HCB 3-Chap 19A: All-Air Systems_Single Zone 1 Chapter 19A: ALL-AIR SYSTEMS: SINGLE ZONE AND SINGLE DUCT Agami Reddy (July 2016) 1)Introduction and common system types 2)Single zone systems- CAV and VAV operation 3)CAV systems without reheat- summer design 4)Air stream heating due to fans 5)CAV systems with reheat under summer design 6)CAV systems- winter design 7)Part-load operation of CAV systems 8)Part-load operation of VAV systems 9)Control issues and benefits of VAV systems

HCB 3-Chap 19A: All-Air Systems_Single Zone 2 Introduction – Large number of all-air system variations to fit different requirements for comfort in buildings, process applications and also for special applications requiring close control of temperature and humidity (clean rooms, computer rooms, hospital operating rooms, …) – An all-air system provides complete sensible and latent cooling, preheating, and humidification capacity to the building supply air – No additional cooling or dehumidification required at zone (but heating may be required) – Basically two categories: (i) single-duct systems: for single zone and multiple zone buildings all heating, cooling and dehumidification is done within a common duct distribution system (ii) dual- duct systems: for multiple buildings only (being phased out) Two ducts- one carrying cold air and another hot air stream

HCB 3-Chap 19A: All-Air Systems_Single Zone 3 Common Types of All-Air Systems 1)Single duct (a) Constant Air Volume (CAV): - single space - multiple-zone reheat (b) Variable Air Volume (VAV): - variable volume only - reheat - induction - fan powered 2) Dual duct/stream (a) Dual duct: - constant air volume - variable air volume (b) Multizone:- constant air volume - variable air volume - three-deck or “Texas” multizone

HCB 3-Chap 19A: All-Air Systems_Single Zone 4 Single Zone Systems -Operating principle of CAV and VAV under peak design cooling and heating conditions -Operation of CAV under part load -Operation of VAV under part load

HCB 3-Chap 19A: All-Air Systems_Single Zone 5 Single Zone Systems Air from room is recycled for energy efficiency FIGURE 19.2 The three sets of dampers and their locations which are meant to inversely vary the return and exhaust air flows in unison, and also the outdoor air ventilation amount as needed.

HCB 3-Chap 19A: All-Air Systems_Single Zone 6 Single Zone Single Duct CAV Systems - Summer Peak sized so that no reheat is required FIGURE 19.1 Schematic diagram of the basic single zone HVAC system with some of the major energy equipment. The important psychrometric state points (0-9) and the return, supply, exhaust and ventilation flows are also shown.

HCB 3-Chap 19A: All-Air Systems_Single Zone 7 Example illustrating analysis during peak or design conditions of CAV system

HCB 3-Chap 19A: All-Air Systems_Single Zone 8 FIGURE The air-conditioning process diagram on a psychrometric chart. The state points correspond to those in Fig while the numerical values apply to Example 19.1.

HCB 3-Chap 19A: All-Air Systems_Single Zone 9 Solution This allows point 1 to be fixed

HCB 3-Chap 19A: All-Air Systems_Single Zone 10

HCB 3-Chap 19A: All-Air Systems_Single Zone 11 Air Stream Heating Due to Fans

HCB 3-Chap 19A: All-Air Systems_Single Zone 12 For spaces with high latent loads- reheat may be required even under peak summer design conditions FIGURE 19.7 Process diagram for single duct CAV system to air-condition a single space with high latent loads. The state points correspond to Fig while the numerical values apply to Example Note that the supply fan reheat is neglected.

HCB 3-Chap 19A: All-Air Systems_Single Zone 13

HCB 3-Chap 19A: All-Air Systems_Single Zone 14

HCB 3-Chap 19A: All-Air Systems_Single Zone 15 Note: -substantial increase in supply air flow rate (from 17,140 lb a /h to 26,667 lb a /h -substantial additional cooling (from 178 kBtu/h to kBtu/h) - The increase in cooling is equal to reheat = 69 kBtu/h

HCB 3-Chap 19A: All-Air Systems_Single Zone 16 FIGURE 19.5a Schematic diagram of the complete single-duct single zone CAV system with ducted return. The preheater and the steam humidifier would operate during winter while the cooling coil and the reheat coil would be active when cooling loads are to be met. Single Zone Single Duct CAV System Winter Operation Cooling coil is inactive

HCB 3-Chap 19A: All-Air Systems_Single Zone 17

HCB 3-Chap 19A: All-Air Systems_Single Zone 18 Figure 19.5b

HCB 3-Chap 19A: All-Air Systems_Single Zone 19

HCB 3-Chap 19A: All-Air Systems_Single Zone

HCB 3-Chap 19A: All-Air Systems_Single Zone 21 6.

HCB 3-Chap 19A: All-Air Systems_Single Zone 22 FIGURE 19.8 (a) Schematic of the winter heating and humidification system with injection of liquid water. The cooling coil is not shown. (b) Psychrometric process diagram with state points. Alternative system Not requiring a steam boiler

HCB 3-Chap 19A: All-Air Systems_Single Zone 23 Part-load operation of CAV

HCB 3-Chap 19A: All-Air Systems_Single Zone 24

HCB 3-Chap 19A: All-Air Systems_Single Zone 25 Note: Energy penalty due to reh eat 6.

HCB 3-Chap 19A: All-Air Systems_Single Zone 26 Great energy savings compared to CV systems under part-load operation: VAV systems modulate air flow as load reduces while keeping supply air temperature constant at cooling coil leaving air temperature. This reduces both cooling load on coil, fan power and avoids reheat Supply air flow modulation is achieved by having terminal boxes at each zone AND having a variable speed supply fan Variable Air Volume (VAV) Systems FIGURE Terminal reheat configuration for a VAV system for a single zone ducted return system for a small laboratory

HCB 3-Chap 19A: All-Air Systems_Single Zone 27 Part-load operation of VAV System

HCB 3-Chap 19A: All-Air Systems_Single Zone 28

HCB 3-Chap 19A: All-Air Systems_Single Zone 29 For CAV = 7.03 T No reheat needed

HCB 3-Chap 19A: All-Air Systems_Single Zone 30 Comments: In the above example, the reduced airflow has eliminated the need for reheat but has somewhat compromised occupant comfort due to the slightly elevated humidity in the space (RH increased from 45% to 49%). The flow reduction of ( /17,140) = is right at the lower end of the norm allowed. Had it been lower, the proper operating strategy would be to provide a small amount of reheat at the entrance to the space

HCB 3-Chap 19A: All-Air Systems_Single Zone 31 Different types of VAV Operation: -Cooling only - VAV reheat (because minimum supply air flow reached) -Heating only Design flow: about cfm/ft 2 FIGURE 19.9 Modulation of room supply air flow for a single zone VAV system as space loads vary over the year. Note the three modes of operation: (a), (b) and (c).

HCB 3-Chap 19A: All-Air Systems_Single Zone 32 FIGURE 19.6 Cutaway drawing of a packaged air-handler unit (AHU).

HCB 3-Chap 19A: All-Air Systems_Single Zone 33 Outcomes Understanding of the common all-air system types used for air distribution in buildings Understanding the operation of the basic CAV and VAV systems for single zones Knowledge of air stream heating across fans Familiarity with different configurations of CAV and VAV systems Be able to analyze single zone CAV with reheat, CAV without reheat, and VAV under summer and winter design conditions Be able to analyze the performance of CAV and VAV under part-load operation Understanding of the various control issues and energy efficiency benefits of using VAV system