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    HVAC(heating, ventilation, and air conditioning) is the technology ofindoor and vehicular environmental comfort. HVAC system design is asub discipline of mechanical engineering, based on the principlesof thermodynamics, fluid mechanics, and heat transfer.

    Air conditioning has implied cooling or at the most improvement ofthe indoor environment during warm months of the year.

    The science of air-conditioning started at the beginning of the twentiethcentury.

    It is in the modern times that this term has taken a more literal

    meaning, thereby referring to control of temperature moisture content,cleanliness, indoor air quality and air circulation.

    Today air-conditioning is no longer considered a luxury but a must forany modern building, be it an office space, shopping center orinstitutional building.

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    The principles at work in air-conditioning is evaporation. In therefrigeration cycle this principle is put towork by causing a liquid refrigerant toevaporate in a cooling coil (evaporator).

    This refrigerant is a specially chosensubstance, which has the property ofevaporation at very low temperature.

    For example, the commonly usedrefrigerant, R-2, would start evaporatingat 40 degrees even under normalatmospheric pressure.

    The cooling coil in which the refrigerant

    evaporates, is in contact with the air( orwater in chilled water systems)surrounding it, thereby cooling that aswell. Once cooled, this air (or water) isthen directed to the spaces, whichrequire comfort cooling.

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    Under atmospherictemperature and pressure therefrigerant is in gaseous form.

    The cooling takes place whenliquids evaporate to become

    gas. Therefore, the refrigerant gas

    must first transform into theliquid form.

    Most gases can be made intoliquid form by raising their

    pressure. The equipment that increases

    the pressure of the gas bycompressing it is called thecompressor

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    During compression,however ,the refrigerantbecomes hot. This is owing totwo factors:

    Because of the work done onit (remember how warm thehand pump becomes when

    pumping air into your bicycletires?)

    Because the refrigerant isconverted from gas to liquidreleasing its latent heat.

    This heat has to be removedto enable the gas to condenseinto a liquid easily. Theequipment that removes theheat is called the condenser.

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    There are two ways in which the condenser is cooled

    A)by blowing or sucking air through it in air-cooled condenser and

    B)by pumping water through it in a water-cooled condenser.

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    An air cooled condenser: itconsists of a set of finedcopper tubes and fan todraw or blow the air

    through the condenserinside the tubes, while airis blown or suckedthrough the fined tubearrangement by a fan.

    The air which is normallyat a temperature 10degrees to 12 degreeslower than the gas, picksup the heat from the gasmaking it condense insidethe tube.

    Air cooled condensers are

    very common for window,split and packaged air-conditioners and are nowbecoming popular forcentral plants as well.

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    Water-cooled condenser: in this, water is pumped through thetubes of a shell and tube condenser using a water pump, and therefrigerant is passed through the shell.

    This condenser is also called a heat exchanger because this is

    where the refrigerant and the water exchange heat with eachother.

    On giving away some heat in the heat exchanger travels to thecooling tower where part of the water evaporates in contact withair, cooling the remaining water, which is once again circulatedthrough the heat exchanger.

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    From the condenser, the liquid refrigerant is ready to go to work.This refrigerant can remove heat when it starts evaporating.

    The liquid refrigerant from the condenser is injected through ametering device called the capillary or expansion valve into the

    cooling coil, which is a bundle of tubes. Inside the cooling coil the pressure is low, because of the metering

    or throttling device on one side. Under low pressure, the liquidrefrigerant starts evaporating rapidly.

    While evaporating it needs sensible heat to transform itself from

    the liquid to the gaseous state. Thus it soaks up heat from the surrounding tubes, and from the

    air, with which the tubes are in contact. This is what causes thecooling

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    TON

    The term ton is used in connection with heat load or capacity of the air-conditioning equipment.

    The term ton (TR) in refrigeration and air conditioning is a unit indicating acertain quantity of heat is different from temperature, which only says how hotthe substance is but how much heat it contains.

    The air conditioning engineer uses the term, ton refrigeration popularlyreferred to as ton, when associated with refrigeration and air conditioning.

    One ton refrigeration was originally conceived to describe the quantity of heatremoved to freeze one ton (2240 pounds) of water at 32 degrees F to ice at thesame temperature in 24 hours.

    The two most common units stating the heat quantity are the British thermalunit (btu) and the calorie(cal). The Btu is the quantity of heat needed to raise thetemperature of 1lb of water 1 degree F

    The calorie is the needed metric unit of heat quantity. It is the heat indeed toraise the temperature of one gram of water by 1degree C. since calorie is a verysmall measurement, it is practical to use thousand calories as the unit for air-conditioning and it is expressed as kilo calorie(K Cal)

    One ton refrigeration equals to 12,000 Btu or 3000 K Cal.

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    One of the most important factors in delivering comfort is thefreshness of the conditioned air.

    Substantial research has been done by the American society ofheating refrigeration and air-conditioning engineers (ASHRAE) to

    determine the optimum requirement of fresh air for differentapplications and the air-conditioning engineer designs the plantaccordingly.

    Usually the fresh air requirements are stipulated as cubic feet perminute (cfm) per person or minimum air changes per hour.

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    The basic branches of air-conditioning are non-ducted productsand ductable systems.

    Non-ducted products: this refers to those air-conditioners that donot use any air ducting to cool the conditioned space. Window

    mounted room air-conditioners and the non-ducted split-conditioners fall into this category. These products are suitablewhere air throw is limited to around 4m. (13ft) and small spacesare involved. Where large spaces are involved, multiple units areused to distribute the air. Alternatively in order to distribute theair uniformly using less units, ductable systems are preferred.

    Ductable systems: it can be further subdivided into packaged air-conditioners and central plans. The term ductable implies any air-conditioning system suitable for ducting. The design engineermay prefer not use ducting by employing fan coil units instead.

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    Room air-conditioners are the ubiquitous machines, which can beseen mounted in window air-conditioners.

    In case of window air-conditioners the compressor condenser fan,condenser and evaporator are all enclosed in a single cabinet.

    The unit is to be installed in a wooden frame either in a windowor in a hole in the wall.

    The air being blown through the condenser must pass freelythrough without restriction.

    These air-conditioners come in cooling capacities of 0.5,0.75,1,1.5

    and 2 tons, adequate for a room between 5 and 20 sq.m in size. Larger spaces may be handled by using multiple units of this type.

    While window air-conditioners are economical and mostconvenient to install, they could be noisy for some applications.

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    As the name implies, the air-conditioner is split into two basic components, theindoor unit and the outdoor unit.

    These two units are connected by refrigeration tubing and electrical wires that

    can pass through a hole in the wall barely 10cm in diameter. Since the relatively noisy components such as the compressor and condenser fan

    are in the outdoor unit, the conditioned space tends to be quiet.

    There are situations where it is not possible to mount a window air-conditionerbecause of obstructions from neighboring walls or non availability of a suitablewindow.

    In such cases, the split air-condtioner is used because the outdoor unit can bemounted on the roof or on a ledge some distance away from the room to be air-conditioned.

    Though split air-conditioners are more expensive than the window mountedtype, they are preferred for their low noise levels.

    The outdoor unit houses the compressor, condenser and condenser fan. The indoor unit consists of the evaporator (cooling coil) and the evaporator

    blower.

    Since the noiser components are outside the building the conditioned space ismuch quieter.

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    While the outdoor units of split air-conditioner is split into two basic

    components, the indoor unit and the outdoor unit. These two units are connected by refrigeration tubing and electrical wires that

    can pass through a hole in the wall barely 10cm in diameter.

    Since the relatively noisy components such as the compressor and condenserfan are in the outdoor unit, the conditioned space tends to be quiet.

    There are situations where it is not possible to mount a window air-conditioner because of obstructions from neighboring walls or non availabilityof a suitable window.

    In such cases, the split air-conditioner is used because the outdoor unit can bemounted on the roof or on a ledge some distance away from the room to beair-conditioned.

    Though split air-conditioners are more expensive that the window mountedtype, they are preferred for their low noise levels.

    The outdoor unit houses the compressor, condenser and the condenser fan.

    The indoor unit consists of the evaporator (cooling coil) and the evaporatorblower.

    Since the noisier components are outside the building the conditioned space ismuch uieter.

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    Central plants are usually large air-conditioning plantsassembled at the site. These plants are used for bigbuildings, such as hotels, theatres, hospitals, large officecomplexes and factories.

    They are designed for accurate control of all the parameters

    of comfort. As the name implies the central plant is housed in a central

    location, usually in a plant room. This room could be in a basement or adjacent to the building

    to be air-conditioned.

    Central plants comprise equipments such as one or morecompressors, condensers, air handling units , water chillersand cooling towers. For central plants the main division arethose that use direct expansion(DX) and those that usechilled water.

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