Presentation on theme: "Abstract ABSTRACT The increasing demand for comfort air-conditioning has brought with in the need for greater numbers of practical, technical & sales."— Presentation transcript:
ABSTRACT The increasing demand for comfort air-conditioning has brought with in the need for greater numbers of practical, technical & sales personal who have should training in basic principles and applications of modern air-conditioning. The technical information presented in this work is intended to satisfy the immediate and fundamental concepts in this work is intended to satisfy the immediate and fundamental concepts and relevant principles in the field of air-conditioning. The subject of air-conditioning has come to stay with the universities in the country.
The industries and commercial establishment are experiencing the need of air-conditioning as an instrument and commercial establishment are experiencing the need of air-conditioning as in instrument of efficiency and increased output rather than of comfort alone. The practice of air-conditioning is making rapid strides and its increase practical use has brought in new problems, which is required to be tackled by scientific research. As a thumb rule practice in air-conditioning do not lead to thorough understanding or correct solution of new problems. The project material has been prepared to help to meet this by providing the fundamental process and procedures.
For the completion of this project an easy and simple methodology is adapted by preparing a met lab program to generalize and making it simple for any multistory building cooling/heating load estimation. For estimation,first work sheets are prepared for different existing conditions. Then by using this program the total cooling/heating load for the entire complex has been easily calculated.
1. Definition of Air Conditioning: Air conditioning is defined as the "the process of treating air so as to control simultaneously temperature, humidity, cleanliness and distribution to meet the requirements of the conditioned space." As indicated in the definition, the important actions involved in the operation of Air conditioning systems are:
Temperature control - Room temperature is controlled to the pre designed dry bulb temperature by cooling or heating room air. Humidity control – Room air is controlled to the pre designed relative humidity by humidifying or dehumidifying the room air. Air filtering, cleaning and purification – Room air is cleaned by removing dust and dirt from the air. Air movement and circulation – Air which is controlled in temperature and humidity and cleaned is distributed evenly throughout in a room. As a result, room air can be maintained evenly in temperature humidity conditions.
Temperature, humidity, cleanliness and distribution of air are called "four elements of air conditioning". By controlling these four elements, room air can be comfortably maintained regardless of outdoor temperature. Should these four elements can be replaced with the work of air conditioner, the room air is drawn in the air conditioners, where dust and dirt are removed from the air by the air filter (cleanness of air) and it is sent to evaporator, where temperature of the air is reduced by evaporation of the refrigerant (temperature), and at the same time, humidity in the air is removed as condensation (humidity). As a result, the air distributed from the air conditioner is cool and crisp and can be distributed throughout the room by the evaporator fan (distribution of air system). Such works are repeated so as to perform air conditioning.
1.1 COMFORTABLE AIR: The heat and coldness that the men feel depend not only on air temperature (dry bulb temperature), but also on humidity and distribution of air. In addition the general comfortable zone air conditions are within the comfortable zone, the room air is not always optimum. For example, if temperature differs between indoor and outdoor is nearly 10oC because room air controlled so as to be within this is my computer zone, one feels coolness and heat strongly when he enters in and out of a room, which makes him feel uncomfortable. Such uncomfortable ness is called "cold shock" consequently, it is important to control room air temperature so as not to feel "clock shock" during cooling by adjusting the thermostat. The optimum temperature difference between indoor and outdoor is from 3 to 6oC in consideration with health and economy.
1.2. NEED OF ACCURATE HEAT LOAD ESTIMATION: The primary function of air-conditioning is to maintain conditions that are: Conductive to human comfort. Required by a product, or process within space. To perform this function, equipment of the proper capacity must be installed and controlled throughout the year. The equipment capacity is determined by the actual instantaneous speak load requirement; type of control is determined by the conditions to be maintained during peak and partial load. Generally, it is impossible to measure either the actual peak or the partial in any given space; these loads must be estimated.
CHAPTER-2 (Cooling and Heating Load Considerations)
2: PRELIMINARY CONSIDERATIONS: The importance of accurate load calculations for air- conditioning design and selection of equipment can never be overemphasized. In fact, it is on the precision and care exercised by the designer in the calculations of the cooling load for summer and the heating load for winter that a trouble-free successful operation of an air-conditioning plant after installation would depend. An important consideration in this exercise is the date and time for which these calculations are made. The date would depend on the local climatic conditions. Although the longest day in summer is June 21, hottest and most humid day may occur in July. Similarly, the coldest day may occur in January or even February instead of on December 21.
Again, though the maximum temperature may occur outside at 1or 2 p.m. the maximum heat gain of the room may occur at 3 or 4 p.m. due to the direct solar radiation through glass on the west side, or even later due to the time lag for the heat transfer through the structure. Further, the application for which the building is intended to be used would also govern the choice of time. For example, for an office building in winter that is not used at night, the time for load calculations may be taken during the early hours of the morning, although the maximum heating load may occur at night. Similarly, an office building in summer may have the maximum cooling load at 7 p.m. due to the time lag, but since no occupants would be present at that time, the time for load calculations may be taken as 4 or 5 p.m.
The major components of load in buildings are due to the direct solar radiation through the west glass, transmission through the building fabric or structure and fresh air for ventilation. In the case of applications such as theatres and auditoriums, the occupancy load is predominant. A detailed discussion of the solar radiation incident on a surface and its transmission through glass has been given in the literature1. Further, in literature2, we have studied the methods of calculating heat transmission and infiltration through structures. These form the components of load on the building from the external environment. The internal and system heat gains or losses also form the major components of other loads. In this chapter, the methods for the evaluation of the above mentioned and other individual loads are first presented, followed by a summary of all loads at the end along with an example and a calculation sheet illustrating the procedure that is followed by practicing engineers. In the first instance here cooling load estimation is given followed by that of the heating load.
2.1: INTERNAL HEAT GAINS: The sensible and latent heat gains due to occupants, lights. appliances, machines, piping, etc., within the conditional space. form the components of the internal heat gains. 2-2: Occupancy Load The occupants in a conditioned space give out heat at a metabolic rate that more or less depends on their rate of working. The relative proportion of the sensible and latent heats given out, however, depends un the ambient dry bulb temperature. The lower the dry bulb temperature, the greater the heat given out as sensible heat. The values for restaurants include the heat given out by food as well. It will be seen that the sensible heat (S) gain does not vary much with activity, more and more heat being liberated as latent heat (L) thus making up for total heat. The usual problem in calculating the occupancy load lies in the estimation of the exact number of people present.
Table 2.1 :1 Heat liberated due to occupancy Metabolic Heat liberated, W ------------------------------------------------------------------ Activity Rate Room dry Bulb temp. Cº ------------------------------------------------------------------ W 20 22 24 26 _____________________________________________ S L S L S L S L Seated at rest 115 90 25 80 35 75 40 65 50 Office work 140 100 40 90 50 80 40 70 70 Standing 150 105 45 95 55 82 68 72 78 Eating in restaurant 160 110 50 100 60 85 75 75 85 light work in factory 235 130 105 115 120 100 135 80 155 Dancing 265 140 125 125 140 105 160 90 175
2.3: Lighting Load Electric lights generate a sensible heat equal to the amount of the electric power consumed. Most of the energy is liberated as heat and the rest as light which also eventually becomes heat after multiple reflections. Lighting manufacturers give some guidance as to the requirement of power for different fittings to produce varying standards of illumination. In connection with fluorescent tubes, it may be stated that the electric power absorbed at the fitting is about 25 percent more than necessary to produce the required lighting. Thus a 60 W tube will need 75 W at the fitting. The excess of 15 W is liberated at the control gear of the fitting. As a rough calculation one may use the lighting load equal to 33.5 W/m2 to produce a lighting standard of 540 lumens/m2 in an office space. After the wattage is known, the calculation of the heat gain is done as follows: Fluorescent: Q = Total watts × 1.25 Incandescent: Q = Total watts
2.4: Appliances Load Most appliances contribute both sensible and latent heats. The latent heat produced depends on the function the appliances perform, such as drying, cooking, etc. Gas appliances produce additional moisture as a product of combustion. Such loads can be considerably reduced by providing properly designed hoods with a positive exhaust system or suction over the appliances. Electric motors contribution sensible heat to the conditioned space. A part of the power input is directly converted into heat due to the inefficiency of the motor and is dissipated through the frame of the motor. This power is Power (W) = (Input) (I -motor efficiency) The rest of the power input is utilized by the driven mechanism for doing work which may or may not result in a heat gain to the space. These depend on whether the energy input goes to the conditioned space or outside it.
Table 2:2 Appliance load, W Appliance sensible latent total Coffee brewer 0.5 gal 265 65 329 Warmer 0.5 gal 71 27 98 Egg boiler 353 235 60 Food warmer /m² of plate 1150 1150 2300 Griddle frying with frying top of 46 cm *36 cm 912 500 1412 Toaster, 360 slices / h 1500 382 1882
2.5: Piping, Tanks, Evaporation of Water from a Free Surface and Steam Heat is added to the conditioned space from running pipes carrying hot fluids due to heat transfer. On the other hand, cold pipes take away heat from the space. Open tanks containing warm water contribute both sensible heat and latent heat to the space due to evaporation. This can be calculated by knowing the rate of evaporation and energy balance. In industrial air conditioning, products have often to be dried. This involves the sensible heat gain to the space from the hot surfaces of the dryer and the latent heat gain depending upon the drying rate. For these calculations, knowledge of the heat and mass transfer coefficients is essential. When steam is entering the conditioned space, the sensible heat gain is very little. It is equal to only the difference in the enthalpy of steam at the steam temperature and the enthalpy of water vapour at the room dry-bulb temperature. The main load is in the form of the latent heat gain. Thus SHG = (kg/s) (tstcam- ti)(1.88)kW (2.1) LHG = (kg/s) (2500) kW (2.2)
2.6: Product Load In the case of cold storages. the enclosures are insulated with at least 10 - 15 cm of thermocole and are almost completely sealed. Thus, many of the loads present in buildings for comfort air conditioning are either absent or lessened in the case of cold storages. However, in addition to the heat removed from products at the time of initial loading, there is also the heat produced by the commodities during storage. This heat of respiration forms a sizable product load even at a storage temperature of 0'C. At higher temperatures, it is more. The approximate rate of evolution of heat by various products at different temperatures.
2.7: Process load: The procedure of calculating the cooling and heating load for various industrial air-conditioning processes is specific for each process. The requirements for the process may involve the control of one or more of the following factors: Regain of moisture content by hygroscopic materials, such as cotton. silk, tobacco, etc., and the accompanying heat liberated. Drying load. Rate of chemical and biochemical reactions. Rate of crystallization, freezing, freeze-drying, etc. Sensible cooling load. For details of these loads, one may refer to the ASHRAE Handbook
2.8: SYSTEM HEAT GAINS The system heat gain is the heat gain (or loss) of an air-conditioning system comprising its components, viz., ducts, piping, air-conditioning fan, pumps, etc. This heat gain is to be initially estimated and included in the total heat load for the air-conditioning plant. The same should be checked after the whole plant has been designed 2.9: Supply Air Duct Heat Gain and Leakage Loss The supply air normally has a temperature of 10 to 15°C. The duct may pass through an unconditioned space having an ambient temperature of 40°C. This results in a significant heat gain till the air reaches the conditioned space even though the duct may be insulated. The heat gain can be calculated using the following expression Q = UA (ta- ts) (2.3) Where U is the overall heat-transfer coefficient and A is the surface area of the duct system exposed to the ambient temperature ta. As a rough estimate. a value of the order of 5 percent of the room sensible heat may be added to the total sensible heat if the whole supply duct is outside the conditioned space, and proportionately less if some of it is within the conditioned space.
It has been found that duct leakages are of the order of 5 to 30 percent depending on the workmanship. Air leakages from supply ducts result in a serious loss of the cooling capacity unless the leakages take place within the conditioned space. If all ducts are outside the conditioned space, a 10 percent leakage is to be assumed which should be considered as a complete loss. When only a part of the supply duct is outside the conditioned space, then only the leakage loss of this portion is to be included. The fraction of 10 percent to be added in such a case is equal to the ratio of the length outside the conditioned space to the total length of the supply duct.
2.10: Heat Gain from Air-Conditioning Fan: The heat equivalent of an air-conditioning fan horsepower is added as the sensible heat to the system. If the fan motor is outside the air stream, the energy lost due to the inefficiency of the motor is not added to the air. There are two types of air supply systems. 1-Draw-through system In the draw-through system, the fan is drawing air through the cooling coil and supplying it to the conditioned space. This is the most common system. In this system, the fan heat is in addition to the supply air heat gain. The heat should therefore be added to the room sensible heat. 2-Blow-through system In the blow-through system, fan blows air through the cooling coil before being supplied to the conditioned space. In this system, the fan heat is added after the room to the return air. Thus the fan heat is a load on the cooling coil. The heat should therefore be added to the grand total heat. The fan efficiencies are of the order of 70 percent for central air conditioning plant fans and about 50 percent for package air-conditioner fans. The fan horsepower depends on the quantity of air supplied and the pressure rise. The supply air quantity in turn depends on the dehumidified rise, which is of the order
of 8 to 14°C. The fan total pressure depends on the system pressure loss which comprises the pressure drop through the duct-work, grilles, filters, cooling, etc. Once the supply air-rate and pressure developed are known, the fan power can be calculated. But these cannot be known until the load calculations have been completed. Hence the procedure is to initially assume fan heat between 2.5 and 7.5 percent of the room sensible heat and check the value after the design has been completed
2.11: Return Air Duct Heat and Leakage Gain: The calculation of the heat gain for return air ducts is done in exactly the same way as for supply air ducts. But the leakage in this case is that of the hot and humid outside air into the duct because of suction within the duct. If the ducts are outside the conditioned space, an in leakage up to 3 per cent may be assumed depending on the length of the duct. If there is only a short connection between the conditioning equipment and the space, this leakage may be neglected
2.12: Heat Gain from Dehumidifier Pump and Piping: The horsepower required to pump water through the dehumidifier adds heat to the system and is to be considered like that of other electric motors. For this purpose pump efficiencies may be assumed as 50 percent for small pumps and 70 percent for large pumps. The heat gain of dehumidifier piping may be calculated as a percentage of the grand total heat as follows: (i) Very little external piping: 1 % of GTH (ii) Average external piping: 20% of GTH (iii) Extensive external piping: 4% of GTH. Note: Percent Addition to Grand Total Heat: It is to be noted that all heat gains after the room are not to be added to room heat gains but to the grand total heat load that directly falls on the conditioning equipment. These include the return air duct heat
and leakage gain, dehumidifier pump power, dehumidifier and piping losses, as out- lined above and the fan sensible heat in the case of the blow-through system. 2.13: Safety Factor Safety factor is strictly a factor of probable error, in the estimation of the load. For the purpose, additional 5 percent heat should be added to the room sensible and latent heats.
Abha, the city in kingdom of Saudi Arabia is situated at 18.23 N latitude and 42.65 E longitudes in the south west of the kingdom. Its height from sea level is 1,500 meter. Here the weather condition is very good. The average temp. round the year is about 26 ºC.So throughout the year, it is very comfortable. But due to global change in weather condition the weather is becoming sour and hot.For human comfort we need to air- condition our residential and commercial places. For the air- conditioning we have selected the workshop complex of Mechanical Engg. Department of KKU, ABHA.
3. BUILDING SURVEY: An accurate survey of the load components of the space to be air-conditioned is a basic requirement for a realistic estimate of cooling and heating loads. The completeness and accuracy of this is my computer survey is the very foundation of the estimate, and its importance cannot be overemphasized. Mechanical and architectural drawings, complete field sketches and in some cases, photographs of import aspects are part of a good survey. The following physical aspects must be considered.
3.1:Orientation of building – Location of the space to nbr air conditioned with respect to: Compass points – sun and wind effects. Nearby permanent structures – shading effects. Reflective surfaces – water, sand, parking lots etc. 3.2:Use of space (s) – Office, hospital, department store, specialty shop, machine shop, factory, assembly plant etc. 3.3:Physical dimensions of space (S) Length, width and height. 3.4:Ceiling height – Floor to floor height, floor to ceiling, clearance between suspended ceiling and beams. 3.5:Columns and beams – Size depth also knee braces. 3.6:Construction materials – Materials and thickness of walls, roof, ceiling, floors and partitions and their relative position in the structure. Surrounding conditions – Exterior color of walls and roof, shaded by adjacent building or sunlit. Attic spaces – invented or vented. Surrounding space conditioned or unconditioned – temperature of non – conditioned adjacent spaces such as furnace or boiler room and kitchens, floor on ground, crawl spaces, and basement.
3.7:Windows – Size and location wood or metal sash, single or double hung. Type of glass – single or multipane. Type of shading device. Dimensions of reveals and overhangs. 3.8:Doors – Location, type, size and frequency of use. 3.9:Stairways, elevators and escalators – Location, temperature of space if open to unconditioned area. Horse power of machinery, ventilated or not. 3.10:People – Number, duration of occupancy, nature of activity, any special concentration at time, it is required to estimate the number of people on the basis of square feet per person, or on average traffic.
3.11:Lighting – Wattage at peak. Type – incandescent, fluorescent, recessed, exposed. If the lights are recessed, the type of airflow over the lights, exhaust, return or supply, should be anticipated. At time, it is required to estimate the wattage on a basis of watts per sq. due to lack of exact information. 3.12:Motors – Location, nameplate and brake horsepower and usage. The latter is of great significance and should be carefully evaluated. The power input to electric motors is not necessarily equal to the rated horsepower divided by the motor efficiency. Frequently these motors may be operating under a continuous overload, or may be operating at less than rated capacity. It is always advisable to measure the power input whenever possible. This is my computer is especially important in estimates for industrial installations where the motor machine load is normally major portion of the cooling load. 3.13:Appliance, business machines, electronic equipment – Location, rated wattage, steam or gas consumption, exhaust air quantity installed or required and usage. Avoid pyramiding the head gains from various appliances and business machines. For examples, a toaster or a waffle iron may not be used during the evening, or the fry kettle may not be used during moming, or not all business machines in a given space may be used at the same time.
3.14:Ventilation – CFM per person, CFM per Esq. ft. Scheduled ventilation (agreement with purchaser) Excessive smoking, floor orders, code requirement. Exhaust fan-type, size, speed, and CFM delivery. 3.15:Thermal storage – includes system operating schedule (12, 16 or 24 hours per ay. Specifically during peak out door conditions, permissible temperature swing in space during a design day, rugs on floor. 3.16:Continuous or intermittent operation – Whether system be required to operate every business day during cooling season, or only occasionally, such as churches and ballrooms. If intermittent operation, determine duration of time available for precooling or pull down.
4. DESIGN CONDITIONS: Since the need of air conditioning is primarily a function of our body's reaction to the climate, we will begin our study of load estimating by looking at outdoor and indoor design conditions. Establishing these conditions for a specific application, locality, and time will fix the magnitude of head gain or loss essentially establishes the potential for head to flow and can be equated to establish the voltage for an electrical circuit.
4.1:OUTDOOR DESIGN CONDITIONS: There are several sources of data that can be secured to establish outdoor design. Three common ones are. ASHRAE Handbook of Fundamentals. Engineering Weather Data. Carrier System Design Manual. Each source contains data based on average weather conditions available at the time of publication. It is commonly acknowledged that ASHRAE has come to be regarded as the industry standard when it comes to outside design data for abha. ASHRAE data is based on detailed records from official weather stations of the abha. Weather Bureau, abha. To illustrate the outdoor design data is taken from carrier system design manual. New Delhi will be used to illustrate the values published.
4.2:USE OF OUTDOORS DESIGN CONDITIONS Summer design condition in ABHA(KINDOM OF SAUDI ARABIA) Summer DB34oC Summer WB21oC Daily Range12oC 4.3:INSIDE DESIGN CONDITIONS: The human body considers itself comfortable it can maintain an average body temperature between 360C and 37.70C. To accomplish this is my computer body exchanges heat with its environment by evaporating body fluids and
exchanging heat thru stable body temperature, and the mind perceives itself as comfortable when body temperature can easily maintained. It becomes the task of air-conditioning to maintain the environment around the body within this is my computer comfort zone of conditions. The following variables, all of which affect the ability of body to exchange heat with surrounding and perceive itself comfortable. Surrounding Dry Bulb Temperature Surrounding Relative Humidity Surrounding Mean Radiant Temperature Surrounding Air Velocity
4.4: DESIGN CONDITIONS At this point we have sufficient information to complete the design portion of the calculation sheet form for our workshop complex building in the department of Mechanical Engineering,KKU,Abha. Assuming we are going to do a block load for our selected building for May at 4 P.M. the outdoor design conditions from table 1 has been determined to 340C DB and 21oC WB. Plotting this point on the psychometric chart results in finding a corresponding 74 GR/LB moisture content of the air. Considering the ASHRAE comfort zone, let's pick inside design conditions of 22 oC DB and 50% RH (relative humidity). The difference between outdoor air DB of 34oC and room (indoor) DB of 22oC IS 12oC. This indicates for each CFM of outdoor air entering the building for ventilation purposes, the OA must be cooled 12oC.
4.5:LOAD COMPONENTS: The load components are one of the two general types: Sensible Latent 4.5.1:A Sensible Load result when heat entering the conditioned space causes a dry bulb temperature increase. 4.5.2: A Latent Load result when moisture entering the space causes the humidity to increase. A load component may be all sensible, all latent or a combination of two. Additionally load components can be classified into one of the following three categories. SKIN LOAD INTERNAL LOADS OTHER LOADS Skin loads originate from heat sources outside or external to the conditioned space. Internal loads have their sources within the space itself. Other Loads occur from head gains or losses associated with moving cool fluids to and from the conditioned Space design conditions.
4.5.3:SKIN LOADS: 4.6:SOLAR GAIN THRU GLASS: The sun rays pass through the glass windows as radiant energy and are absorbed within the space. Solar head gain typically reduced by the space. Solar heat fain is typically reduced by the use of internal or external-shadin g devices, reveals, overhangs or shadows cost by adjacent buildings. 4.6.1:SOLAR AND TRANSMISSION GAIN THRU WALLS AND ROOF: Heat is caused to flow through external wall and roofs by two sources: Sun rays striking the external surfaces. The high outdoor air temperatures.
4.6.2:TRANSMISSION THRU GLASS, PARTITIONS, CEILING AND FLOORS: When an adjacent area is at a temperature higher than the space to be air conditioned, heat will flow through windows, ceilings, partitions, or floors by means of transmission. 4.6.3:INFILTERATION: Wind blowing against the side of building causes the outdoor air, which is higher in temperature and moisture content, to infiltrate thru the cracks around doors and windows. This results in localized sensible and latent heat gains. 4.6.4;VENTILATION: Should ventilation air for odor removal be introduced directly into the space, it will appear as load in the space. It could be considered as forced infiltration and would result in localized sensible and latent heat gain.
4.7:INTERNAL LOADS: 4.7.1:PEOPLE: The human body through metabolism generates heat within itself and releases it by radiation, convection, evaporation from the surface, and by convection and evaporation in the respiratory tract. The amount of eat generated and released depends on surrounding temperatures and the activity level of the person. Both sensible and latent heat loads will enter the space. 4.7.2:LIGHTS: Illuminates convert electrical power into light and sensible heat. Lighting is either fluorescent or incandescent.
4.7.2:LIGHTS: Illuminates convert electrical power into light and sensible heat. Lighting is either fluorescent or incandescent. 4.7.3:EQUIPMENT: Within the conditioned spaced powered equipment can produce localized sensible and / or latent loads. Such devices would include calculators, computers, motors, popes, tanks, or product from a process. 4.7.4:ROOM LOADS: The room includes the entire space inside the building. Adding all the sensible loads together results in the room sensible heat gain (RSH). Similarly, the sum of all the latent heat gain (RLH). Finally the sum of RSH and RLH is the room total (RTH).
4.8:OTHER LOADS – SUPPLY AIR SIDE: If there was no heat gained or lost between the coil of the air handling unit and supply air terminal, the temperature of the air leaving the coil would be the same as that of the air entering the room. In a real system the following losses exists. 4.8.1:SUPPLY DUCT HEAT GAIN: Should be supply air duct pass thru a space whose temperature is higher than that of the air being transmitted, a sensible heat gain will be experienced. 4.8.2:SUPPLY DUCT LEACKAGE LOSS: The supply air is transmitted under to the room. Depending on the quantity of ductwork installation, leaks at the joints will exits to some degree resulting in a loss of sensible as well as latent capacity.
4.8.3:SUPPLY AIR HEAT: In air handling units whose fan is located downstream of the coil, the does the work on the air resulting in a sensible heat gain to supply air. In addition the motor losses could show up on the supply air of the motor is located on the air stream. 4.9:BYPASSED OUTDOOR AIR: Because the coil is not a perfect device, a position of the entering air passes through the coil completely unaltered in temperature or humidity, resulting in a sensible and latent loss of supply air. 4.10: EFFECTIVE LOADS: Adding the supply air sensible losses to the RSH results in air load laving the coil referred to as effective room sensible heat (ERSH). In similar manner latent losses of the supply air plus RLH results in effective room latent heat by the coil. These loads are referred to as effective since both the coil leaving air temperature and humidity level must effectively be lower than conditions required at the room on order to: Absorb the losses along the way and Absorb the room loads
4.11: EQUIVALENT TEMPERATURE DIFFERENCE: Heat flow though an exterior wall is due to the combined effect of two heat sources: Sun's rays striking the wall resulting in solar insert gain. Outside air temperature higher than the inside temperature resulting in transmission thru the wall. Since the wall has mass, the storage, affect of wall makes the flow of heat through it time related. Determination of actual amount of eats entering the space is therefore a rather complex calculation. To get a look for the movement of heat thru wall under these circumstances a look at time related temperature profiles across a wall is beneficial. Assume air temp. on both sides of a walls maintained at 23.88oC. With no sun shining on the wall temp thru wall is constant. As sun shines on wall, radiant energy is converted to heat at surface to outdoor air and into wall. With time the surface temp. rises as well as temp in the wall, as heat flows into wall and then on to the interior.
With continued sun shining on the wall, a steady state heat flow situation will occur where amount of energy striking the surface equals the amount of heat given off to outdoor air plus the amount of heat entering the interior. When sun cease to shine on the wall, stored energy in wall continues to flow to the outside and inside until temp. Throughout the wall equalizes. Whether the sun is shining on the wall heat is always flowing in two directions. Transmission heat flow thru wall behaves in a manner similar to solar flow of heat. As the outdoor temperature rises-causing heat to flow into wall. If outdoors temp. remained at 34oC for a long time a steady state heat transfer condition would exist. Under this condition heat entering the indoor space would define by equation: Q = U*A* (Temperature Difference)
Should the outdoor air temp. fall quickly, energy stored in wall quickly flow outward in both directions. Under these conditions the flow of heat is no longer steady state and above equation no longer applies. Since the solar intensity striking the outside air temperature is continually changing, simple heat transmission equation cannot be used. The equivalent temperature difference is a factious number used to describe the flow of heat thru the wall at a given point in time.
4.12: INFILTRATION, VENTILATION AND EXHAUST: Infiltration is leakage of untreated outdoor air through porous exterior walls, floors, roofs etc. the amount of leakage is not controllable by the occupants in the building, and can results in rather large heat gains or losses. The rate of which the leakage takes place is dependent upon the pressure differential across the exterior surface. The pressure difference is in turn caused by wind velocity, difference in air density, or pressurization caused by mechanical supply and exhaust systems. Due to the tremendous variability in building geometry, wind patterns, and construction quality, accurate evaluation becomes the task of the designer to use his educated judgment based on the information available to provide for this potential load source. The following text provides a partial in sight into this phenomenon, however, further research by the reader is necessary to properly evaluate infiltration-particularly in tall building (i.e. those over 30 m high).
4.13:VENTILATION: The introduction of outdoor air for ventilation of conditioned spaces is necessary to dilute odors given off by people, smoking, or other internal air contaminants. Local codes usually determine the minimum amount of ventilation required, and may be specified either as CFM / person or CFM/Ft2 of net floor area. It is customary to minimize the amount of outdoor air introduced into the space since this can result in a substantial heating or cooling load. With high-energy costs this can translate into significant operating costs. The people density of the building is 50 sq. ft. / person/ as we know that 15 CFM / Person is a good minimum ventilation rate. Thus the building requires.
4.14:EXHAUST: Codes require that some odour producing areas in a building must be positively exhausted. Such area would be toilets or kitchen hoods over grilles. This does not mean that extra outdoor air must be supplied for the purpose of exhausting. Room air in most cases is perfectly satisfaction for exhaust requirements. 4.15:AIR CHANGE: As the example building is a shopping complex application, therefore 1 air change is given in order to maintain the proper air composition in the conditioned space. 4.16: INTERNAL HEAT GAIN: Within the conditioned space people, lights, generate sensible loads, powered equipment, and appliances. Internal latent load sources are recorded further down the load calculation sheet.
4.16.1:PEOPLE: Heat is generated within the human body by oxidation – commonly called metabolic rate. This heat is carried to the surface of the body and dissipated by: Radiation from the body to the surrounding colder surfaces. Convection from the body and respiratory tract to the surrounding air. Evaporation of moisture from the body surface and in the surrounding air. from the carrier system design manual shows average dissipation of Sensible and latent heat from people at different level of activity.
4.16.2:LIGHTS: Lights generate sensible heat by the conversion of electrical power into light and heat. The heat is then dissipated by radiation to the surrounding surfaces, by Conduction into adjacent materials, and by convection to the surrounding air. Lights are typically specified as lamp watts/ Ft2 of floor area. Incandescent lights convert approximately 10% of the power input into light. The rest appears as heat within the bulb. The heat then makes its way into the space 80% by radiation and 10% by conduction and convection florescent lights are more commonly used in commercial buildings. About 20% of the input power(E) is converted to light by florescent bulbs – thus they are more efficient than incandescent lights. 20% of the power input (E) is dissipated by convection and conduction to the space. Additionally, 20% of the input power is generated as heat in the ballast of the lamps. The above values very from manufacturer to manufacturer and more efficient lights significantly reduce the ballast loss. Generally, however, the following equations are used to give a good approximation of the heat gain to the space.
TYPE:HEAT GAIN TO THE SPACE: FLORESCENT:RATED LAMP WATTS x 1.25 X 3.413 BTUH / WATT INCANDESCENT:RATED LAMP WATTS X 3.143 BTUH / WATT 1.25 ACCOUNTS FOR BALLAST HEAT GAIN
4.16.3:STORED EVERGY FROM LIGHTS: The radiant energy from lights has the potential to be stored in the mass of the building and appear as a load later in the space. This is the same process that occurs with solar radiant energy. It is normal practice to neglect the storage impact when calculating the space load due to lights. Most comfort applications result in less than 2% reduction in the load, and therefore, storage is neglected. The basic reason behind this practice is as follows: Carpeting is widely used. The primary target for radiant energy from the lights is the floor. Carpet insulates the floor from the radiant light rays. The radiant energy strikes the carpet and is converted into heat. The heat is then dissipated almost immediately into the space. Only a small portion of the energy is stored in the floor.
Unless the air conditioning equipment is run for an hour or two before the light are turned on, the mass of the building will already be saturated with stored heat from the night before, therefore, as light heat is observed in the mass, the equivalent amount of energy is almost simultaneously released to the space: In order to see a significant reduction in heat of lights due to storage the following Condition should be present. The air conditioning equipment should be run longer then 12 hours per day to remove solar energy from the mass of the building, and also reduce or eliminate morning pull down loads. The lights should be turned on 1 (preferably 2) hours after the air conditioning equipment in the morning. Determining the storage impact can be done by the use of tables 12 in the carrier system design manual.
4.17:BYPASSED OUTDOOR AIR: Since the cooling coil is not a perfect heat exchanger device, some of the air entering the coil passes through the coil untreated. This represents a loss air leaving the coil must overcome before entering the room. It is a load equivalent to an infiltration load in the room, and is calculated in that manner. The percentage of air (expressed as a decimal) that passes through the coil untreated is referred to as the bypass factor and is typically figured at 0.5. it is really a function of the type of the coil and equipment used. A more accurate value can be assigned, as the designer becomes more familiar with cooling coil used in air conditioning equipment.
4.18:EFFECTIVE ROOM SENSIBLE HEAT (ERSH): Adding the supply duct sensible losses to the room sensible heat (RSH) results in a load known as the effective room sensible heat (ERSH). This load is used in determining the CFM of air required across the cooling coil the air must be effectively higher than the normal quantity to absorb the room load and the losses encountered along the way from the cooling coil to the room.
4.19: LATENT LOADS: The latent counterparts of infiltration, internal loads are now calculated order to determine the room latent heat (RLH) and effective room latent heat (ERLH). Vapor transmission is one new load source encountered. 4.20:VAPOUR TRANSMISSION: Water vapor flows thru building structures, resulting in a latent load whenever a vapor pressure difference exists across the structure. The latent load from this source is usually insignificant in comfort application and need be considered only in low or high dew point applications.
Water vapor flows from high to lower vapor pressure at rate determined by the permeability of the structure. Further details on this subject can be found in the carrier system design manual. For the air conditioning the available plan of the workshop complex building of Mechanical Engineering,KKU,ABHA,KINDOM OF SAUDI ARABIA has been selected for cooling load calculation.The plan of the complex as shown in fig.1.The other details for the building are taken as below. Building located at 18.2o N Latitude, the following data are given Plaster on inside wall =1.25 cm Out side wall construction=20 cm concrete block =10 cm brick veneer Partion wall construction =33 cm brick Roof construction =20 cm RCC slab with 4 cm asbestos cement board Floor construction =20 cm concrete Densties, brick =2000kg/ m Concrete =1900kg/m Plaster =1885kg/m Asbestos board =520kg/m
Fenestration (weather-stripped, =2mx1.5m glasses loose fit l U=5,9wm k doors 1.5mx2 m wood panels U=.63w m k out door design condition =34C DBT,22C WBT Indoor design condtion 25 DBT, 50% RH Daily rang 220C TO 34 ºC =12 ºC Occupancy =200 Light 15000 w fluorescent 4,000 w tungsten Assume bypass factor of cooling coil =0.15 Find room sensible and latent heat load and also the grand total heat load.