Analysis and design of room air distribution systems.
A building requires supply of fresh air and removal of heat, gases, and particles that are emitted in the building. It is not sufficient to supply clean air to an individual room in a building with the correct temperature and flow rate--it is also necessary to design an air distribution system in the room in such a way that occupants experience high air quality and thermal comfort in the occupied zone. Therefore, it is important that the occupied zone has the optimum climate with respect to air temperature, air velocity, temperature and velocity gradients, mean radiant temperature, and asymmetric radiant temperature. It is also important that the supply air reaches all parts of the occupied zone without the presence of stagnant zones. An important element in a design procedure for an air distribution system is to ensure an environment free from draft in the occupied zone. Some design procedures predict the velocity where the flow passes imaginary surfaces that define the occupied zone, and other methods make a direct prediction of the velocity close to people in the occupied zone. The design procedures may be based on semi-analytical flow element models, on computational fluid dynamics (CFD) predictions, or on full-scale experiments.
FLOW ELEMENTS AND AIR DISTRIBUTION
The principle behind design with flow elements is to divide the flow in the room into areas that can be treated independently of the surrounding flow (ASHRAE 2005; Awbi 2007; Etheridge and Sandberg 1996). Flow elements such as the velocity from a given diffuser [u.sub.x], for example, are found from experiments. This flow element can be used in a design procedure to establish the velocity [u.sub.ocz], where the flow passes the imaginary horizontal surfaces that define the occupied zone (see Figure 1a). The next step in the design procedure is to select a diffuser with such a capacity that this velocity has an acceptable level in the occupied zone. The design is often based on a situation where the flow is isothermal and the jet penetrates the whole room. The design process can also include the real non-isothermal flow situation. In this case, the penetration length of the jet should be restricted to have at least a minimum length [x.sub.s], for example, half of the room length. The different design procedures for mixing ventilation using flow elements are addressed by Nielsen et al. (2001) and Awbi (2007).
[FIGURE 1 OMITTED]
The flow element describing the stratified flow from a wall-mounted low-velocity diffuser is used in the design of the air distribution in a room with displacement ventilation. For radial flow the velocity [u.sub.x] will decrease with the distance from the diffuser. The selection of diffuser, flow rate, and temperature are made in such a way in the design process that the velocity [u.sub.ocz] at the imaginary vertical surfaces defining the occupied zone close to the diffuser (Figure 1b) has an acceptable level.
Some new air distribution systems cannot be designed by the use of flow elements because it is difficult to describe the element in a general way. This is, for example, the case for the vertical air distribution system described later.
Experiments in a full-scale room show that the velocity [u.sub.ocz] is not always a good expression of the maximum velocity in the occupied zone [u.sub.rm] (Figure 1). The connections between the velocities are dependent on the actual air distribution system. This is a problem when flow elements are used in the design method because the method is often based on the velocity [u.sub.ocz] on the imaginary surfaces that define the occupied zone.
The draft (the design limits for the system) is not always generated by the flow from the supply openings; it can also be generated by the heat load in the room. This is, for example, the case for a diffuse ceiling inlet where the whole ceiling, or part of the ceiling, is the inlet. Full-scale experiments and CFD predictions are used in the design process in such cases, as is shown later in this paper, and in such situations it is not possible to use flow elements for the air distribution system in a design procedure.
One of the aims of the design of an air distribution system is to find the limits of the diffuser regarding possible flow rates into the room and temperature differences between the supply and return temperatures, i.e., to find the limits that maintain an acceptable comfort level with low draft and low temperature gradients in the room. To make the decisions more clear, a design chart has been developed and used (see Nielsen , Jacobsen et al. , and Nielsen et al. ). This design chart has been utilized in connection with several ventilation principles used in the same test room (see Nielsen et al. ). By use of the design chart it becomes possible to compare different systems to find a system usable for the demands in a certain situation.
Figure 2 describes the idea behind a design chart for air distribution in rooms. The chart is based on the minimum and maximum allowable flow rate [q.sub.o] to the room and also on the maximum temperature difference between return and supply, expressed as a [q.sub.o]-[DELTA][T.sub.o] chart. The figure indicates that it is necessary to have a minimum flow rate of fresh air into the room to obtain a given air quality. This flow rate is constant and independent of [T.sub.o] when the air distribution system generates mixing, but it can be a modest function of [DELTA][T.sub.o] in case of stratification, as in displacement ventilation. It is typical of air distribution systems based on the supply of momentum flow that this flow generates draft when the flow rate is above a certain level. The systems, on the other hand, also generate a mixing in the occupied zone, which is important for the creation of uniform conditions when the heat load is high. Some systems, such as a diffuse ceiling inlet, will not generate any air movement and will therefore not show the limit of [q.sub.o] or [T.sub.o]. Draft is generated by the heat load in the room, and this will limit the product of [q.sub.o] and [T.sub.o] (the thermal load).
[FIGURE 2 OMITTED]
The temperature difference [DELTA][T.sub.o] between return and supply is also restricted, as indicated in Figure 2. A too-high temperature difference may either cause draft in the occupied zone or create a too-large temperature gradient in the room.
Figure 2 indicates an area for [q.sub.o] and [DELTA][T.sub.o] that supplies sufficiently fresh air and ensures draft-free air movement in the occupied zone plus a restricted vertical temperature gradient. This area is the design area for a given air distribution system. The limits for this area in the design chart are often defined as a maximum air velocity of 0.15 m/s in the occupied zone, a maximum vertical temperature gradient of 2.5-3.0 K/m, and a minimum flow rate of 10 L/s per person, but other reference values can of course also be selected. Draft is not only dependent on the velocity but also on the air temperature and turbulence intensity. This is considered by expressing the obtained draft rating (DR) as a function of the flow rate for the different systems.
The ventilation system will show a limit for the maximum flow rate in connection with the design of the duct system. This limit can also be shown in a design chart, as illustrated in Figure 3. There is also a restriction on the temperature difference [DELTA][T.sub.o] and the flow rate [q.sub.o] in connection with the maximum cooling capacity, and the temperature may be restricted, for example, in cases where low temperature outdoor air is used for cooling. Other limits for [T.sub.o] and [q.sub.o] can also be incorporated in the design chart, such as the noise level from the system or the diffuser (see Figure 3).
[FIGURE 3 OMITTED]
The horizontal axis can have other variables as supply velocity [u.sub.o] for a given diffuser (Nielsen 1980) or fan speed ([min.sup.-1]) of a fan in the ventilation system (Larsen et al. 2007).
Three important parameters are considered in the design of room air distribution. The parameters are the air velocity (draft), the vertical temperature gradient, and the asymmetry in the mean radiant temperature. The air velocity can either be the velocity [u.sub.ocz] when it penetrates the upper boundaries of the occupied zone (1.8 m above the floor), the velocity [u.sub.01] close to the ankles of a person, or the velocity [u.sub.11] at head height of a seated person.
The limits shown in the design chart (Figure 2) can in some situations be found from a design procedure using flow elements such as, for example, those addressed by Nielsen (1980) and Jacobsen et al. (2002). Some new air distribution systems cannot be designed by the use of flow elements because it is difficult to describe the elements in a general way. The supply flow may also occupy a large area of the room with the consequence that different types of important flow occur between the flow elements. Experiments and the similarity principle are used in this case. The similarity principle shows that any nondimensional velocity in the room can be given as a unique function of the Archimedes number, Ar, if the flow in the room is a fully developed turbulent flow (high Reynolds number flow) (see Tahti and Goodfellow ). The similarity principle is used to obtain a formulation of the velocity level for [u.sub.ocz], [u.sub.01], and [u.sub.11].
[u.sub.ocz]/[u.sub.o] = [func.sub.1](Ar) (1)
[u.sub.01]/[u.sub.o] = [func.sub.2](Ar) (2)
[u.sub.11]/[u.sub.o] = [func.sub.3](Ar) (3)
The Archimedes number, Ar, is given by
Ar = [[[beta]gd[DELTA][T.sub.o]]/[u.sub.o.sup.2]], (4)
where [beta], g, d, and [DELTA][T.sub.o] are the thermal expansion coefficient, gravitational acceleration, reference length, and temperature difference between return and supply flow, respectively; [u.sub.o] is the supply velocity given as a face velocity [q.sub.o]/[a.sub.o]; [q.sub.o] is the flow rate to the room; and [a.sub.o] is a reference area of the diffuser.
It is possible to find the limits in the design chart from Equations 1-3, even if the experiments are made at conditions different from the boundaries in the chart, as explained in Figure 4, which shows a given experiment ([q.sub.o], [T.sub.o]). The curve corresponds to a constant value of [T.sub.o]/[q.sub.o.sup.2], which is also a constant Archimedes number. The three equations are therefore constant along the curve, and it is possible to find the position of the limiting [q.sub.o] for a given reference velocity (as indicated in the upper part of the figure) from the expression (Equations 1-3)
[FIGURE 4 OMITTED]
u = ([q.sub.o]/[a.sub.o]) const, (5)
although no measurements have been made at this flow rate.
The draft is not always generated by the momentum flow from the supply openings; it can also be generated by the heat load in the room. Full-scale experiments and CFD predictions are used in the design process in such cases and it is not possible to use Equations 1-3. The maximum load in the room can be found from
u = [func.sub.4](Q), (6)
where u is a velocity in the occupied zone and Q is the thermal load removed by the ventilation system. A reference velocity such as 0.15 m/s will give the corresponding load [Q.sub.max].
DIFFERENT AIR DISTRIBUTION PRINCIPLES
This paper addresses five air distribution systems: mixing ventilation from a wall-mounted terminal, mixing ventilation from a ceiling-mounted diffuser, mixing ventilation from a ceiling- mounted diffuser with swirling flow, displacement ventilation from a wall-mounted low-velocity diffuser, and a low-impulse system based on a textile terminal. All these systems are summarized in Table 1 (Nielsen et al. 2003, 2005, 2006).
Table 1. Five Different Air Distribution Systems System Supply Return Mixing End wall-mounted Return opening [??] ventilation below supply terminal Mixing Ceiling swirl End wall-mounted [??] ventilation diffuser below ceiling Mixing Radial ceiling End wall-mounted [??] ventilation diffuser below ceiling Displacement End wall-mounted End wall-mounted [??] ventilation below ceiling Vertical Ceiling-mounted End wall-mounted at [??] ventilation low-impulse textile floor level terminal
The systems are all tested in the same full-scale room. The dimensions of the room are in accordance with the requirements of the International Energy Agency Annex 20 work (Lemaire et al. 1993), with length, width, and height equal to 4.2, 3.6, and 2.5 m, respectively.
Figure 5 shows the furnishings and the heat load of the room (an office layout). The heat load consists of two PCs, two desk lamps, and two manikins producing a total heat load of 480 W.
[FIGURE 5 OMITTED]
The thermal load is constant in all experiments, and the air change rate has been raised from 2.75 [h.sup.-1] up to 10.45 [h.sup.-1], with a variation in the temperature difference [DELTA][T.sub.o] between 12 K and 3.5 K. The room temperature is close to 22.9[degrees]C in all of the experiments.
COMPARISON BETWEEN FIVE DIFFERENT AIR DISTRIBUTION SYSTEMS
It is possible to make a direct comparison between the different air distribution systems shown in Table 1 because the experiments are made in the same room with the same office equipment (two computers, two desk lamps, and two manikins). The results are given in Figure 6.
[FIGURE 6 OMITTED]
The limiting design parameter for mixing ventilation generated by a wall-mounted diffuser is the velocity [u.sub.ocz] of the jet when it penetrates the upper boundaries of the occupied zone in the case of non-isothermal flow. The maximum penetration velocity through the occupied zone [u.sub.ocz] is equal to 0.4 m/s because this corresponds to a level where [u.sub.01] and [u.sub.11] are up to 0.15 m/s. In this case, the vertical temperature gradient is not considered to be important.
The experiments with the radial diffuser show that [u.sub.01] equal to 0.15 m/s corresponds to both a [u.sub.ocz] value close to 0.3 m/s and a maximum [DELTA][T.sub.o] value of 10 K. It can thus be concluded that [u.sub.ocz] equal to 0.3 m/s combined with [DELTA][T.sub.o] = 10 K should be the maximum entering velocity in the occupied zone and the maximum temperature difference for a design with a radial diffuser.
The experiments with the swirl diffuser show that [u.sub.01] equal to 0.15 m/s corresponds to a [u.sub.ocz] value in the range of 0.3 to 0.4 m/s. It can thus be concluded that [u.sub.ocz] equal to 0.35 m/s should be the maximum entering velocity in the occupied zone for a design with a ceiling-mounted diffuser with swirl.
The design area for the diffuser with swirling flow is located at lower airflow rates than the design area for the radial diffuser. A swirl diffuser with large supply openings could, if necessary, move the design area toward large airflow rates because an increase in the supply area results in a decrease of the corresponding velocity level in the occupied zone. There is no upper limit for [DELTA][T.sub.o] for this diffuser. The high mixing rate in the initial flow removes temperature differences in the flow and eliminates a large part of the buoyancy forces.
The limiting design parameter for vertical ventilation from a ceiling-mounted textile terminal is the velocity [u.sub.ocz] of the downward-directed jet established when the flow is non-isothermal. Draft-free conditions around the manikins correspond to [u.sub.ocz] [less than or equal to] 0.2 m/s. The vertical temperature gradient is not important for this type of flow because the thermal plumes from heat sources and the downward-directed displacement flow from the terminal generate a large mixing of the room air.
Displacement ventilation has a high air velocity in the stratified flow at the floor. Reduced velocity in the occupied zone is therefore obtained by restricting the velocity in the stratified flow where it enters the vertical boundary of the occupied zone in front of the diffuser. This is expressed by the length of the adjacent zone [l.sub.n], which is the distance from the diffuser to a given velocity level in the stratified flow (see Nielsen  and Skistad et al. ). The vertical temperature gradient is also important in displacement ventilation and should be kept below 2.5-3.0 K/m.
The length of the adjacent zone [l.sub.n] is given as 1 m with a reference velocity of 0.2 m/s in the case of displacement ventilation, and the temperature difference [DELTA][T.sub.o] is in principle restricted to 12.5[degrees]C in the room, giving a gradient of 3 K/m. The curves in Figure 6 show the corresponding area for fulfilment of thermal comfort in the [DELTA][T.sub.o], [q.sub.o] chart. It should be mentioned that there is a possibility to reduce the minimum airflow rate for the displacement ventilation system because the stratification of supply air improves the air in the inhalation zone when the contaminant source in the room is associated with a heat source far away from the occupant. This effect has not been considered in Figure 6.
Figure 6 indicates that to some extent the room has the same level of comfort (in terms of maximum velocity and temperature gradient) in the case of mixing ventilation with a wall-mounted diffuser or displacement ventilation with a wall-mounted low-velocity diffuser. The figure also shows that the vertical ventilation (low impulse) systems are superior to both mixing ventilation based on a wall-mounted diffuser and displacement ventilation (the [q.sub.o]-[T.sub.o] curve is located to the right of the curves of the two other systems). In connection with vertical ventilation it should be noticed that the layout with two work places in a small room may result in the ability to handle a higher load compared with the situation in a large room with vertical ventilation. (The lower curve in Figure 6 for vertical ventilation is obtained for one manikin in the room, corresponding to two persons in a larger room.)
Mixing ventilation generated by a radial ceiling-mounted diffuser is able to handle a higher heat load than any of the other systems. This is indicated in Figure 6, with a [q.sub.o]-[T.sub.o] curve located above and to the right of the curves of the two other systems.
It should also be emphasized that the design chart, Figure 6, to some extent is dependent on room size, room layout, and layout and design of the terminal units (number, location, etc.).
It is possible to use the method behind the design chart in rooms larger than the test room in this paper as well as for other types of diffusers. It is necessary to work with modified flow elements (wall jet, penetration length, stratified flow, etc.) corresponding to the actual room dimension, which results in a specific design chart for the room and the diffuser.
The design models discussed in the previous section can only give the limits for the operation of the air distribution system. It is necessary to consider thermal comfort for all flow rates if the system has to be optimized.
The thermal environment often shows temperature gradients, velocity gradients, different turbulence levels, and an asymmetric radiant temperature distribution. The local discomfort, which is the result of this environment, is found from measurements of the local values of air temperature, air velocity, and turbulence level and from measurements of surface temperatures or asymmetric radiant temperatures (see Fanger and Langkilde , Olesen et al. , Fanger et al. , and Toftum et al. ).
The number of dissatisfied persons because of draft, the draft rating (DR), is used as a measure of local discomfort. DR is defined as
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (7)
where [t.sub.a], u, and Tu are ambient air temperature, air velocity, and turbulence intensity, respectively (see Toftum et al. ), and d is a factor that is dependent on direction.
Equation 7 is especially valid for a situation where the draft velocity is exposed to the back of the neck of the persons. In this paper the highest velocity is often found at ankle level, and the sensitivity at this position is slightly lower than that of the back of the neck. The DR given from Equation 7 therefore represents a conservative estimate.
Figures 7a and 7b show the DR for the persons at positions A and B (see Figure 5). DR is dependent on the location of the work place in mixing and displacement ventilation, which, however, is not the case for the vertical ventilation system. Position A is the work place located farthest from the wall with the inlet openings, and position B is the work place closest to this wall. Generally, the best results are obtained for displacement ventilation with a work place far away from the diffuser, while mixing ventilation with a wall-mounted diffuser has a high DR. The vertical ventilation system gains from the high level of buoyant flow in the room, which in this case protects the manikins from draft. Mixing ventilation with a ceiling-mounted diffuser that generates flow with swirl has a DR close to the best result obtained by displacement ventilation at position A. The DR for mixing ventilation with a ceiling-mounted radial diffuser also shows a low value at both positions under the optimal conditions around 0.055 [m.sup.3]/s. All systems with ceiling-mounted diffusers (radial diffuser, swirl diffuser, textile terminal) are superior to systems with end-wall-mounted diffusers.
[FIGURE 7 OMITTED]
The air distribution system can in many cases be designed by a dimensional analysis method based on flow elements, a procedure that is mentioned in this paper. Many new air distribution systems are difficult to define from a combination of flow elements and other methods must, therefore, be used. It is shown that results obtained by experiments or by CFD can be expressed in a [q.sub.o]-[T.sub.o] design chart. It is possible to compare measurements on different air distribution systems in such a design chart if the experiments are made in the same room with the same heat load geometry.
It is shown that air distribution systems based on mixing ventilation and ceiling-mounted diffusers are able to generate comfortable conditions up to a thermal load that is slightly superior to systems with mixing ventilation from wall-mounted diffusers, displacement ventilation with a wall-mounted low-velocity diffuser, and vertical ventilation systems with ceiling-mounted textile terminals.
It is characteristic that a mixing ventilation system with a ceiling-mounted swirl diffuser does not restrict the temperature difference between return and supply within the level of traditional design practice. This effect is seen in Figure 6 as a lack of limitation of [DELTA][T.sub.o]. A similar effect can be seen for the vertical ventilation system, which is without restrictions on both temperature difference [DELTA][T.sub.o] and flow rate [q.sub.o].
The DR is low for a mixing ventilation system with an optimal design and restricted heat load. A mixing ventilation system with an end wall diffuser and a displacement ventilation system with an end-wall-mounted low-velocity diffuser both have a higher DR under the design conditions.
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Peter V. Nielsen, PhD
Peter V. Nielsen is a professor in the Department of Civil Engineering, Aalborg University, Denmark.
Received March 29, 2007; accepted August 13, 2007
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|Author:||Nielsen, Peter V.|
|Publication:||HVAC & R Research|
|Date:||Nov 1, 2007|
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