HCB 3-Chap 19A: All-Air Systems_Single Zone Chapter 19A: ALL-AIR SYSTEMS: SINGLE ZONE AND SINGLE DUCT Agami Reddy (rev May 2017) Introduction and common system types Single zone systems- CAV and VAV operation CAV systems without reheat- summer design Air stream heating due to fans CAV systems with reheat under summer design CAV systems- winter design Part-load operation of CAV systems Part-load operation of VAV systems Control issues and benefits of VAV systems HCB 3-Chap 19A: All-Air Systems_Single Zone

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

Common Types of All-Air Systems 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 - three-deck or “Texas” multizone HCB 3-Chap 19A: All-Air Systems_Single Zone

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

HCB 3-Chap 19A: All-Air Systems_Single Zone Single Zone Systems Air from room is recycled for energy efficiency Fig. 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

HCB 3-Chap 19A: All-Air Systems_Single Zone Single Zone Single Duct CAV Systems - Summer Peak sized so that no reheat is required Fig. 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

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

HCB 3-Chap 19A: All-Air Systems_Single Zone Fig. 19.3. The air-conditioning process diagram on a psychrometric chart. The state points correspond to those in Fig. 19.1 while the numerical values apply to Example 19.1. HCB 3-Chap 19A: All-Air Systems_Single Zone

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

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

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

HCB 3-Chap 19A: All-Air Systems_Single Zone For spaces with high latent loads- reheat may be required even under peak summer design conditions Fig. 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. 19.1 while the numerical values apply to Example 19.3. Note that the supply fan reheat is neglected. HCB 3-Chap 19A: All-Air Systems_Single Zone

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

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

HCB 3-Chap 19A: All-Air Systems_Single Zone Note: -substantial increase in supply air flow rate (from 17,140 lba/h to 26,667 lba/h -substantial additional cooling (from 178 kBtu/h to 246.7 kBtu/h) - The increase in cooling is equal to reheat = 69 kBtu/h HCB 3-Chap 19A: All-Air Systems_Single Zone

Single Zone Single Duct CAV System Winter Operation Cooling coil is inactive Fig. 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. HCB 3-Chap 19A: All-Air Systems_Single Zone

HCB 3-Chap 19A: All-Air Systems_Single Zone Example 19.1. HCB 3-Chap 19A: All-Air Systems_Single Zone

HCB 3-Chap 19A: All-Air Systems_Single Zone Fig. 19.5 Schematic diagram (b) Psychrometric process HCB 3-Chap 19A: All-Air Systems_Single Zone

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

HCB 3-Chap 19A: All-Air Systems_Single Zone 4. 5. HCB 3-Chap 19A: All-Air Systems_Single Zone

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

HCB 3-Chap 19A: All-Air Systems_Single Zone Alternative system: Not requiring a steam boiler Fig. 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. HCB 3-Chap 19A: All-Air Systems_Single Zone

HCB 3-Chap 19A: All-Air Systems_Single Zone Part-load operation of CAV (Example 19.1) HCB 3-Chap 19A: All-Air Systems_Single Zone

HCB 3-Chap 19A: All-Air Systems_Single Zone Fig. 19.1 HCB 3-Chap 19A: All-Air Systems_Single Zone

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

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

HCB 3-Chap 19A: All-Air Systems_Single Zone Variable Air Volume (VAV) Systems Fig. 19.10 (a) Terminal reheat configuration for a VAV system for a single zone ducted return system for a small laboratory 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 HCB 3-Chap 19A: All-Air Systems_Single Zone

HCB 3-Chap 19A: All-Air Systems_Single Zone Part-load operation of VAV System Example 19.5 HCB 3-Chap 19A: All-Air Systems_Single Zone

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

HCB 3-Chap 19A: All-Air Systems_Single Zone For CAV = 7.03 T For CAV = 3.24 T For CAV =10.27 T No reheat needed HCB 3-Chap 19A: All-Air Systems_Single Zone

HCB 3-Chap 19A: All-Air Systems_Single Zone 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 (10.208.3/17,140) = 0.595 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

Different types of VAV Operation: Cooling only Fig. 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). Design flow: range 0.8 - 1.2 cfm/ft2 Different types of VAV Operation: Cooling only VAV reheat (because minimum supply air flow reached) Heating only HCB 3-Chap 19A: All-Air Systems_Single Zone

Single Zone CAV Pros: Used for simple rooms with similar conditioning requirements over year (interior rooms) Simple to build and operate Lower initial cost Used in: Small commercial Strip malls Elementary Small health clinic Auditoriums Theaters Cons: Higher energy costs during operation Not energy efficient for rooms whose loads vary over year HCB 3-Chap 19A: All-Air Systems_Single Zone

Single-zone VAV Can be used for single zone (many rooms) Pros: Used for simple rooms whose conditioning requirements are similar but varies during day Simple to build (inexpensive) and operate Used in: Small commercial Strip malls with different types of stores Small health clinic with different specialization Cons: Requires reheat if rooms with different conditioning requirements since supply air cannot be reduced beyond a certain amount Total amount of outside air may be difficult to maintain due to variable volume HCB 3-Chap 19A: All-Air Systems_Single Zone

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

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