Chapter 19B: ALL-AIR SYSTEMS FOR MULTIPLE SPACES Agami Reddy (rev May 2017) CAV terminal reheat CAV multizone and dual-duct VAV terminal reheat Example of sizing an all-air system Part-load performance of all three systems- solved example Fan powered VAV systems: primary and secondary air flows Measures to improve energy efficiency - Fan control - Outdoor air economizer - Heat recovery devices - Deck reset 8) Advantages of all-air systems HCB 3-Chap 19B: All-Air Systems_Multizone

HVAC Systems for Multiple Spaces Different zones have different sensible and latent loads and need different supply air conditions. Important generic types: Terminal reheat Multi-zone system Dual Duct System VAV System Constant CFM, varying DBT and W Wasteful in energy Varying CFM with fixed or varying DBT and W Saves both thermal energy as well as fan electricity HCB 3-Chap 19B: All-Air Systems_Multizone

CAV Terminal Reheat Systems CFM to each zone kept fixed, supply temperatures varied with load Reheat at individual zones can be hydronic or electric In summer, cooling and humidity control needed In winter, some zones may still need cooling Fig. 19.15 (b) Plenum return single-duct VAV system for a two-zone building HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone CAV Multi-Zone Units Pro: Simple and low initial cost Con: High operating cost, lack of flexibility and proper humidity control HCB 3-Chap 19B: All-Air Systems_Multizone

Multi-zone Central Air System HCB 3-Chap 19B: All-Air Systems_Multizone

Fig. 19.16 CAV Multi-Zone Dual-Duct Systems Pro: Flexibility (compared to multi-zone system) Con: High operating cost and poor humidity control HCB 3-Chap 19B: All-Air Systems_Multizone

Single duct VAV secondary system sizing (Design or peak conditions) Example 19.8 Cooling HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone Other Specifications HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone Assumptions HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone Eq. 19.11 Eq. 16.34 IP HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone Example 19.9- Part Load Conditions (a) Single duct VAV system HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone (b) Single duct CAV system HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone Summary table comparing all three HVAC systems   Ideal loads Single duct CAV Single duct VAV Dual duct CAV Cooling (Tons) 27.6 37.7 29.0 34.5 Heating (Btu/hr) - 93,947 53,726 Fan power (HP) 12.0 8.41 Thermal efficiency 0.61 0.95 0.71 -VAV system outperforms the other two systems, and is very close to the ideal loads. It is the best choice since it requires the least cooling energy, a smaller heating energy and lower fan electricity use. If we define efficiency in terms of the ideal loads, then for the VAV system it is (27.6/29) = 0.95, while that for the single duct CAV = [(27.6 Tons x 12,000 Btu/h-ton) / (37.7 Tons x 12,000 Btu/h-ton+ 93,947)= 0.61. The two CAV systems are similar thermodynamically, and the differences in energy use determined are partly due to the manner we selected the hot deck temperature, the way we adjusted for the supply fan being upstream, and also due to slightly different resulting humidity levels in the space which affect the latent loads. HCB 3-Chap 19B: All-Air Systems_Multizone

Fan-Powered VAV Terminal Boxes The basic VAV system with single supply fan evolved into fan-powered VAV boxes with one fan at each box (fan power about 1/3rd to ½ HP) with terminal reheat. The reason was that low supply flows (down to say 0.5 cfm/ft2) compromised IAQ (poorer mixing, more stagnant air spaces, poorer filtration) Thus, fan-powered VAV maintain higher air circulation thru room at low thermal loads while retaining some of the advantages of VAV systems Plenum return needed Fig. 19.12 Fig. 19.11. Induction VAV system HCB 3-Chap 19B: All-Air Systems_Multizone

Ducted and Plenum Return Air Supply air is always ducted Ducted Plenum HCB 3-Chap 19B: All-Air Systems_Multizone

The two types of fan powered VAV boxes Fan located within primary airstream and runs continuously when zone is occupied Fan located outside primary airstream- this allows intermittent fan operation Fig. 19.13 (a) Schematic diagrams of series and parallel fan-powered VAV boxes Series: fan runs continuously Parallel: fan intermittent HCB 3-Chap 19B: All-Air Systems_Multizone

Fan-Powered VAV Terminal Boxes The secondary air flow is adjusted so that the total supply air flow is kept at the design flow Especially used in perimeter zone In colder climates, terminal heat has to be supplemented by baseboard heaters Three types of designs evolved which allowed higher room supply flow rates without adversely impacting thermal and fan energy: - Induction type - Series fan powered box (consumes slightly more energy) Parallel fan powered box (consumes less energy) (good equip manufacturer web site to check is TITUS) Nowadays VAV terminal fans have ECM (electronically commutated motors) which are very efficient and consume little energy HCB 3-Chap 19B: All-Air Systems_Multizone

Series Fan-Powered VAV Fan powered VAV boxes allow plenum air (called secondary air) to be drawn in, thereby increasing supply air flow rate to room Fig 19.12 Two zone space conditioned by a series fan powered VAV system Primary air flow Secondary air flows Plenum return needed HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone Series fan powered Almost constant air flow to space- can be used for both perimeter and interior spaces Fig. 19.14 Air flow characteristics Parallel fan powered Intermittent fan operation- Primary air modulated in response to cooling demand primary air used for perimeter zones with auxiliary hot water or electric heat HCB 3-Chap 19B: All-Air Systems_Multizone

All-Air Systems: Energy Efficiency Measures VAV instead of CAV Energy efficient operation: fan control Economizer operation and benefits Heat recovery devices Deck (or supply air ) temperature reset The temperature set points of the heating and cooling coils are changed with changes in outdoor DBT (on which the building loads depend) HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone Fan Control- Three different commonly used methods of controlling fan speed in VAV systems- the VSD is most energy efficient but most costly Empirical curve-fit equations are used for modeling such control methods Figure 16.23 Part-load fan characteristics for outlet damper, inlet vane, and variable-speed control methods. HCB 3-Chap 19B: All-Air Systems_Multizone

Traditional HVAC System Variants Similar to a reheat systems but w/o requiring heat - Control is a little more difficult Humidity control may be poor Fig. 19.20 Recycle air bypass - Used in the 60-70’s - No longer used - Replaced by fan- powered VAV systems Fig. 19.19 HVAC system modifications (a) Mixed air bypass (b) face and bypass damper arrangement HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone Outdoor Air Economizer operation Basic concept: Increase the outdoor air flow intake under certain outdoor air conditions so that “free cooling” brought in can reduce active cooling Two types of economizers: - sensible - enthalpy In any case, the outdoor air flow has to be modulated properly, otherwise the benefit of reduced energy is lost Fig. 19.21 Enthalpy and temperature economizer operating ranges. The constant-enthalpy line represents the room enthalpy criterion. HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone About 65 F FIGURE 19.22 Outside airflow characteristic for fixed-volume system with economizer. HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone Outdoor DBT when outdoor mass flow reaches its minimum value: Example 19.11: Building process—The economizer - Return air temperature from a zone is 25° C (77° F) At least 25 percent outdoor air is required at all times to meet ventilation requirements, design cooling system air supply temperature is to be 13° C At which outdoor temperature will outdoor inlet damper be at its minimum setting? 25 HCB 3-Chap 19B: All-Air Systems_Multizone

Heat Recovery Devices from Exhaust Air Effectiveness ~ 0.75 Effectiveness ~ 0.6 Fig. 19.23 (a) Rotary total energy exchanger Fig. 19.23 (b) Plate-type heat exchanger HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone Example 19.12: Energy benefit of exhaust air energy recovery   Assume a building with 100% outdoor air. The exhaust air temperature is always kept constant at 75oF year-round. The building supply temperature is assumed to be 55oF and 100% saturated during summer and 90oF during winter operation. The ventilation and exhaust air streams are both equal to 10,000 cfm.  (a) Determine the capacity of the cooling and heating coils without energy recovery under the following peak or design outdoor air conditions: winter: 40o F and 40% RH summer: 98o F and 40% RH (b) Determine the capacity of the cooling and heating coils when an energy recovery unit of effectiveness of 0.6 is included. Fig. 19.24 Sketch of the HVAC system with energy recovery unit for Example 19.12. HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone

Table 19.4 Advantages/Disadvantages of All-Air Systems Best choice for close control of zone temperature and humidity May limit the extent to which energy use can be reduced in low or zero energy buildings Major equipment is centrally located allowing for nonintrusive maintenance Duct space requirements add to building height Providing heating and cooling in different zones simultaneously is easily achieved Air balancing is difficult Can handle space churn to some extent May be more expensive than other secondary systems in first cost and operating cost Seasonal changeover is easily achieved by HVAC controls Noise in fan operation may be a problem in certain types of spaces Well suited for air-side economizer, heat recovery and large outside air requirements May not be satisfactory for perimeter spaces in cold locations No drain pipe or power wiring in occupied areas Difficult to correct indoor conditions in individual rooms if improperly designed initially Less property damage if air ducts leak May lead to shoddy construction since air leakage from ducts causes no visible damage and so may be overlooked HCB 3-Chap 19B: All-Air Systems_Multizone

HCB 3-Chap 19B: All-Air Systems_Multizone Outcomes Knowledge of the different types and working principles of CAV multizone systems Be able to solve and analyze problems involving CAV and VAV multizone systems under peak design conditions and under part load operation Knowledge of the different types, operating principles and features of fan-powered VAV systems Familiarity with differed ways by which energy efficiency can be enhanced in all-air systems Be able to analyze the energy benefits of outside air economizer cycles and heat recovery devices Understanding the advantages and disadvantages of all-air systems HCB 3-Chap 19B: All-Air Systems_Multizone