Heating , cooling, lighting - Lechner ch 16
1. Fan-coil system - basically consists of a fan and a coil within which water circulates. the units are often in the form of cabinets for placement under windows. The fan blows room air across coils through which either hot or cold water circulates. thermostatically controlled valves regulate the flow of water through the coils. in the less expensive but also less comfortable two-pipe system, hot water circulates during the winter and cold water in the summer. In such systems, it is not possible for an occupant to choose either heating or cooling. A three-pipe (hot, cold, and a common return) system also exists but wastes energy because hot and cold water return through the same pipe.
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2. water - loop Heat-pump system. with this system, each zone is heated or cooled by a separate water-to-air heat pump. A thermostat in each zone determines whether the local heat pump extracts heat from a water loop (heating mode) or injects heat into the water loop (cooling mode)
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A heat pump is a special kind of air conditioner running in reverse during the winter. heat pumps are appropriate where both summer cooling and winter heating are required.
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A radiatn floor will heat the space by both radiation and natural convection.
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Air systems are of two types: constant-air-volume (CAV) and variable-air-volume (VAV).
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Air-water system - supply both air and water to each zone of a building. Although this increase the complexity of the mechanical systems, it greatly decreases the size of the equipment because of the immense heat-carrying capacity of water as opposed to air. air is supplied mainly because of the need for ventilation.
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All water systems - systems supply no air, they are appropriate when a large amount of ventilation either is not necessary or can be achieved locally by such means as opening windows.
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All-Air System - the advantage is complete control over air quality is possible. the main disadvantages are that all-air systems are very bulky and a significant part of the building volume must be devoted to the delivery of air. they are also less efficient because moving large quantities of air requires a great deal of power. it must be noted that for clarity, only the supply ducts are shown on each plan in the following examples. there could also be a sizable return-duct system on each floor.
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Chillers produce chilled water that is used in air-handling units and fan-coil units.
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Design Guidelines for Mech sys.
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Fan-coil units are most appropriate for air-conditioning building for air-conditioning buildings with small zones, such as apartments, condominiums, motels, hoetls, hospitals, and schools. besides the under-window location, fan-coil units are sometimes located above windows (valanceunits), in small closets, or in the dropped ceiling above a bath room or hallway.
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Geo-exchange heat pumps are in many cases the most energy-efficient, environmentally clean, and cost-effective active space-conditioning systems available. also know as geothermal or ground coupled heat pumps.
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Heat recovery ventilators - heat recovery ventilators, also known as heat exchangers or air-toair heat exchangers, can capture much of the heat that is ordinarily lost during ventilation. when in winter air is exhausted from a building to make room for fresh outdoor air, a large amount of both sensible and latent heat is lost. the cold, dry outdoor air must be heated and humidified, and in summer, the outdoor air must be cooled and dehumidified. some heat recovery ventilators use fixed plates or heat pipes to capture only the sensible heat. Other ventilators are more sophisticated and recover both latent and sensible heat.
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Heat-recovery units are excellent devices for minimizing heat loss from ventilation. both sensible and latent heat can be recovered from these air-to-air heat exchangers.
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Heating systems: a. hot-water (hydronic) systems are efficient and comfortable but generally are not useful for cooling. b. Hot-air systems are appropriate when cooling is also required. c. electric-resistance heating should be used as little as possible because it is most wasteful of energy. d. heat pumps are an efficient way to heat and cool with electricity. Geothermal (geo-exchange)heat pumps are more efficient than air-to-air heat pumps. e. combined heat and power (CHP) systems can supply heat at low cost.
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Hot air furnace uses a heat exchanger to prevent combustion air from mixing with room air. a blower and filter are standard, while a humidifier and cooling coil are optional.
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Hot-air heating system are especially popular where summer cooling is also required. thos hot-air systems that supply air at or near floor level around the perimeter of the building are most suitable for cold climates, where the heating season is the main consideration.
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Hot-air systems - air systems are popular because they can perform the whole range of air conditioning functions: heating, cooling, humidification, dehumidification, filtering, ventilation, and air movement to eliminate stagnant and stratified air layers.
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Location Guidelines 1. for medium-sized building: a. place the equipment on the roof or b. use a MER centrally located to minimize duct sizes, and place it along an outside wall for easy servicing 2. for large multistory buildings: a. place the centralized mechanical equipment in the basement, or the roof, or on intermediate floors b. the cooling tower should be placed on the roof or on out-ofthe way adjacent land c. any additional MER on each floor should be centrally located to minimize duct sizes and distance air has to be moved. if the MERs require large amounts of outdoor air, they should be located along an outside wall.
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Low energy heating and cooling - 1. design the building itself so that i needs very little heating and cooling (e.g., orientation, form, passive heating and cooling, standing, a great thermal envelope, etc) 2. use geo-exchange (geothermal) heat pumps. 3. use air-to-air heat pumps. 4. use an air system only for ventilation. 5. use water or refrigerant rather than air to heat and cool a building. 6. use chilled beams where appropriate. 7. use radiant floor heating where appropriate. 8. use large diameter, short length, and straight run ducts. 9. all ductwork must be on the indoor side of the thermal envelope. 10 use heat recovery ventilators (heat exchangers) 11 use displacement ventilation where appropriate. 12. consider combined heat and power systems 13. consider thermal storage systems to shift load from day to night.
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Mechanical cooling should supply only the small amount of cooling still required after all the heat avoidance (tier one) and passive cooling (tier two) techniques have been employed.
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Mechanical equipment rooms (MERs) should be centrally located to minimize the quantity and complexity of the duct work.
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The vapor compression refrigeration cycle - 1. a large amount of heat of vaporization is required to change a liquid into a gas. of course, this heat is released again when the gas condenses back into a liquid. 2. the boiling/ condensation temperature of any material is a function of pressure. for example. 212°F is the boiling point of water only at the pressure of sea level (14.7 lb/in sq.) when the pressure is reduced, the boiling point is also reduced.
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Variable-air-volume (VAV) systems are efficient but are limited to either heating or cooling a building at any one time
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a great advantage of all the devices shown in figure 16,5a is that they allow many heating zones to be easily established - each room or part thereof can be a separate]ate zone.
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a large office building would require at least five zones based on differences in exposure. each zone will take its own thermostat.
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air-to-air and geothermal (geo-exchange) heat pumps can cool as well as heat. four energy sources/ sinks are possible: ponds well water, vertical ground loops, and horizontal ground loops.
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all but the smallest buildings have more than one thermal zone.
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almost all refrigerants were made of chlorofluorocarbons (CFCs) until 1987, when the world agreed through the Montreal protocol to phase out these ozone-damaging compounds.
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although the compression refrigeration cycle is more efficient than the absorption refrigeration cycle, and inexpensive source of heat can make the absorption cycle attractive.
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an air system requires a substantial amount of a building's volume for ducts and air-handling equipment (1-5%), while water and electrical systems require far less.
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baseboard convectors are liner units placed parallel to exterior walls, while cabinet convectors concentrate the heating where it is most needed-under windows to conteract the cold down-draft and low MRT of the windows. when there is a large area of glazing from floor to ceiling, a below-floor convector can be used.
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because convectors rely on natural convection, they must be placed low in a room. however, if a fan is used for forced convection, any mounting position is possible.
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because the absorption refrigeration cycle is inherently inefficient, the cycle is economical only when an inexpensive source of heat is available.. solar heated water may become such a source.
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coefficient of performance (COP) COP = energy out/ energy in
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combined heat and power (CHP) systems, formerly known as cogeneration systems, can be more efficient than buying electricity and heating energy separately.
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cooling - heat to flow inward when the outdoor temperature is higher than the indoor temperature. the only way to remove the heat is with a machine that pumps heat from a lower temperature to a higher temperature. such device is called a refrigeration machine.
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cooling systems use refrigeration machines to pump heat from lower temperatures indoors to higher temperatures outdoors.
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diffusers and registers mix supply air and room air without causing drafts. both have dampers to control the volume of air. grilles might or might not direct the flow of air, and they do not contain dampers.
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displacement ventilation is more efficient than conventional ventilation in eliminating pollutants.
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district heating and cooling systems are more efficient than smaller individual systems.
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duct-work should always be on the indoor side of the thermal envelope!
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ductless split systems come in three types: mini-split, multi-split, and multi-split with variable refrigerant flow (VRF)
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evaporative cooler often replace air conditions in dry climates, most types do not remove total heat from a building. instead, they convert sensible heat into latent heat, which in dry climates creates thermal comfort very economically.
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exposing mechanical equipment and ducts on the interior is efficient and has aesthetic potential. exposing ducts and pipes on the outdoor side of the thermal envelope is very inefficient and not sustainable. ducts, pipes, and the structure should always be located inside the thermal envelope.
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fan-coil systems provide many zones at modest cost, but they provide fresh air only if they are located on an outside wall.
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for heating single spaces, a wall furnace can be a practical solution because no ductwork is required.
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for slab-on-grade construction in cold climates, the loop perimeter system offers the greatest thermal comfort. the supply air heats the slab where it is coldest-at the edge. thus, this system offers the benefits of both hot-air and radiant slab heating. the main disadvantage is the high initial cost of the system.
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hot-water (hydronic) heating - hot water can distribute the heat throughout the building in several ways. when concrete slabs are used, the coils can be cast right into the slab. radiant-floor fot-water heating systems are excellent for cold climates.
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imagine that the valve is closed and the compressor has pumped most of the refrigerant into the condenser coil. when the valve is slightly opened, only a small stream of liquid refrigerant can enter the partial vacuum of the evaporator coil. the refrigerant boils (evaporates) because of the very low pressure. to change state, the liquid will require the large amount of heat called heat of vaporization. thus, the evaporator coil will cool as it gives up its sensible heat to allow the liquid refrigerant to change into a gas. a high pressure gas collects at point A. since any gas under pressure heats up, the condenser coil gets hot. as the coil loses heat, the high-pressure refrigerant gas will e able to condense and give up its heat of vaporization.
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in the compression cycle, the refrigerant moves heat from the evaporator coil to the condenser coil.
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indoor-air quality (Id of pollution (e.g. volatile organic compounds [VOCs]). b. directly exhausting the source of pollutants. c. using ventilation to dilute pollutants. d. using living plants to clean air.
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internal duct insulation, sound traps, and sections of flexible ducts control mechanical noise and vibration.
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large quantities of heat are efficiently dumped into the atmosphere by evaporative coolers and cooling towers.
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lighting can be efficiently use for heating. air is heated by returning it through the lighting fixtures. the heated air can then be used to heat the perimeter areas of a building.
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most hot-water systems use convectors to transfer the heat from the water to the air of each room. most convectors consist of fin-tubes or fin-coils to maximize the heat transfer by natural convection.
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new refrigerants made of hydrofluorocarbons (HFCs) are 90% less damaging to the ozone but are still significant greenhouse gases.
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packaged system - preengineered, self-contained units where most of the mechanical equipment is assembled at the factory. systems offer low installation, operating, and maintenance costs. rooftop versions are the most common, with each unit serving a separate zone. system can also heat a building via electric strips, a heat pump, or a gas furnance.
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packaged units allow a maximum of mechanical fabrication to be performed in the factory.
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radiant floor heating is a good match for active solar because solar collectors can effeciently produce the relatively low wter temperatures needed (about 90°)
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refrigeration cycles: the refrigeration machine, which pumps heat, is the critical element of any cooling system. there are basically three refrigeration methods: vapor compression, absorption, and thermoelectric. the compression cycle is the most common, but the absorption cycle is often appropriate when a source of low-cost heat is available.
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resistance-heating devices to convert electricity directly into heat. the exceptions are the heat pump and heat from the lighting system.
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round ducts are most efficient, but oblong and rectangular ducts require less headroom. flat oval ducts are a compromise.
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sound traps can be added to ducts instead of lining the ducts with sound-absorbing insulation.
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split systems - most homes and many other small to medium-sized buildings find the split system to be most appropriate. in this system, the compressor and condenser coils are in an outdoor unit, while the AIR-HANDLING UNIT (AHU) with the evaporator coil is indoors.
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split systems are, in effect, two packaged systems.
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the absorption refrigeration cycle - depends on the same two properties of matter described with third property: 3. some liquids have a strong tendency to absorb certain vapors. for example, water vapor is absorbed by liquid lithium bromide or ammonia. these materials are also known as desiccants.
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the air-handling units with their evaporator coils and heating systems are in a mechanical equipment room (MER).
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the baseboard units heat by natural convection, while the unit heater have fans for forced convection. radiant heating is possible at three different intensities. because of their large areas, radiant floors and ceiling can operate at rather low temperatures (80° and 110°F). Radiant panels on walls or ceiling must be hotter (about 190°F) to compensate for their smaller areas. they are used to increase the MRT near large areas of glazing or other cold spots.High intensity infrared lamps operate at over 1000°F and k therefore, can be quite small. these high intensity infrared heaters are designed not to heat air but rather solid objects, such as walls, furniture, and people. these heaters can be used outdoors for purposes like keeping people warm in front of hotel or theater entrances. It also can be powered by gas instead of electricity.
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the economizer cycle uses cold outdoor air when available to cool a building.
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the extended-plenum system is appropriate for buildings because it enables the supply ducts to run parallel and between the joists, thereby saving much space and headroom.
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the four major categories of cooling systems: direct refrigerant, all air, all water, and combination air-water.
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the greatest benefit of geo-exchange heat pumps is their high efflicency. the use about 40% less energy than air-to-air heat pumps and about 70% less energy than standard air conditioning with electric resistance heating.
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the number of required zones has a great impact on the choice of a mechanical system.
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the radial-perimeter system is more suitable for crawl space or attic construction.
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the resistance-heating devices = one unit of energy in the form of electricity is turned into unit of heat - a poor choice. However, with a heat pump, about three units of heat energy can be created for every unit of electrical energy used. the reason is that electricity is not converted into heat, but is used instead to pump heat from outdoors to indoors.
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the sing-duct system - return air can travel either through the above-ceiling plenum or through the return air duct shown in the section.
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thermal-storage systems can take advantage of low night electricity rates and the greater efficiency of running refrigeration machines in cool night air. the energy storage (heat sink) can be in the form of chilled water, ice, or the mass of the building itself.
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to prevent condensation on ducts, a vapor barrier must always be placed on the outside of the insulation. for noise control, porous fiberglass insulation must be exposed to the system.
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when heat pumps are coupled with the ground instead of with out-oor air, they are called geo-exchange (geothermal) heat pumps.
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the absorption refrigeration machine requires no pumps or other moving parts, but it does require a source of heat, such as a gas flame or the waste heat from an industrial process.
in chamber A, water evaporartes and in the process draws heat from the chilled water coil (output). the water vapor migrates to chamber B, where it is absorbed by the lithium bromide. Consequently, the vapor pressure is reduced, and more water can evaporate to continue the cooling process. Eventually, the lithium bromide will become too dilute to further absorb water. However, some of the lithium bromide flows into chamber C, where an extenal heat source boils the water off the lithium bormide. the concentrated lithium bromide is then returned to chamber B while the water vapor is condensed back into water in chamber D. The last step is to return the liquid water back to chamber A so that cycle can continue.
fireplaces are highly inefficient unless they have a heat exchanger, outdoor combustion air, and doors stoves are much more efficient than even the best fireplaces.
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mechanical heating should supply only the small amount of heat still needed after a tight thermal envelope (tier one) and passive solar energy (tier two) have been fully utilized
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