VARIABLE-AIR-VOLUME (VAV) SYSTEMS The VAV system is based on the principle of matching the load by varying the supply air volume rather than varying the supply air temperature, with the intent of saving fan power as compared with a CAV system. VAV system is a centralized all-air system, either single zone or multi-zone. The common configurations include the following: -
cooling or heating only VAV reheat VAV dual duct fan-powered VAV
Applications: office complexes, department stores, supermarkets, auditoriums and etc. ( i.e. spaces with fluctuating loads)
1. VAV COOLING COOLING ONLY ONLY SYSTEMS SYSTEMS 1.1
Cooling only (multi-zones): (multi-zones):
The variable-air-volume VAV cooling system as shown in Fig. 1 is an all-air, multi-zone, single-duct system. It only provides year-round supply of cold air to the following areas: (i) (ii) (iii)
interior zone in commercial and public buildings. conditioned space where the internal loads will exceed the winter heating load. perimeter zone for summer cooling operation or for outdoor ventilation air supply.
1.2
System Description
In Fig. 1, the supply air from the supply duct of a VAV cooling system is discharged to the various VAV terminal boxes through some flexible ducts or connected ducts. In the VAV box, the volume flow rate is modulated according to the condition of the space load at zone 1, 2, ... n. The modulated air from the VAV box will flow to one or several slot diffusers via the flexible duct and then it is supplied to the various subdivided zones. The space return air at point r is actually a mixture of space return air from the various subdivided zones. During the winter mode operation, if the cold supply air is required to offset the space cooling load, then the outdoor air at point O is mixed with the recirculating air to form a mixture at point m. The mixture will then flow through the supply fan, before it is supplied to the various subdivided zones at point s.
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(a)
Schematic diagram
Summer
Winter
(b)
Air-conditioning cycles
Fig. l Variable-air-volume VAV cooling system
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1.3
VAV Boxes
A variable-air-volume (VAV) box is a terminal device in which the supply volume flow rate is modulated by varying the opening of the air passage by means of a single-blade butterfly damper, a multi-blade damper, or an air valve, as shown in Fig. 2. A VAV box may have a single outlet or multiple round outlets. A single-blade damper VAV box, as shown in Fig. 2(a) and Fig. 2(b), has a simple construction and is simple to operate. A typical damper closes at an angle 30 degrees from vertical and rotates in counterclockwise direction to an angle of 60 o in the fully open position. An air valve , as shown in Fig. 2(c), is a piston damper moving horizontally inside a hollow cylinder. The opening of the air passage can be adjusted. The main advantage of an air valve is its almost linear relationship between the modulated air volume and the displacement of the piston damper. The sizes of the VAV boxes made by one manufacturer range from a volume flow rate 100 l/s to about 1800 l/s. The pressure drop of these VAV boxes at nominal maximum volume flow rate when the damper is fully open usually varies from 50 to 125 Pa. In order to provide the required amount of outdoor air to the conditioned space, in a VAV cooling system, a VAV box usually reduces its volume flow to a minimum setting, typically 30 % while the space is occupied. In practice, each VAV box is connected to one or more slot diffusers via flexible ducts. The high entrainment ratio and good surface effect of the slot diffusers improve the air movement at low supply flow.
1.3.1
Direct Digital Control of VAV Boxes
In a DDC (direct digital control) single-blade damper VAV box, as shown in Fig.2(a) & 2(b), the temperature sensor sends a signal to a DDC controller. It actuates the motorized operator, moves the actuator to a certain displacement, and rotates the damper through the linkages. It then opens the single-blade damper wider. The damper is closed either by spring force or the reverse rotation of the motorized operator. The direct digital control of an air valve is similar to that of a single-blade damper. A DDC VAV box can provide many sophisticated control functions and can communicate with the central computer.
1.3.2
Influence of Duct Static Pressure on VAV Boxes
VAV boxes can be classified as pressure-dependent or pressure-independent. In a pressure-dependent VAV box, as shown in Fig. 2(a), the variation of the duct static pressure at the inlet of the VAV box caused by the opening and closing of the dampers connected to the same main supply duct influences the modulation of its supply volume flow rate. They are less expensive and used when the duct static pressure is more stable.
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A pressure-independent VAV box modulates its supply volume flow rate regardless of the variation of duct static pressure at its inlet. A typical pressure-independent VAV box is shown in Fig. 2(b) and 2(c). The controller receives the signals from both the temperature sensor and the velocity probe near the inlet of the VAV box. It will actuate the damper to widen or reduce the opening based on combined effects of the two signals so that the space temperature can be maintained within the predetermined limits. Even if the static pressure at the VAV box inlet varies from 125 to 750 Pa, the volume flow is maintained according to the required value called for by the temperature sensor and the controller.
Fig.2 VAV boxes: (a) single-blade, pressure-dependent; pressure-independent; and (c) air valve, pressure-independent.
(b)
single-blade,
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1.3.3
Sound Power Level of a VAV Box
The sound power level of a VAV box depends mainly on the following: (i) Volume flow of supply air for a specific size box. (ii) Difference in static pressure ∆ ps, across the VAV box to provide a specific volume flow. (iii) The configuration of the VAV box, flexible duct(s), and diffuser(s). The greater the ∆ ps , the larger the static pressure at the box inlet. The smaller the damper opening, the higher the sound power level of the VAV box. Noise generated by the VAV box can be transmitted to occupants in the conditioned space by duct-borne path through flexible ducts and diffusers, or a radiated path from its casing through the ceiling or plenum.
1.4
Control Systems for a Typical VAV Cooling System
The control systems for a typical VAV cooling system will provide, the following types of control: (i)
Space air temperature control When the temperature sensor T1 for a specific zone senses the space temperature, as shown in Fig. 1(a), it will send a signal to the controller C1. C1 will modulate the damper position through a controlled device, a damper motor, to vary the opening of the damper and the supply volume flow rate. Hence, the space temperature is maintained within the required limits.
(ii)
Supply air temperature control Thermostat T3 controls the cooling coil’s capacity to maintain a constant supply air temperature.
(iii)
Minimum outdoor air control An electronic type of velocity probe or pitot tube is located after the outdoor air damper to sense the volume flow rate of the outdoor air Vo. When Vo drops as the supply volume flow rate is being reduced, the signal from controller C4 will open wider the outdoor air damper and the required volume flow rate of the outdoor air intake is maintained.
(iv)
Supply duct static pressure control A pressure sensor P is usually located at approximately 2/3 of the length between the first and last tee offs in the main duct. It senses the static pressure and actuates the inlet vanes or the speed of the supply fan through the controller in order to maintain a predetermined static pressure at the sensing point. 5
1.5
VAV Cooling System Characteristics
The air-conditioning cycles for both the summer mode and winter mode operations for a VAV cooling system are shown in Fig. 1 (b). The block volume flow rate Vsb, which is the total supply volume flow rate of the air-handling system, is also the maximum sum of the supply volume flow rates for the individual zones Vsn at the same time, that is Vsb =
Vsn
Moreover, the coil's load is the block load based on the block volume flow rate. It is given by: qc = Vsb
s [(hm –
hcc) - (wm – wcc) Cpw tcc]
It is important to note that the size of the VAV box for a specific zone must meet the peak load requirements of this zone. Furthermore, the size of the air duct must also be able to accommodate the peak volume flow rates taking care by such a duct section at the same time. The condition of the space air can be calculated according to the weighted average of the space temperature and the humidity ratio. During the part-load operation, the space temperature is usually maintained at a nearly constant value. If the supply air condition s remains the same at part-load operation, the space relative humidity ∅np will be greater than, equal to, or smaller than that at the full-load operation. This will depend on whether the sensible heat ratio of the space condition line SHR sp is less than, equal to, or greater than that at the full-load operation.
2.
VAV Heating-only System
The VAV heating-only system has similar structure to that of VAV cooling-only system, with the cooling coil replaced by heating coil. But the application is rather rare in Hong Kong.
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3.
VAV REHEAT SYSTEM
The VAV reheat system shown in Fig. 3 first uses a reduction of the supply volume flow rate of the air supplied to the conditioned space and then, the supply of the heated air from the reheating coil to maintain the space temperature within the predetermined limits during part-load operation and winter heating.
Fig. 3 VAV reheat system
3.1
Control sequence
As the cooling load drops, the damper of the VAV box closes progressively to reduce the supply air flow until about 30% of the full supply. At this point, the supply rate remains constant and the reheat coil is activated to maintain the room temp setting. 3.2
System characteristics
The VAV reheat system is simple and effective in space temperature control for both the interior and the perimeter zone. It overcomes some deficiencies of the cooling-only VAV system since it provides adequate air distribution & ventilation all the time without paying the extent of energy penalty incurred in constant volume reheat applications. If the reheat energy comes from heat recovery system then the VAV reheat system will not be considered as energy inefficient when compared with other VAV systems.
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4.
VAV DUAL-DUCT SYSTEM
4.1
Basic Scheme
A dual-duct VAV system employs two supply air ducts -- a warm duct and a cold duct, as shown in Fig. 4. The warm air in the warm duct and the cold air in the cold duct are supplied to the perimeter zone through a VAV mixing box.
Fig.4
VAV Dual Duct System
Dampers of a VAV mixing box are arranged such that the warm and cold air flow rates drop appreciably before the other stream begins to supply air.
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4.2
VAV Mixing Box
Fig. 5 A pressure-dependent mixing VAV box
Figure 5 shows a pressure-dependent mixing VAV box using a specific terminal DDC controller. This box consists of two separate equal-sized air passages arranged in parallel - one for warm air and another for cold air. Each has a single-blade damper. These two air passages are then combined together and the mixture of warm and cold air is discharged through single or multiple outlets to the diffusers through a flexible duct. A temperature sensor is connected to the DDC controller to modulate the dampers in the warm air and cold air passages separately, using two actuators. If the mixing VAV box is pressure-independent, two additional velocity probes are added to the inlets of cold and warm air so that each damper is modulated separately by the DDC controller according to the signal from the velocity probe reset by the zone temperature sensor. In a dual-duct VAV system using a mixture of cold and warm air, the minimum setting of zone supply volume flow from the mixing VAV box should follow the guidelines for VAV systems specified in ASHRAE/IESNA Standard 90.1, that is, 30 % of peak supply volume.
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An example of supply air control scheme: (i)
space temp. ≤ 21°C, maximum warm air supply
(ii)
space temp. ≥ 25°C, max cold air supply
(iii)
Tr 1 < space temp. < Tr 2, mixed air supply
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4.2.1 Worked Example
In a VAV dual-duct system, full warm air supply = 0.8 kg/s at Rm temp Tr ≤ 2 l°C full cold air supply = 1. 1 kg/s at Tr ≥ 25°C Same slope of air flow-to-space temperature lines for warm & cold streams (-s & +s respectively) Min. supply air to room = 0.3 kg/s Determine the space temperatures (Trl & Tr2) that the cold & warm air flows fall to zero respectively. Solution:
For cold air mc = s Tr + Cc (Conisder y = mx + c)
...
(1)
...
(2)
For warm air mh = -s Tr + Ch
At full air flow of either stream: 1.1 = 25 s + Cc 0.8 = -21 s + C h
... ...
(3) (4)
At zero flow of cold stream (min flow) 0 = sTr l + Cc 0.3 = -s Tr 1 + Ch
... ...
(5) (6)
Solve equations (3) & (4), (5), (6) gives Cc= -8.9, Ch=9.2, s=0.4 & Tr 1 = 22.25°C From equation (2), m h = 0 ⇒ 0 = -0.4 Tr 2+ 9.2
∴ Tr 2= 23°C 11
4.3
System Characteristics
An air leakage of 3% to 7% of the supply volume flow rate must be considered for the shut-off damper in the VAV mixing box. The additional volume flow rate of the supply air Vad due to the air leakage can be given by: Vad = K Vc (Th – Tr )
during cooling
Tr – Tc = K Vh (Tr – Tc )
during heating
Th – Tr where K - leakage factor ≅ 5% 5% represents the mean value of air leakage Th & Tc - warm/cold supply air temp. at mixing box Then actual supply volume flow: For cooling:
V = ( 1 + K ) V c + Vad
For heating:
V = ( 1 + K ) Vh + Vad
where Vc & Vh, are the required supply flow of cold/warm air streams based on summer/winter design load & assuming no leakage.
4.4
Alternate Scheme Serving Both Perimeter & Interior Zones
Perimeter zone: cooling load may drop to only 20% of max load; heating is required in winter. Interior zone:
cooling load is usually not less than 50% of the maximum (always cooling) & supply flow is usually not less than 40% of design flow.
Possible design scheme: (i)
Interior zone
-VAV cooling
(ii)
Perimeter zone -VAV dual duct
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4.5
System Description
For a single supply fan dual-duct VAV system, as shown in Fig. 6 and Fig. 7 , the mixture of outdoor and recirculating air at point M will enter the blow-through supply fan. At the fan discharge outlet A , the air is divided into two streams -- the warm air stream and the cold air stream. The warm air stream will flow through the heating coil (which is not energized in summer). It is then discharged from the hot deck to the warm duct. The cold air stream will flow through the cooling coil where the air is cooled and dehumidified. It is then discharged from the cold deck to the cold duct. The streams of warm or cold air are then modulated in the VAV mixing box before it is supplied to the perimeter zone at point sx. In the interior zone, however, only the cold air discharged from the cold duct will be supplied through the VAV box at point si. The return air from the perimeter zone at point Rx will then mix with the air from the interior zone at point Ri in the ceiling plenum. The air at the mean point N is then returned to the multi-zone AHU to mix with the outdoor air again at point O. A portion of the space air is then exhausted to the outside atmosphere. In the interior zone, due to the closing of the flow passage in the VAV box during part-load operation, the volume flow rate of the cold air supply will also be reduced. However, because of the smaller variation of the space cooling load in the interior zone, the minimum volume flow rate of the cold air supply is usually not lower than a predetermined minimum setting.
Fig. 6
VAV Dual Duct System Schematic
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Si, Sx
Summer Full load (max. cooling): heating coil not energized
Winter Full load:
Fig. 7
Perimeter zone- max. heating Interior zone- cooling
VAV Dual Duct System : Air Conditioning Cycles
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4.6 Example of Dual Duct System Serving Both Perimeter & Interior Zones
A single supply fan dual-duct VAV system employed for a typical floor of a high-rise office building is designed for following summer conditions: Outdoor air condition Indoor air condition Max. space cooling load (i) perimeter zone (ii) interior zone
Min. space cooling load (i) perimeter (ii) interior
33°C DB, 24°C DB,
28°C WB 50% RH
QSx max. QLx max. QSi max. QLi max.
150 kW 8 kW 38 kW 8 kW
QSx min. QLx min. QSi min. QLi min.
36 8 20 7
kW kW kW kW
Min. outdoor ventilated air
1.6 m3/s
Supply system heat gain
fan power 1°C cold air duct 1°C
Return system heat gain
2.5°C
It is assumed that the air leakage of the shut-off damper at the VAV box is K = 5%, a warm air supply temp Th = 28.2 °C and the relative humidity of the air leaving the cooling coil is 95%. Determine: (i)
max. & min. supply air volume flows in both interior & perimeter zones,
(ii)
volume flow ratios (VFR) of the perimeter & interior zones respectively, and
(iii)
max. cooling coil's load.
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Solution:
0.0102 kg/kg
0.00873 kg/kg
(i)
Interior Zone
Under max. load, SHRi =
38 = 0.83 38 + 8
On psy. chart, locate pt. R i & draw R iC such that C is 1 °C DB away from 95% RH Pt. B is then located. TC ∅C hC wC νC
= = = = =
13 °C 90 % 34 kJ/kg 0.0084 kg/kg 0.822 m3/kg
TB ∅B hB wB
= = = =
12 °C 95 % 33.1 kJ/kg 0.0084 kg/kg
Max. supply vol. flow: Vsimax =
Qsi
ρ C pa (Tr – Tsi ) = 38 x 0.822 1.02 x (24 -13) = 2.78 m3/s
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Min. supply vol. flow: Vsimin =
20 x 0.822 1.02 x (24 -13)
= 1.465 m3/s Perimeter Zone 150 x 0.822 Vcmax = 1.02 x (24 - 13) = 11 m3/s Vcmin = 36 x 0.822 1.02 x (24 -13) = 2.64 m3/s To offset the damper air leakage, Vad = K Vcmax (Th – Tr ) Tr – Tc = 0.05 Vcmax (28.2 – 24) 24 - 13 = 0.0191Vc max
∴
Vsxmax = Vcmax + Vad + KVcmax = Vcmax + 0.0191Vc max + 0.05Vcmax = 1.0691 Vc max = 1.0691 x 11 = 11.76 m3/s
∴
Vsxmin = Vcmin + Vad + KVcmin = Vcmin + 0.0191Vc min + 0.05Vcmin = 1.0691 Vc min = 1.0691 x 2.64 = 2.82 m3/s
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(ii)
Volume flow ratios VFR i = Vsimin = 1.465 Vsimax
2.78
VFR x = Vsxmin = Vsxmax
(iii)
SHRx =
= 0.527
2.82
= 0.240
11.76
150 = 0.95 150+8
On psy. chart, from C draw SHR x = 0.95 to R x at 24 °C DB
also
Wrx reads 0.0086 kg/kg Wri reads 0.0093 kg/kg
VsimaxWri + VsxmaxWrx then W N = Vsimax+ Vsxmax = 2.78 x 0.0093 + 11.76 x 0.0086 2.78 + 11.76 = 0.00873 kg/kg
∴ Pt. N is located (T N = 24°C DB) Pt. E can be located such that TE = T N + 2.5 = 26.5 °C On psy. chart, ME = Vo = 1.6 = 0.110 OE Vsimax + Vsxmax 2.78 +11.76 Pt. M is then located and TM reads 27.2°C. Locate Pt. A on psychrometric chart such that: TA = TM + 1 = 28.2 °C WA =WM = 0.0102 kg/kg hA reads 54.3 kJ/kg 18
Maximum volume flow through cooling coil: Vsbmax = Vsimax + ( Vc max + Vad) = 2.78 + ( Vc max + 0.0191Vc max ) = 2.78 + Vc max ( 1 + 0.0191) = 2.78 + 11 ( 1.0191) = 13.99 m3/s Maximum cooling coil's load: = Vsbmax ρ [(hA- hB) - (wA- wB) C pw TB] = 13.99 [(54.3 - 33.1) - (0.0102 – 0.0084 ) x 4.185 x 12] 0.822 = 359.3 kW
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5.
Fan Powered VAV System
The VAV fan-powered system, as shown in Fig.8, is a combination of the VAV cooling system and the fan-powered VAV terminal units which are mainly used to deliver the warm plenum air or heated air to the perimeter zone.
Fig. 8 Fan Powered VAV System The fan-powered VAV terminal unit consists of a small centrifugal fan, a heating coil & an inlet damper (or air valve). Its operation is governed by the control signal from the thermostat Tx. In summer, fan and heater are normally off. fan-powered VAV terminal unit works like a common VAV box; supply volume flow not less than min. ventilation requirement. under very low load, fan is energised to extract warm plenum air to increase the supply air temp. In winter, both fan & heater operate. The control scheme is therefore a combination of both variable volume & variable temp., sometimes known as VVT. 6. Comparison of VAV System and CAV System 20
When compared with a CAV system of the basic central system with terminal reheat or multi-zone system, the multi-zone VAV system has the following advantages: (i)
The initial cost for the fans, duct work, refrigeration plant and water system is lower because the supply volume flow rates of the AHU and the main duct are based on the block volume flow rate.
(ii)
The energy requirement and the operating cost for the fan power input is lower because of the part-load operation and the lower block volume flow rate.
(iii)
The energy consumption and the operating cost for the refrigeration plant and the water system is lower because of the lower block volume flow rate.
(iv)
VAV system is self-balancing; therefore the air balance and commissioning work is much simpler and easier.
(v)
There is more flexibility in the sub-division of the new zones and the reallocation of the VAV boxes and flexible duct when there is a change in the layout of the room arrangement due to the change of tenants.
(vi)
The space relative humidity is lower when compared with the CAV multi-zone system at part-load condition.
(vii)
The air supply can be shut off when certain space is not occupied.
The main disadvantages of the multi-zone VAV systems are: (i)
Insufficient air movement may occur at a very low supply volume flow rate. Therefore, a high entrainment ratio and good surface effect outlets such as the slot diffusers are preferable. Usually, the design volume flow intensity should not be lower than 3.8 l/s.m 2.
(ii)
An insufficient supply of outdoor ventilation air may happen at a low supply volume flow rate. Hence, a minimum outdoor air control is often necessary. A lower limit is often preferable for the volume flow reduction and the shut-off type VAV boxes are not recommended.
(iii)
There is the possibility of fan surge at the reduced volume flow rate. Hence, the fan operating performance both at the maximum and minimum volume flow rates have to be checked.
(iv)
Additional noise may be generated from the VAV box and the fan-powered terminal units. Hence, the sound levels should be chec ked during both maximum and minimum operating conditions.
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7.
Comparison Between Various Types of VAV Systems
For the typical floor of a high-rise office building having both the perimeter and interior zones, a comparison can be made between the following VAV systems: (i)
VAV reheat system;
(ii)
VAV dual-duct system; and
(iii)
Fan-powered VAV system.
The factors which must be considered in the comparison are: (i)
Energy consumption and the operating cost The fan-powered VAV system often consumes less energy due to the provision of sequence control in the perimeter zone, especially in intermediate seasons, and also the absence of air leakage.
(ii)
Installation cost The VAV dual-duct system is more expensive, more complicated in system control, and requires more space for operating the warm air duct in the perimeter zone.
(iii)
Environmental control The VAV dual-duct system is superior in terms of air cleanliness due to the provision of a greater amount of filtered air. It also provides comparatively greater air movements due to the greater amount of air supply.
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