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In most comfort air-conditioning systems, usually only the space temperature is controlled within limits. A slight variation of the space relative humidity during the operation of the air system is often acceptable.
Therefore, the store effect of moisture is ignored except in conditioned spaces where both temperature and relative humidity need to be controlled or in a hot and humid area where the air system is operated at night shutdown mode.
In most cases, latent heat gain is considered equal to latent cooling load instantaneously. It is also assumed that the superposition principle holds.
When a number of changes occur simultaneously in the conditioned space, they will proceed as if independent of each other. The total change is the sum of the responses caused by the individual changes.
The energy analysis program compares the total energy use in a certain period with various alternatives in order to determine the optimum one. Convective heat, latent heat, and sensible heat gains from infiltration are all equal to cooling loads instantaneously.
Space cooling load is a component of the cooling coil load. The Rigorous Approach The rigorous approach to the calculation of the space cooling load consists of 1 finding the inside surface temperatures of the building structures that enclose the conditioned space due to heat balance at time t and 2 calculating the sum of the convective heats transferred from these surfaces as well as from the occupants, lights, appliances, and equipment in the conditioned space at time t.
This conductive heat can be found by solving the partial differential equations or by numerical solutions. The number of inside surfaces i is usually equal to 6, and surface i is different from j so that radiative exchange can proceed.
Using a rigorous approach to find the space cooling load requires numerous computer calculations. It is laborious and time consuming. The rigorous approach is impractical and is suitable for research work only. Coefficients vn and wn are called transfer function coefficients, or weighting factors. Weighting factors are used to weight the importance of the effect of current and previous heat gains as well as the previous space sensible cooling load on the current space sensible cooling load qrs,t.
Mitalas and Stevenson and others developed a method for determining the transfer function coefficients of a zone of given geometry and details of the calculated space heat gains and the previously known space sensible cooling load through rigorous computation or through tests and experiments. In DOE 2. Sowell and Spitler et al. The space cooling load is calculated directly by multiplying the heat gain qe with CLTD, SCL, or CLF instead of first finding the space heat gains and then converting into space cooling loads through the room transfer function.
Finite Difference Method Since the development of powerful personal computers, the finite difference or numerical solution method can be used to solve transient simultaneous heat and moisture transfer in space cooling load calculations. Wong and Wang emphasized the influence of moisture stored in the building structure on the cool-down load during the night shut-down operating mode in locations where the summer outdoor climate is hot and humid.
The finite difference method is simple and clear in concept as well as more direct in computation than the transfer function method. The conversion of space heat gains to space cooling loads takes place by 1 averaging the radiative heat gains to the current and successive hours according to the mass of the building structure and experience and 2 adding the instantaneous convective fraction and the allocated radiative fraction in that time period.
Conduction Heat Gains Following are the principles and procedures for the calculation of space heat gains and their conversion to space cooling loads by the TFM.
The R value, expressed in hr. Sol-air temperature Tsol is a fictitious outdoor air temperature that gives the rate of heat entering the outer surface of walls and roofs due to the combined effect of incident solar radiation, radiative heat exchange with the sky vault and surroundings, and convective heat exchange with the outdoor air. There are two types of shading devices: indoor shading devices and outdoor shading devices.
Indoor shading devices increase the reflectance of incident radiation. Venetian blinds and draperies are the most widely used indoor shading devices. Most horizontal venetian blinds are made of plastic, aluminum, or rigid woven cloth slats spaced 1 to 2 in. Vertical venetian blinds with wider slats are also used. Draperies are made of fabrics of cotton, rayon, or synthetic fibers. They are usually loosely hung, wider than the window, often pleated, and can be drawn open and closed as needed.
Draperies also increase thermal resistance in winter. External shading devices include overhangs, side fins, louvers, and pattern grilles.
They reduce the sunlit area of the window glass effectively and therefore decrease the solar heat gain. External shading devices are less flexible and are difficult to maintain. Shading Coefficient SC. The shading coefficient is an index indicating the glazing characteristics and the associated indoor shading device to admit solar heat gain.
Heat Gain through Window Glass. It is simple and more convenient if they are calculated separately. Internal Heat Gains Internal heat gains are heat released from the internal sources. Lights Heat gain in the conditioned space because of the electric lights, qe.
The pressure difference is probably caused by wind pressure, stack effect due to outdoor—indoor temperature difference, and the operation of an air system s. Today new commercial buildings have their external windows well sealed. If a positive pressure is maintained in the conditioned space when the air system is operating, infiltration is normally considered as zero.
When exterior windows are not well sealed, the outdoor wind velocity is high at winter design conditions, or there is a door exposed to the outdoors directly, an infiltration rate of 0. When the volume flow rate of infiltration is determined, the sensible heat gain due to infiltration qs. It becomes space cooling load instan- taneously. Ventilation air is often taken at the AHU or PU and becomes sensible and latent coil load components. RTF is affected by parameters like zone geometry; wall, roof, and window construction; internal shades; zone location; types of building envelope; and air supply density.
Space Air Temperature and Heat Extraction Rate At equilibrium, the space sensible heat extraction rate at time t, qxs,t, is approximately equal to the space sensible cooling load, qrs,t, when zero offset proportional plus integral or proportional-integral-derivative control mode is used.
Space air temperature Tr can be considered an average reference temperature within a time interval. It is used to size and select the heating equipment.
In heating load calculations, solar heat gain, internal heat gains, and the heat storage effect of the building envelope are usually neglected for reliability and simplicity. An air handling unit AHU handles and conditions the air, controls it to a required state, and provides motive force to transport it. An AHU is the primary equipment of the air system in a central air- conditioning system.
The basic components of an AHU include a supply fan with a fan motor, a water cooling coil, filters, a mixing box except in a makeup AHU unit, dampers, controls, and an outer casing. A return or relief fan, heating coil s , and humidifier are optional depending on requirements. The supply volume flow rate of AHUs varies from to about 60, cfm. AHUs are classified into the followings groups according to their structure and location.
They need more space and are usually for large units. In vertical units, as shown in Figure 9. They are often comparatively smaller than horizontal units. Air is evenly distributed over the coil section, and the fan discharge can easily be connected to a supply duct of nearly the same air velocity. In a blow-through unit, as shown in Figure 9. It usually has hot and cold decks with discharge dampers connected to warm and cold ducts, respectively. Factory-Fabricated and Field Built-Up Units Factory-fabricated units are standard in construction and layout, low in cost, of higher quality, and fast in installation.
Field built-up units or custom-built units are more flexible in construction, layout, and dimensions than factory-built standardized units. Rooftop and Indoor Units A rooftop AHU, sometimes called a penthouse unit, is installed on the roof and will be completely weatherproof. It is a once- through unit.
There is no return air and mixing box. Packaged Units A packaged unit PU is a self-contained air conditioner. It conditions the air and provides it with motive force and is equipped with its own heating and cooling sources. The packaged unit is the primary equipment in a packaged air-conditioning system and is always equipped with a DX coil for cooling, unlike an AHU.
R, Ra, and others are used as refrigerants in packaged units. The portion that handles air in a packaged unit is called an air handler to distinguish it from an AHU. Like an AHU, an indoor air handler has an indoor fan, a DX coil indoor coil , filters, dampers, and controls.
Packaged units can be classified according to their place of installation: rooftop, indoor, and split packaged units. Rooftop Packaged Units A rooftop packaged unit is mounted on the roof of the conditioned space as shown in Figure 9.
Auxiliary electric heating is provided if necessary. Rooftop packaged units are single packaged units. Their cooling capacity may vary from 3 to tons with a corresponding volume flow rate of to 80, cfm. Rooftop packaged units are the most widely used packaged units.
Indoor Packaged Units An indoor packaged unit is also a single packaged and factory-fabricated unit. It is usually installed in a fan room or a machinery room. A small or medium-sized indoor packaged unit could be floor mounted directly inside the conditioned space with or without ductwork.
The cooling capacity of an indoor packaged unit may vary from 3 to tons and volume flow rate from to 40, cfm. Source: Mammoth, Inc. Reprinted by permission. Split Packaged Units A split packaged unit consists of two separate pieces of equipment: an indoor air handler and an outdoor condensing unit. The indoor air handler is often installed in the fan room. Small air handlers can be ceiling hung.
The condensing unit is usually located outdoors, on a rooftop or podium or on the ground. A split packaged unit has its compressors and condenser in its outdoor condensing unit, whereas an indoor packaged unit usually has its compressors indoors.
The cooling capacity of split packaged units varies from 3 to 75 tons and the volume flow rate from to 30, cfm.
It is the summarized single index of part-load efficiency of PUs based on weighted operations at several load conditions. It is the total heating output of a heat pump during its annual usage period for heating, in Btu, divided by the total electric energy input to the heat pump during the same period, in watt-hours.
Chilled water, brine, and refrigerants that are used to cool and dehumidify the air are called coolants. Coils consist of tubes and external fins arranged in rows along the air flow to increase the contact surface area.
Tubes are usually made of copper; in steam coils they are sometimes made of steel or even stainless steel. Copper tubes are staggered in 2, 3, 4, 6, 8, or up to 10 rows. Fins are extended surfaces often called secondary surfaces to distinguish them from the primary surfaces, which are the outer surfaces of the tubes. Copper, steel, or sometimes stainless steel fins are also used. Fins are often in the form of continuous plate fins, corrugated plate fins to increase heat transfer, crimped spiral or smooth spiral fins that may be extruded from the aluminum tubes, and spine pipes, which are shaved from the parent aluminum tubes.
Corrugated plate fins are most widely used. Fin spacing Sf is the distance between two fins. In a water cooling coil, water circuits or tube feeds determine the number of water flow passages. The greater the finned width, the higher the number of water circuits and water flow passages. Direct Expansion DX Coil In a direct expansion coil, the refrigerant, R, Ra, or others, is evaporated and expanded directly inside the tubes to cool and dehumidify the air as shown in Figure 9.
Refrigerant is fed to a distributor and is then evenly distributed to various copper tube circuits typically 0. Air and refrigerant flow is often arranged in a combination of counterflow and cross flow and the discharge header is often located on the air-entering side. Finally, the vapor refrigerant is discharged to the suction line through the header.
At such a temperature, the surface temperature of the coil is often lower than the dew point of the entering air. A condensate drain pan is necessary for each vertically banked DX coil, and a trap should be installed to overcome the negative pressure difference between the air in the coil section and the ambient air.
Face velocity of the DX coil va, in fpm, is closely related to the blow-off of the water droplets of the condensate, the heat transfer coefficients, the air-side pressure drop, and the size of the air system. For corrugated fins, the upper limit is fpm, with an air-side pressure drop of 0. A large DX coil is often divided into two refrigerant sections, each with its own expansion valve, distributor, and discharge header.
They are spaced at a center-to-center distance of 0. These tubes may be staggered in 2, 3, 4, 6, 8, or 10 rows. Chilled water coils are often operated at a pressure of to psig. As in a DX coil, the air flow and water flow are in a combination of counterflow and cross flow. Then the water cooling coil becomes a dry—wet coil with part of the dry surface on the air entering side and part of the wet surface on the air leaving side. A dry—wet boundary divides the dry and wet surfaces.
A condensate drain pan is necessary for a dry—wet coil. This coil becomes a sensible cooling—dry coil, and the humidity ratio of the conditioned air wa remains constant during the sensible cooling process. Water Heating Coil The construction of a water heating coil is similar to that of a water cooling coil except that in water heating coils hot water is supplied instead of chilled water and there are usually fewer rows, only 2, 3, and 4 rows, than in water cooling coils. Steam Heating Coil In a steam heating coil, latent heat of condensation is released when steam is condensed into liquid to heat the air flowing over the coil, as shown in Figure 9.
Steam enters at one end of the coil, and the condensate comes out from the opposite end. For more even distribution, a baffle plate is often installed after the steam inlet.
Steam heating coils are usually made of copper, steel, or sometimes stainless steel. For a steam coil, the coil core inside the casing should expand or contract freely. The coil core is also pitched toward the outlet to facilitate condensate drainage. Coil Accessories and Servicing Coil accessories include air vents, drain valves, isolation valves, pressure relief valves, flow metering valves, balancing valves, thermometers, pressure gauge taps, condensate drain taps, and even distribution baffles.
They are employed depending on the size of the system and operating and serving requirements. Coil cleanliness is important for proper operation. If a medium-efficiency air filter is installed upstream of the coil, dirt accumulation is often not a problem. If a low-efficiency filter is employed, dirt accumu- lation may block the air passage and significantly increase the pressure drop across the coil.
Coils should normally be inspected and cleaned every 3 months in urban areas when low-efficiency filters are used. Drain pans should be cleaned every month to prevent buildup of bacteria and microorganisms. Outdoor air should be guided by a baffle plate and flow in an opposite direction to the recirculating air stream so that they can be thoroughly mixed without stratification. Run the chilled water pump for the idle coil with a water velocity of 2. A better method is to drain the water completely.
For a hot water coil, it is better to reset the hot water temperature at part-load operation instead of running the system intermittently. A steam heating coil with inner distributor tubes and outer finned heating tubes provides better protection against freeze-up. Air Filters Air Cleaning and Filtration Air cleaning is the process of removing airborne particles from the air. Air cleaning can be classified into air filtration and industrial air cleaning. Industrial air cleaning involves the removal of dust and gaseous contaminants from manufacturing processes as well as from the space air, exhaust air, and flue gas for air pollution control.
In this section, only air filtration is covered. Air filtration involves the removal of airborne particles presented in the conditioned air. The purpose of air filtration is to benefit the health and comfort of the occupants as well as meet the cleanliness requirements of the working area in industrial buildings.
An air filter is a kind of air cleaner that is installed in AHUs, PUs, and other equipment to filter the conditioned air by inertial impaction or interception and to diffuse and settle fine dust particles on the fibrous medium. The filter medium is the fabricated material that performs air filtration. Test Methods The performance of air filters is usually tested in a test unit that consists of a fan, a test duct, the tested filter, two samplers, a vacuum pump, and other instruments.
Three test methods with their own test dusts and procedures are used for the testing of low-, medium-, and high-efficiency air filters. The weight arrestance test is used for low-efficiency air filters to assess their ability to remove coarse dusts. Standard synthetic dusts that are considerably coarser than atmospheric dust are fed to the test unit.
By measuring the weight of dust fed and the weight gain due to the dust collected on the membrane of the sampler after the tested filter, the arrestance can be calculated. The atmospheric dust spot efficiency test is used for medium-efficiency air filters to assess their ability to remove atmospheric dusts.
Atmospheric dusts are dusts contained in the outdoor air, the outdoor atmosphere. Untreated atmospheric dusts are fed to the test unit. Air samples taken before and after the tested filter are drawn through from identical fiber filter-paper targets. By measuring the light transmission of these discolored white filter papers, the efficiency of the filter can be calculated. The DOP penetration and efficiency test or simply DOP test is used to assess high-efficiency filters removing dusts particles of 0.
According to U. By measuring the concentration of these particles in the air stream upstream and downstream of the tested filter using an electronic particle counter or laser spectrometer, the penetration and efficiency of the air filter can be calculated. These filters are usually in panels as shown in Figure 9.
Their thickness varies from 1 to 4 in. Activated Detergents may be used to wash off dusts so that the filter media can be cleaned and reused — they are therefore called viscous and reusable. The filter medium is discarded when its final pressure drop is reached — dry and disposable.
The face velocity of the panel filter is usually between and fpm. The initial pressure drop varies from 0. WG and the final pressure drop from 0. They are usually dry and disposable. Air velocity through the medium is 6 to 90 fpm. Face velocity of the air filter is about fpm to match the face velocity of the coil in AHUs and PUs. WG and final pressure drop from 0. Its filter media are made of glass fibers of submicrometer diameter in the form of pleated paper mats.
The medium is dry and disposable. The surface area of the HEPA filter may be 50 times its face area, and its rated face velocity varies from to fpm, normally at a pressure drop of 0. WG for clean filters. The final pressure drop is 0. Sealing of the filter pack within its frame and sealing between the frame and the gaskets are critical factors that affect the penetration and efficiency of the HEPA filter.
Both its sealing and filter media are more efficient than those of a HEPA filter. Activated Carbon Filters These filters are widely used to remove objectional odors and irritating gaseous airborne particulates, typically 0. Adsorption is physical conden- sation of gas or vapor on the surface of an activated substance like activated carbon.
Activated substances are extremely porous. One pound of activated carbon contains 5,, ft2 of internal surface. Activated carbon in the form of granules or pellets is made of coal, coconut shells, or petroleum residues and is placed in trays to form activated carbon beds as shown in Figure 9.
Low-efficiency prefilters are used for protection. When air flows through the carbon beds at a face velocity of to fpm, the corresponding pressure drop is 0. Humidifiers A humidifier adds moisture to the air. For comfort air-conditioning systems, a steam humidifier with a separator as shown in Figure 9. Steam is supplied to a jacketed distribution manifold. It enters a separating chamber with its condensate.
Steam then flows through a control valve, throttles to a pressure slightly above atmospheric, and enters a dry chamber. Due to the high temperature in the surrounding separating chamber, the steam is superheated. Dry steam is then discharged into the ambient air stream through the orifices on the inner steam discharge tubes. For an air system of cold air supply with humidity control during winter mode operation, an air washer is economical for large-capacity humidification in many industrial applications.
An air washer is a humidifier, a cooler, a dehumidifier, and an air cleaner. An air washer usually has an outer casing, two banks of spraying nozzles, one bank of guide baffles at the entrance, one bank of eliminators at the exit, a water tank, a circulating pump, a water filter, and other accessories as shown in Figure 9. Outer casing, baffles, and eliminators are often made of plastics or sometimes stainless steel. An eccentric inlet connected to the discharge chamber of the spraying nozzle gives centrifugal force to the water stream and atomizes the spraying water.
Water is supplied to the spraying nozzle at a pressure of 15 to 30 psig. The distance between two spraying banks is 3 to 4. The air velocity inside an air washer is usually to fpm. The cooling and heating capacities of an AHU can be varied by using coils of different numbers of rows and fin densities. The size of a PU is determined by its cooling capacity. Normally, the volume flow rate per ton of cooling capacity in PUs is to cfm. In most packaged units whose supply fans have belt drives, the fan speed can be selected so that the volume flow rate is varied and external pressure is met.
Selected equipment in a size larger always means a waste of energy and investment. It raises the pressure of refrigerant so that it can be condensed into liquid, throttled, and evaporated into vapor to produce the refrigeration effect.
It also provides the motive force to circulate the refrigerant through condenser, expansion valve, and evaporator. According to the compression process, refrigeration compressors can be divided into positive dis- placement and nonpositive displacement compressors.
A positive displacement compressor increases the pressure of the refrigerant by reducing the internal volume of the compression chamber. Reciprocating, scroll, rotary, and screw compressors are all positive displacement compressors. The centrifugal com- pressor is the only type of nonpositive displacement refrigeration compressor widely used in refrigeration systems today. Currently used refrigeration compressors are reciprocating, scroll, screw, rotary, and centrifugal compressors.
Reciprocating Compressors In a reciprocating compressor, as shown in Figure 9. The refrigeration capacity of a reciprocating compressor is a fraction of a ton to about tons. Refrigerants R and Ra are widely used in comfort and processing systems and sometimes R- in industrial applications.
The maximum compression ratio Rcom for a single-stage reciprocating compressor is about 7. Capacity control of reciprocating compressor including: on-off and cylinder unloader in which discharge gas is in short cut and return to the suction chamber.
Although reciprocating compressors are still widely used today in small and medium-sized refriger- ation systems, they have little room for significant improvement and will be gradually replaced by scroll and screw compressors. Scroll Compressors A scroll compressor consists of two identical spiral scrolls assembled opposite to each other, as shown in Figure 9.
One of the scrolls is fixed, and the other moves in an orbit around the motor shaft whose amplitude equals the radius of the orbit. The two scrolls are in contact at several points and therefore form a series of pockets. Vapor refrigerant enters the space between two scrolls through lateral openings. The lateral openings are then sealed and the formation of the two trapped vapor pockets indicates the end of the suction process. The vapor is compressed and the discharge process begins when the trapped gaseous pockets open to the discharge port.
Compressed hot gas is then discharged through this opening to the discharge line. In a scroll compressor, the scrolls touch each other with sufficient force to form a seal but not enough to cause wear.
The upper limit of the refrigeration capacity of currently manufactured scroll compressors is 60 tons. A scroll compressor also has only about half as many parts as a reciprocating compressor at the same refrigeration capacity. Few components result in higher reliability and efficiency. A scroll compressor also operates more smoothly and is quieter. Rotary Compressors Small rotary compressors for room air conditioners and refrigerators have a capacity up to 4 tons.
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