18 آبان 1398

Scale of air conditioning system in building installations

Starting from the view that taxonomy can help to understand the complex system, the classification of air conditioning systems is presented. Air conditioners are usually classified by scale as follows.

air conditioning
Local air conditioning: A local system is usually intended for use in one area (the area usually consists of one or more rooms). It is a self-sufficient system (components, distribution, delivery and control are generally in one package). The system is usually located in the desired space (with implications for aesthetics and flexibility). The system is usually small in size and capacity (with effects on efficiency). The system is usually not controlled from a central location (this aspect can be considered positive or negative). A window air conditioning unit is an example of a local HVAC system.
Central air conditioning system: A central system is used for multiple areas of a location (or a remote location). A distribution system is essential to transfer the influence of heating and cooling from its origin (such as a facility room) to system areas. The system ranges greatly from a single-family residence to thousands of square feet (square meters) of office or laboratory. A building may be used with one central system or several central systems. The Variable Air Volume System (VAV) is an example of a central system.
Regional air conditioning system: A regional air conditioning system is used in several buildings. Usually the regional system provides the heating and cooling effect for the environment or areas of the special facility. The building usually has its own central air conditioning system. Scale economies are possible with large equipment indexing a regional system (this may include the purchase of bulk fuel or electricity, custom operational control sequences, outstanding maintenance quality and trained operators). Ball State University's ground source heat pumping system (Ball State University, 2013) is a creative example of a regional system.

Long-distance steam distribution has been in use for more than a century; the development of high temperature water distribution (HTW) and cold water distribution throughout the building is a new development. High-pressure hot and cold water systems are widely used in the US Air Force base, airports, and a range of buildings, such as hospital complexes and universities. Much effort was made to install new heating / cooling networks at fossil fuel power plants. More than half of the fossil fuel inputs are wasted at the plant, and the heating system uses a lot of cooling fuel.

If enough water is kept at high pressure, it will not enter the steam. Water is then pumped through the sweep lines and through branches into heat pump heat exchangers, which generate conventional low pressure hot water systems and steam, and perform several thermal functions. The pressure is of the order of 400 pounds per square inch, a temperature of [2800 kPa] and a temperature of about 300 ° F (150 ° C). During this cycle, water sometimes drops to 150F (83C °) and 60 pounds per square inch (414 kPa). The cross-section of Figure 12.60 illustrates this common arrangement.

High-temperature water has a number of advantages over steam for particular installations. In this case, a two-pipe distribution system is used and the temperature drop in the line often goes up to 10F (5.5 ° C). At high water speeds, the size of the main pipe can be reduced to about half the size required for steam distribution without the need for a steam trap and pressure reducing valve. To adjust the condensation, the pipes do not need to be screwed down to the same degree as the steam, but they cross the ground. Although it costs more to install, it costs less to operate steam. Inlet water treatment is negligible and corrosion is minimal. The problem of expansion and insulation is similar to other underground systems. The large reciprocating ring is compatible with expansion between fixed points and underground piping is embedded in the effective heat insulating layer. Cold water systems also have advantages. Large central chillers are likely to use the heat lost in a non-CFC absorption refrigeration cycle. Cold natural resources are possible; Toronto District Corporation of Canada from Lake Ontario Water, derived from a 1.6-mile (2.6 km) pipe at a depth of 200 feet (61 m) with year-round temperatures of 40 ° F (4.5 ° C). Uses. The lake water reaches the city water source through a heat exchanger (with cold water) and then is treated.

With district heating / cooling, all source components are placed together in a remote unit. This leaves other buildings free from the necessary space and visible effects of filter columns, boilers, fuel storage, water chillers and cooling towers. Heat, humidity, polluted air and noise are present in the remote unit. When such a system serves commercial customers, hot or cold water is measured. When owned by the individual (such as on campus), it is usually not measured. This can become a problem when trying to identify

Building energy savings and subsequent savings.

Distribution environment

The distribution of the heating / cooling effect (other than steam) is done using either water or air or weather. As a result, there are three distinct categories of central air conditioning systems:

All-Air System: In an all-air system, the heating-cooling effect is distributed from the source to the spaces through the hot or cold air conveyed into the channel; water is not used for heat transfer to ventilated areas. The main advantage of an all-air system is that the air is used to change the air (circular, but this direct andLogical); The main issue in some construction projects is the amount of space that must be allocated to the canal. Air conditioning is sent to different spaces through the diffuser / valves. An all-in air conditioning system (with configuration) must be able to meet the owner's project needs to provide thermal comfort, IAQ, and energy efficiency.

Air-water system: In a climate system, much of the heat-cold impact of the source (s) is distributed to the spaces through hot or cold water in the pipes. The air also enters the spaces from a centralized unit - usually enough air to ensure optimal indoor air quality;



Figure 12.60 Typical arrangement of a high temperature water system

This is often about 10 percent of the airflow of an all-air system. It can also transfer some heat or cold. The main advantage of the air-water system is that it requires less volume of distribution (piping smaller than the conduit for equal heat transfer). An important concern in construction projects is the placement of air-to-air heat exchangers in the occupied spaces. A water-based air conditioning system (with configuration) should be able to easily meet the needs of the project owner in terms of comfort, IAQ and energy efficiency.

All-water system: The heat-cold effect is transmitted from the source to the spaces via hot or cold water via piping and to these spaces through a heat exchange device

It is entered. Air is not used to transfer heat to / from air conditioners and is not introduced into the air by the air conditioning system (air is introduced independently, for example by patio methods). The main advantage of this system is that the volume of space needed to distribute the minimum amount is possible (no channel at all). An important issue with the all-water HVAC system is meeting the needs of the project owner in terms of IAQ.
Anatomy of the air conditioning system

Building Scale

Larger buildings are generally more complex than smaller ones. The same is true for air conditioners. The anatomy of the system begins with a smaller scale building (a building with, for example, one to five thermal zones). A larger-scale building discussion will follow.

Smaller buildings usually have shell (or cover) loads; weather (rather than indoor loads) indicates whether heating or cooling is a major design concern. In some climates, only heat is needed; a building can keep itself cool in hot weather without a mechanical system. In another climate, only cooling is required. In other climates, both heating and cooling are required. It is worth noting that the requirements are from the customer's point of view as well as the climatic expectations of the system to guide design decisions and analysis.

The shell load structure may vary, but at the same time it requires a room-to-room solution for optimal heating, ventilation and cooling. Such a situation suggests acceptance of the approach of local air conditioners. Consider building with north and south facing areas on a cool, sunny winter day; one side absorbs sufficient solar heat, while the other requires additional heat. One of the benefits of local systems is the ability to respond quickly to solitary confinement. The central air conditioning system also has the following advantages: The equipment is housed inside its own space rather than occupying space in each room and maintenance can be done without disturbing the occupation in the occupied room.

Section 12.4 (F) discusses equipment space requirements for large buildings. It is also useful to determine the approximate size of a small building's air conditioning equipment during a schematic design. When the design load of heating and / or cooling is known, refer to the manufacturers' catalog for the dimensions of the proper heating and cooling equipment. An important decision in measuring HVAC equipment is design temperature: What are the appropriate indoor and outdoor temperatures that influence system selection and climate change considerations?

It is not easy to determine the cooling capacity initially. However, a very approximate preliminary estimate can be obtained from the hourly interest rates listed in Table G.3. This type of estimation is likely to be less than the value obtained using the maximum thermal gain clock for which the cooling equipment is measured. Ton is a conventional unit of cooling capacity. One tonne is equivalent to the beneficial cooling effect of one tonne of ice; that is 12000 Btu / h (3516 W). Larger buildings usually have an overwhelming interior load, with lighting, people and equipment showing the overall combination of heating and cooling required. The interior areas of a building often have no thermal connection to the outside environment, all of which are often internal. Buildings have peripheral areas that share a season with the building's exterior. One design challenge is ensuring that the air conditioning system responds to such varied responses across space and time. The following discussion mainly refers to the larger building.



Table 12.4 The main components of each HVAC system and their application

Table 12.4 describes the main components of each HVAC system and focuses on the concepts of source, distribution, delivery (and control).

Emphasizes. There are three common tasks (heating, cooling and ventilation). There is an inlet and outlet valve in each case. Although the final selection of the HVAC system must follow a detailed analysis of the owner's project requirements, some basic concepts (previously introduced and expanded here) will underlie the system selection.

Central systems require one or more large mechanical spaces (often in the basement and / or on the roof), large distribution trees, and sophisticated control systems. Noise, heat and other conditionsThe mechanical room environment can be controlled relatively easily, as conditions are concentrated in several locations without regular occupation. It is easy to maintain uninterrupted maintenance on a regular basis, although the failure of central equipment can stop the whole building. Air quality can be enhanced by placing air valves above street level pollution and regular maintenance of centralized air filters. Longer service life can be expected with regular maintenance. Waste heat recovery is beneficial for energy efficiency. There are many ways to meet the different thermal requirements of many regions by a single central system. An important drawback of the central system is the possible size and length of the distribution tree required to deliver centralized services to many distributed spaces. Other potential forms of planning

Operations are caused by differences in area usage. When an air conditioner needs to be activated to service one area (such as computer server space) while other areas are off, energy may be wasted.

Local systems may even be appealing to large buildings because the planning difference between areas multiplies. Also, obvious differences in other factors, for example, operating (with convenience expectations) or deploying in a building can lead to the selection of a local system (usually multiple local systems). The large space of centralized equipment is not required in local systems, but the source equipment is distributed throughout a building (either on the roof or surrounding ground). Scattering equipment minimizes (or eliminates) the size of distribution trees and greatly simplifies control systems. In addition, system failure affects only a small part of a building. Noise and other effects of local equipment cause discomfort in occupied spaces and challenge quality maintenance (because access to many separate locations may be impaired or restricted). Good air quality depends on regular cleaning of the air filters, which will be more difficult when they are scattered around a building and occupied by occupied spaces. The potential for energy conservation in local systems is promising, largely because the operations and control of the system are local and personal, but there is little opportunity to capture the lost heat as a source. The central system approach does not mean that all spaces must be used from a single place. Several central systems or positions may be used in a building. This is illustrated in Fig. 12.61b, where the central boiler-chiller space is located remotely and the fan housing is located on each floor. This greatly reduces the size of the large air distribution tree; although the distribution tree is wide for hot and cold water, the pipes are much smaller in diameter and are relatively easily replaced and coordinated. The use of a centralized room makes the energy recovery system more feasible.

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