the dominant plume occurs vertically at windows, and, under peak load conditions, can be several times greater in magnitude than the total internal heat gains.

In general, TRACE 700 uses an algorithm to check if the hourly conditions will be producing temperature stratification for each space. The algorithm will dependent on the supply air temperature and the internal loads in the space.

Underfloor Plenum Height

The Underfloor Plenum Height field determines the height of the supply air path in a UFAD system.

The floor-to-floor height entered on the Create Rooms Rooms screen should include the underfloor plenum height. This field will affect the amount of the wall area that the space sees. This input and the conductive resistance field are used to calculate the amount of heat pickup into the underfloor plenum and heat loss from the space. The underfloor plenum is assumed to be a dead air space unless this room has been attached to a UFAD system. The TRACE 700 default is 1.5 feet.

Conductive Resistance of Raised Floor

This field is the combined resistance of the carpet and the raised floor panel and should not include any air film resistances. This field and the underfloor plenum height field are used to calculate the amount of heat pickup into the underfloor plenum and heat loss from the space. The TRACE 700 default is 1 (hr-ft2-F)/Btu.

Workstations

This field defines the number (or density) of computer workstations located in the room. For HVAC system types that rely on thermal (temperature) stratification, this value helps define the number of thermal plumes to drive the stratification.

This field is only used when the room is attached to an underfloor air distribution or displacement ventilation system, or if Watts/workstation or Btuh/workstation is selected as the units for Miscellaneous Loads.

How to model UFAD systems in TRACE 700?

To model UFAD systems in TRACE 700, please look at page 4-84 in the TRACE 700 Users Manual for specific examples or view our eLearning video on this subject.

SOURCES

1) Trane Engineers Newsletter, Underfloor Air Distribution, 2002, ADM-APN001-EN

2) Schiavon, S., F. Bauman, K.H. Lee, and T. Webster, 2010. Development of a simplified cooling load design tool for underfloor air distribution systems. Final Report to CEC PIER Program. CEC Contract No. 500-06-049, July

3) ASHRAE Journal, Cooling Airflow Design Calculations for UFAD, Fred Bauman and Tom Webster, Oct 2007; ASHRAE Research Project 949, System Performance Evaluation and Design Guidelines for Displacement Ventilation,

4) Y.J.P. Lin and P.F. Linden, A model for an underfloor air distribution system, Energy and Buildings 37 (2005) 399-409.

5) Q.A. Liu, and P.L. Linden, The EnergyPlus UFAD module. Proceedings of the Third National Conference of IBPSA-USA, Berkeley, California, US, (2008) 23-28.

6) T. Webster, W. Lukaschek, D. Dickerhoff, and F. Bauman, Energy Performance of UFAD Systems. Part II: Room air stratification full scale testing. Final Report to CEC PIER Program. CEC Contract No. 500-01-035. Center for the Built Environment, University of California, Berkeley, CA, 2007.

7) Q.A. Liu, and P.L. Linden, The fluid mechanics of underfloor air distribution, Journal of Fluid Mechanics 554 (2006) 323-341.

8) Q.A. Liu, The fluid mechanics of an underfloor air distribution system, PhD thesis. University of California, San Diego, US 2006.

9) F. Bauman, H. Jin, T. Webster. Heat Transfer Pathways in Underfloor, Air Distribution (UFAD) Systems, ASHRAE Transactions, Vol. 112, Part 2, 2006.

10) Price Industries, Engineering Guide for Displacement Ventilation, 2011

APPENDIX. Sidewall Displacement Ventilation default % Space Sensible To Occupied calculation

A procedure for calculating the % Space Sensible to Occupied Layer, PctSensToOccupied, is indirectly provided by an ASHRAE publication written by Chen and Glicksman. These equations have been simplified and examples created in the Price Engineering Guide for Displacement Ventilation [10].

First, we need to recognize that a displacement ventilation system can sometimes require a larger fan than an associated mixing ventilation system. Although the return air temperature with the displacement ventilation system will always be higher than that with the mixing ventilation system, the air temperature difference between the air return and supply is smaller. This is because the air supply temperature in the displacement ventilation system is much higher. Typically, for the displacement system,

Treturn Tsupply = 81F 64F = 17F or (27C 18C = 9C)

and for the mixing system,

Treturn Tsupply = 77F 55F = 22F or (25C 13C = 12 K)

In the above example, to remove the same amount of cooling load would require a larger amount of supply air with displacement ventilation.

So let's run through the Example 1 provided by Price and see how the adjustment is made in TRACE 700:

Qoz = 800 Btuh (people)

Qlit = 825 Btuh (overhead lights)

Qex = 450 Btuh (envelope, including infiltration)

Qt = Qoz + Qlit + Qex = 2075 Btuh

Step 1: Determine the air flow rate to meet the cooling load

QHeadToFoot = aoe*Qoz + alit*Qlit + aex *Qex = 428.15 Btuh

= 0.295*Qoz + 0.132*Qlit + 0.185*Qex = 428.15 Btuh

(WARNING: aoe, alit, and aex = 0.295, 0.132, and 0.185 are based on work by Chen and Glicksman. They clearly state that these coefficients stand for the fractions of the cooling loads entering the space between the head and feet of a sedentary occupant. However, the PctSensToOccupied value expected by Trace700 represents the sensible cooling loads entering the space between the floor and average occupant height. You might be tempted to use the weighted average of these coefficients to come up with PctSensToOccupied but that is not the correct approach which is shown below.)

Assume TDHeadToFoot= 5.4 F

QDV_cfm = QHeadToFoot / (k * TDHeadToFoot)

= 428.15 / (1.08*5.4) = 73 cfm

Step 2: Determine fresh air flow rate

Qoz_cfm = 14 cfm

Qs_cfm = Max(QDV_cfm, Qoz_cfm) = Max(73, 14) = 73 cfm

Step 3: calculate supply air temperature

Az = 120 ft2

Ts = Tspace - TDHeadToFoot - (Az * Qt) / (2.456*Qs_cfm^2 + k*Az* Qs_cfm)

= 72 - 5.4 - (120*2075) / (2.456*73^2 + 1.08*120*73)

= 72 - 5.4 - 11 = 55.6F

Step 4: Determine the return air temperature

Te = Ts + Qt / (k*Qs_cfm) = 55.6 + 2075 / (1.08*73) = 55.6 + 26.3 = 81.92 F

Step 5: Adjust for new supply temp

Ts = 63F

QDV_cfm = Qt / [k * (Te-Ts)] = 2075 / [1.08*(81.92 - 63)] = 101.55 cfm

Step 6: Calculate the space sensible load in occupied layer using Tspace - Ts to match the Trace700 assumptions:

QSpaceSensToOccup = k*QDV_cfm*(Tspace - Ts)

= 1.08*101*(72-63) = 981.72 Btuh

Note: QSpaceSensToOccup is what checksums shows as the Grand Total in the Cooling Space Peak column.

Step 7: What is the overall Space Sensible Gains to Occupied Layer percent?

PctSensToOccupied = (QSpaceSensToOccup / Qt) * 100

= (981.72 / 2075) *100 = 47.3%

Of course, the PctSensToOccupied value above would vary depending on the proportions of the different load components for each space so it is better to leave this field blank in TRACE 700 and let the default calculation above take place.