AIR CONDITIONING SYSTEMS 1 All Air Systems An

AIR CONDITIONING SYSTEMS 1

All Air Systems • An all-Air System is defined as a system providing complete sensible and latent cooling capacity in the cold air supplied by the system. • No additional cooling is required • Can be briefly classified and are discussed here in to basic categories: 1. Single duct systems, or 2. Dual duct systems 2

Cont. . • The single duct systems can provide either cooling or heating using the same duct, but not both heating and cooling simultaneously. These systems can be further classified into: 1. Constant volume, single zone systems 2. Constant volume, multiple zone systems 3. Variable volume systems • The dual duct systems can provide both cooling and heating simultaneously. These systems can be further classified into: 1. Dual duct, constant volume systems 2. Dual duct variable volume systems 25 November 2020 Air Conditionig and Refregeration 3

Cont … • All-Air System can be adapted to all types of air conditioning systems for comfort or process work. • It is applied in off, schools, universities , laboratories, hospitals, stores, hotels and ships where they require individual control of conditions. 25 November 2020 Air Conditionig and Refregeration 4

Cont … 1. Single zone system: simplest form of the all-air system is a single conditioner serving a single temperature zone. -applied to small department stores, small individual shops in shopping center, individual class rooms etc A schematic of the single zone central unit is shown below: 25 November 2020 Air Conditionig and Refregeration 5

Cont … 2. Multiple zone systems: for large building it is usually not economical to provide a separate system for each zone. For such cases the basic control system concept is expanded to meet the cooling and heating requirements of the multi zone. 25 November 2020 Air Conditionig and Refregeration 6

Schematic diagram of terminal reheat zone a. Constant-air Volume Systems 25 November 2020 Air Conditionig and Refregeration 7

Cont … b. Variable-air Volume Systems: when air is the medium used to balance the load, the designer may choose between varying the supply air temperature constant volume) or varying the volume (variable volume) as the space load changes. • • • Variable air volume system (VAV) may be applied to interior or perimeter zones with common or separate fan systems, common or separate air temperature control, and with or without auxiliary heating devices. The variable volume concept may apply to volume variation in the main system total air stream and/or to the zones of control. Variation of flow under control of a space thermostat may be accomplished by positioning simple damper or a volume regulating device in a duct, a pressure reducing device, or at the terminal diffuser or grill. 25 November 2020 Air Conditionig and Refregeration 8

Cont … Cooling or Heating: A single stream of cool air serves all the zones, and a thermostat in each zone regulates a damper to control the flow rate of cool air into the zone. Application : in interior of an office building with no heating loads and where only cooling loads prevail. Diagram: 25 November 2020 Air Conditionig and Refregeration 9

Cont ii. – – … Dual Duct : conditions all the air in a central apparatus and distributes it to the conditioned spaces through two parallel mains or ducts. One duct carries cold air and the other warm air, thus providing air sources for both heating and cooling at all times. In each conditioned space or zone, a mixing value responsive to a room thermostat mixes the warm and cold air in proper proportions to satisfy the prevailing heat load of the space. The dual duct system is very responsive to changes in load of the zone and can simultaneously accommodate heating in some zones and cooling in others 25 November 2020 Air Conditionig and Refregeration 10

Schematic diagram of dual duct system 25 November 2020 Air Conditionig and Refregeration 11

Cont … The system experiences a problem at very light cooling loads where the air flow rate drops off so much that poor air distribution and/or ventilation results. The heating only VAV system the same structures that shown above, but a heating coil is used to provide the source for constanttemperature warm air. ii. Cooling –reheat VAV system: • • This system is the same as the above figure except that the branch lines to each zone contains a reheat coil. The control sequence is that as the cooling load drops off, the damper progressively reduces the flow rate of air until about 25 to 30% ful flow rate. At this point the air flow rate remains constant and the reheat coil is activated. 25 November 2020 Air Conditionig and Refregeration 12

Cont … • The VAV reheat system overcomes a number of deficiencies of the cooling only VAV system since it provides a means of obtaining adequate air distribution and ventilation without paying the energy penalty incurred in constant volume reheat applications. • In the VAV dual duct system, the arrangement is similar to the conventional duct system of the above fig. except for the flow characteristics of the mixing boxes. Instead of providing a constant flow rate of mixed air , the dampers are arranged so that the warm and cool air flow rates drop appreciably before the other streams begins to supply air. 25 November 2020 Air Conditionig and Refregeration 13

Cont … • The VAV reheat and dual duct system provide all the flexibilities of the conventional reheat and dual duct systems in the sense that a zone can be accommodated as it switches from heating to cooling requirements and that some zones on a system can be provided with heating at the same that the other zones need cooling. There is some thermal bucking in both the VAV reheat and dual-duct systems but the magnitude of heating and cooling energy cancellation is modest because the low air flow rates at which thermal bucking occurs. 25 November 2020 Air Conditionig and Refregeration 14

AIR-AND -WATER SYSTEMS • Accomplish space conditioning through the use of both air and water source that are distributed to terminal units installed in the habitable space. • The air and water are cooled or heated in central mechanical equipment rooms and distributed to the space in which comfort conditions are being maintained. • The most common types of terminal used with air and water systems is the high pressure induction unit. The air supplied to the induction unit is called primary air. It is distributed from the central mechanical equipment room at high velocities between 10 and 24 m/sec. the water distributed to the induction unit is called secondary water. 25 November 2020 Air Conditionig and Refregeration 15

Cont … • Air-and-Water systems are primarily applicable to multi zone type exterior spaces of buildings where wide range of sensible loads exists and where close control of humidity is not required. • Applications: hospitals, schools, apartment houses , research laboratories • Components: central air conditioning equipment, duct distribution and water distribution systems and a room terminal • The air supply is generally constant volume and provides outside clean air for ventilation 25 November 2020 Air Conditionig and Refregeration 16

Cont…

Cont… • Category : Two-pipe, three-pipe , four-pipe systems. They are basically similar in function and all incorporate both cooling and heating capabilities for year round air conditioning. • Their name is drive from the water distribution system: two-pipe: the water distribution consists one supply and one return pipe three-pie: a cold water supply and warm water supply are combined with a single common return pipe to make the water distribution system four-pipe : has cold water supply, cold water return, warm water supply and warm water return 25 November 2020 Air Conditionig and Refregeration 18

ALL- WATER SYSTEMS • Accomplish both sensible and latent space cooling by circulating chilled water from ac central refrigeration system through cooling coils in terminal units located in building occupied spaces. • Most common terminals: Fan coil units, unit ventilators, and valence units • The all-water system usually provides individual room control, with no cross contamination of recalculated air from one space to another 25 November 2020 Air Conditionig and Refregeration 19

Cont. .

Cont … • Advantage: – Reduced need for building space to the elimination or reduction of central fan room and duct space requirements – The system provides all of the benefits of a central water chilling and heating plant, while retaining the ability to positively shut off local terminals in unused areas • Disadvantage : – Requires much more maintenance than central all-air system and work must be done in the occupied areas – Each unit requires a condensate pan and drain system that must be cleaned and flushed periodically. – It is difficult to clean coils, should this be required. – Filters are low in efficiency, small, and requires frequent changing 25 November 2020 Air Conditionig and Refregeration 21

Duct Design and Fan Performance • Commercial and residential and, industrial air duct system design must consider 1) 2) 3) 4) 5) 6) 7) 8) 9) 25 November 2020 Space air diffusion Space availability Noise level Duct leakage Duct heat gain and losses Balancing Fire and smoke control Initial investment System operating cost Air Conditionig and Refregeration 22

Cont … • Deficiency in duct design can result in systems that operate incorrectly or are expensive to own and operate. • Poor air distribution cause discomfort, less of productivity and even adverse health effects • Lack of sound attenuators may permit objectionable noise level. • Poorly designed duct work can result in unbalanced system 25 November 2020 Air Conditionig and Refregeration 23