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By joining Download. We’re fans of lightweight PDF readers, which tend to load much faster than the big-name tools, even the free ones. It’s free and can serve as your default PDF software. With a ribbon-style toolbar and contrasting highlights, eXpert PDF Reader’s interface has a familiar look and feel, but with some unique touches, such as a Start button in the upper left corner of the interface.

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Google News google-news. FTC: We use income earning auto affiliate links. Check out 9to5Mac on YouTube for more Apple news:. Apple’s tablet debuted in Since the original version, it’s expanded into multiple screen sizes and Pro and non-Pro options. Introduced in by Steve Jobs, iPhone is Apple’s flagship iOS device and easily its most popular product around the world.

You must be logged in to post a comment. This is one great app. This is the first time I see this interesting approach in an iOS App. You may have both apps installed, but only use the Documents app for everything. I have the iPad version, but so far am not able to download the iPhone version through updates. If I click on the link in the article, it directs me to purchase the app.

Has anyone else noticed this? Yes, I notice this as well. I have 4. Which previous version do you need? It suddenly becomes unavailable. May 29, Be sure to check out our homepage for all the latest news, and follow 9to5Mac on Twitter , Facebook , and LinkedIn to stay in the loop. For ordinary buildings, it turns out that the economics and comfort meet at the That is, the heating system will provide thermal comfort For example, the For the estimation of heating load in the present research the system is designed and developed based on the 99 percent level.

The indoor design temperature should be kept relatively low so that the heating equipment will not be oversized. A design temperature of 70 F or 22 C is commonly used with relative humidity less than or equal to 30 percent. Although a relative humidity of 30 percent with a temperature of 70 F or 22 C is in the lower part of the comfort zone, maintaining a higher humidity must be given careful consideration because severe condensation may occur on windows and other surfaces depending on window and wall insulation and construction [29].

The heating or cooling loads of a building represent the heat that must be supplied to or removed from the interior of a building to maintain it at the desired conditions. A distinction should be made between the design load and the actual load of heating or cooling systems [27].

The design or peak heating load is usually determined with a steady-state analysis using the design conditions for the indoors and the outdoors for the purpose of sizing the heating system.

But the energy use of a building during a heating or cooling season is determined on the basis of the actual heating or cooling load, which varies throughout the day. The internal heat load i. Wind increases heat transfer to or from the walls, roof, and windows of a building by increasing the convection heat transfer coefficient and also increasing infiltration.

Therefore, wind speed is another consideration when determining the heating and cooling loads. The recommended values of wind speed to be considered are 15 mph 6. Solar radiation plays a major role on the heating and cooling of buildings, and it is an important consideration in the evaluation of the design heating and cooling loads. It turns out that peak heating loads usually occur early in the mornings just before the sunrise.

Therefore, solar radiation does not affect the peak or design heating load and thus the size of the heating system [27]. However, it has a major effect on the actual heating load, and solar radiation can reduce the annual heating energy consumption of a building considerably.

Heat losses are mainly: a Transmission Losses or heat transferred through the confining walls, glass, Ceiling, floor, or other surfaces. The Expert system for the estimation of heating load is developed by creating various Knowledge base files which consider the components of heat losses in winter.

Walls and roofs of buildings consist of various layers of materials, and the structure and operating conditions of the walls and the roofs may differ significantly from one building to another. Therefore, it is not practical to list the R-values or U-factors of different kinds of walls or roofs under different conditions.

Instead, the overall R-value is determined from the thermal resistance network. The overall thermal resistance of a structure can be determined most accurately in a lab by actually assembling the unit and testing it as a whole, but this approach is usually very time consuming and expensive.

The analytical approach is fast and straightforward, and the results are usually in good agreement with the experimental values. The thermal resistances for different common roofing materials are listed in Table 4. The major difference between the two kinds of roofs is that pitched roofs do not have any built-up roofing. For roofs, along with the roof material the R-value of decking materials and suspended ceilings should also be considered.

The R-value of standard decking material used is 1. For suspended ceilings, an R-value of 0. The standard insulating materials considered over the roofs along with the R-values are given in Table 4. Table 4. Insulation 1 in.

For accomplishing this, the R-value of the roof can be input to the system for calculations. The estimation of heat losses through walls is similar to that of heat looses through roofs except that the wall materials used are different.

In addition to these the insulating materials used are similar to those used on roofs. The common wall materials used in building construction are listed along the R-values in Table 4. For heat loss calculations the inside and outside still air should also be considered.

Sometimes, the temperatures in adjacent spaces can be different, therefore, temperatures adjacent to each of the walls is considered. Finally, the sum of heat losses or gains through each of the walls gives the heat loss through the four walls. The R-value of a wall or roof structure that involves layers of uniform thickness is determined easily by simply adding up the unit thermal resistances of the layers that are in series.

The overall R-value in these cases is determined by assuming 1 parallel heat flow paths through areas of different construction or 2 isothermal planes normal to the direction of heat transfer [29]. Building components often involve trapped air spaces between various layers. Thermal resistances of such air spaces depend on the thickness of the layer, the temperature difference across the layer, the mean air temperature, the emissivity of each surface, the orientation of the air layer, and the direction of heat transfer.

An approximate R-value of 0. For the estimation of heat load these values have been assumed. This is conduction heat transfer because of the direct contact between the walls and the floor, and it depends on the temperature difference between the basement and the ground, the construction of walls and the floor, and the thermal conductivity of the surrounding earth.

There is considerable uncertainty in the ground heat loss calculations, and they probably constitute the least accurate part of heat load estimates of a building because of the large thermal mass of the ground and the large variation of the thermal conductivity of the soil, depending on the composition and moisture content [27].

These walls are based on a soil thermal conductivity of 0. It can be seen that the heat transfer coefficient values decrease with increasing depth since the heat at a lower section must pass through a longer path to reach the ground surface.

The interior air temperature of the basement can vary considerably, depending on whether it is being heated or not. Heat loss through the basement floor is much smaller since the heat flow path to the ground surface is much longer in this case.

Where Ufloor is the overall heat transfer coefficient at the basement floor whose values are listed in table 4. Heat losses from the water heater and the space heater located in the basement usually keep the air near the basement ceiling sufficiently warm. Heat losses from the rooms above to the basement can be neglected in such cases. This will not be the case, if the basement has windows [27].

Many residential and commercial buildings do not have a basement, and the floor sits directly on the ground at or slightly above the ground level. Where Ugrade represents the rate of heat transfer from the slab per unit temperature difference between the indoor temperature Tindoor and the outdoor temperature Toutdoor and per unit length of the perimeter Pfloor of the building.

Typical values of Ugrade are listed in table 4. It can be observed from the table that perimeter insulation of slab-on-grade reduces heat losses considerably, and thus it saves energy while enhancing comfort. This is also the case when basement board heaters are used on the floor near the exterior walls. Heat transfer through the floors and the basement is usually ignored in cooling load calculations.

In a building envelope, windows offer the least resistance to heat flow. In a typical house, about one-third of the total heat loss in winter occurs through the windows [27]. Also, most air infiltration occurs at the edges of the windows.

The solar heat gain through the windows is responsible for much of the cooling load in summer. The net effect of a window on the heat balance of a building depends on the characteristics and orientation of the window as well as the solar and weather data. Workmanship is very important in the construction and installation of windows to provide effective sealing around the edges while allowing them to be opened and closed easily.

Despite being so undesirable from an energy conservation point of view, windows are an essential part of any building envelope since they enhance the appearance of the building, allow daylight and solar heat to come in, and allow people to view and observe without leaving their home.

For low-rise buildings, windows also provide easy exit areas during emergencies such as fire. Important considerations in the selection of windows are thermal comfort and energy conservation. A window should have a good light transmittance while providing effective resistance to heat flow [27]. The lighting requirements of a building can be minimized by maximizing the use of natural daylight. Heat gain and thus cooling load in summer can be minimized by using effective internal or external shading on the windows.

Even in the absence of solar radiation and air infiltration, heat transfer through the windows is more complicated than it appears to be. This is because the structure and properties of the frame are quite different than the glazing. The corresponding U-factors are given in Table 4. These are the factors which contribute to the thermal resistance by the glass area.

The frame area needs to be considered separately. The different types of frames available are generally made up of aluminum, and wood or vinyl. The frame material U- factors for fixed vertical windows for the type of windows discussed above are given in Table 4.

The result is a double pane window, which has become the norm in window construction [27]. This uncontrolled entry of outside air into a building through unintentional openings is called infiltration, and it wastes a significant amount of energy since the air entering must be heated and cooled in summer.

The warm air leaving the house represents energy loss. This is also the case for cool air leaving in summer since some electricity is used to cool that air. That is, about one-third of the heating bill of such a house is due to the air leaks [27]. The rate of infiltration depends on the wind velocity and the temperature difference between the inside and the outside, and thus it varies throughout the year.

The infiltration rates are much higher in winter than they are in summer because of the higher winds and larger temperature differences in winter. Therefore, distinction should be made between the design infiltration rate at design conditions, which is used to size heating or cooling equipment, and the seasonal average infiltration rate, which is used to properly estimate the seasonal energy consumption for heating or cooling.

Infiltration appears to be providing ” fresh outdoor air” to a building, but it is not a reliable ventilation mechanism since it depends on the weather conditions and the size and location of the cracks [27]. The air infiltration rate of a building can be determined by direct measurements by 1 injecting a tracer gas into a building and observing the decline of its concentration with time or 2 pressurizing the building to 10 to 75 Pa gage pressure by a large fan mounted on a door or window, and measuring the air flow required to maintain a specified indoor-outdoor pressure difference.

The larger the airflow to maintain a pressure difference, the more the building may leak [27]. Despite their accuracy, direct measurement techniques are inconvenient, expensive, and time consuming. A practical alternative is to predict the air infiltration rate on the basis of extensive data available on existing buildings. This is known as Crack method.

Item Infiltration Windows 0. This is called air change method, and the infiltration rate in this case is expressed in terms of air changes per hour ACH. A minimum of 0. Usually the infiltration rates of houses are above 0. It may be necessary to install a central ventilating system in addition to the bathroom and kitchen fans to bring the air quality to desired levels [27].

Venting the cold outside air directly into the house will obviously increase the heating load in winter. Such heat exchangers are commonly used in superinsulated houses, but the benefits of such heat exchangers must be weighed against the cost and complexity of their installation. The primary cause of excessive infiltration is poor workmanship, but it may also be the settling and aging of the building.

Infiltration is likely to develop where two surfaces meet such as the wall-foundation joint. Large differences between indoor and outdoor humidity and temperatures may aggravate the problem. Wind exerts a dynamic pressure on the building, which forces the outside air through the cracks inside the building. Infiltration should not be confused with ventilation, which is the intentional and controlled mechanism of airflow into or out of a building.

Ventilation can be natural or forced or mechanical , depending on how it is achieved. Ventilation accomplished by the opening of windows or doors is natural ventilation, whereas ventilation accomplished by an air mover such as a fan is forced ventilation. Forced ventilation gives the designer the greatest control over the magnitude and distribution of airflow throughout a building. The airtightness or air exchange rate of a building at any given time usually includes the effects of natural and forced ventilation as well as infiltration.

Air exchange, or the supply of fresh air, has a significant role on health, air quality, thermal comfort, and energy consumption. Therefore, the rate of fresh air supply should be just enough to maintain the indoor air quality at an acceptable level. The infiltration rate of older buildings is several times the required minimum flow rate of fresh air, and thus there is a high- energy penalty associated with it.

Infiltration increases the energy consumption of a building in two ways: First, the incoming outdoor air must be added or cooled in summer to the indoor air temperature. Second, the moisture content of outdoor air, in general, is different than that of the indoor air, and thus the incoming air may need to be humidified or dehumidified. The latent heat load is particularly significant in summer months in hot and humid regions such as Florida and coastal Texas.

In winter, the humidity ratio of outdoor is usually much lower than that of indoor air, and the latent infiltration load in this case represents the energy needed to vaporize the required amount of water to raise the humidity of indoor air to the desired level.

For the system developed, the rate of air infiltration is taken as cfh when the doors open and close infrequently. For this amount of air the sensible and latent components of infiltration load can be estimated similarly as given by equations 4. Database files door. As shown in fig 4.

The Expert system asks the User about the location where the Space heat load is to be estimated so that the winter design temperature at that location can be used as the Outdoor temperature. Based on the input variables, the expert system retrieves the data stored in the database files and estimates each of the Space heat load components.

Finally, the indoor and outdoor design along with the heat load components in the form of output is available for the User. The Output can be selected in three different types for analysis and design of Heating System. An estimation of heating load in Atlanta is taken into consideration where a building is having a wall construction by 12 in.

Concrete block, an area of four walls as Sq. Also, an insulation of 1. For On-grade basement heat calculations, a poured concrete wall with severe conditions having a perimeter of Sq.

Ft is used. A triple pane window area of Sq. Ft is used for estimation of heat losses through fenestration. For the Problem stated for estimation of space load, the expert system is consulted.

Once the User, usually a plant personnel having knowledge about Building construction and equipment installation is ready for consultation with the Expert system, the User can execute loadex. The following screen appears on the VP-Expert. After running the program, the expert system asks for the indoor design temperature that is required for the Space. A temperature of 72 deg.

For the estimation of outdoor design temperature, based on the weather data for 40 different locations throughout United States Figure 4. For the present problem a heating system is to be Fig. Designed in the vicinity of Atlanta, Georgia. When the User enters the number corresponding Atlanta, Georgia which is 10, the average winter outdoor temperature is displayed.

The expert system then proceeds with the details of Wall Construction to estimate the heat losses through walls. Figure 4. The expert system developed can also be used when adjacent room temperatures are different. If there is any equipment that generates heat or uses heat, the User has the option to enter the temperature adjacent to that wall.

This indicates that there is heat gain through Wall 4 giving a net heat loss is — In other words there is heat gain instead of heat loss when one of the adjacent spaces is at a high temperature. The roofing parameters are requested by the expert system for the estimation of heat losses through the roof as shown in figure 4. After estimating the heat losses through the walls and the roof, the expert system then proceeds for the estimation through on-grade floors or below grade basements. For this, the user is to select the type of basement for the space as shown in fig.

The Expert system requests the user information about the On-grade floors. A poured concrete wall in a location having severe weather Fig. Conditions and no on grade floor insulation of perimeter ft.

The remaining consultation deals with window conduction heat losses as solar heat gain is not considered in the determination of design heat load which usually occurs in the earlier part of the day.

The User has to furnish details about the window frame and window glass types and areas for the estimation of heat losses. The window heat losses are as shown in Figure 4. The Infiltration load is calculated using the crack length method. For accommodating negative pressures that can occur due to infrequent opening and closing of doors, the user can input whether there is infrequent operation of doors.

The amount of air infiltrating due to infrequent opening and closing of doors is assumed to be cu. Ft per hour [26]. Also, in the calculation of latent heat load for infiltration, which is the heat, required to humidify the outdoor air, the relative humidity of indoor and outdoor air is taken as 0.

The Total heat load estimated by the Expert System is The results are obtained by using the same building parameters and dimensions at all the locations with an indoor design temperature of 72 deg. F as exemplified during the expert system consultation as discussed in the previous section. These results are given in Table 4. F IndianaPolis -2 Boston -6 Dallas 18 Memphis 13 Syracuse -3 San Francisco 35 As can be seen from the histogram, the maximum heat load occurs in that location where there is a very low average outdoor temperature in winter.

In Boston, the average winter outdoor temperature is —6 deg. F, hence a larger amount of heating capacity is required to heat a building in Boston than the heat required to heat the same type of building in San Francisco.

Also, from figure 4. To deg. F Figure 4. Fenestration and Infiltration heat loads form the next major part and heat transmission through basement walls and floors appears to be less important. Also, with the use of an expert system, we are able to consider the possible heat losses through adjacent spaces if the spaces are at different temperatures. This is one of the features of Expert systems that can be very useful sometimes. Another point that can be noted is the Expert System accomodation of negative pressure in the Infiltration losses.

The cooling load details are discussed first and then the expert system consultation and results are exemplified. Loads are the heat that must be supplied or removed by the HVAC equipment to maintain a space at the desired conditions.

The calculations are like accounting [28]. One considers all the heat that is generated in the space or flows across the envelope; the total energy, including the thermal energy stored in the space, must be conserved according to the first law of thermodynamics.

The principal terms are indicated in fig. Outdoor air, occupants, and possibly certain kinds of equipment contribute both sensible and latent heat terms.

Load heat Conduction roof, walls, glazing supplied Conduction ground or Space removed Air exchange, sens. Heat gain is the rate at which energy is transferred to or generated within a space. It has two components, sensible heat and latent heat, which must be computed and tabulated separately. The Cooling load is the rate at which energy must be removed from a space to maintain the temperature and humidity at the design values.

While considering heavy constructed buildings, the cooling load will generally differ from the heat gain because the radiation from the inside surface of walls and interior objects as well as the solar radiation coming directly into the space through openings does not heat the air within the space directly. Only when the room air receives the energy by convection does this energy become part of the cooling load [29].

For most purposes and for reasonable estimation, the sum of all the heat gains can be considered as the Cooling load. The heat extraction rate is the rate at which the energy is removed from the space by the cooling and dehumidifying equipment. Again it is not reasonable to design for the worst conditions on record because a great excess of capacity will result. The daily range of temperature given in table 4. The daily range is usually larger for the higher elevations, where temperatures may be quite low late at night and during the early morning hours.

The daily range has an effect on the energy stored by the structure. The designer should be alert for unusual circumstances that may lead to uncomfortable conditions. Certain activities may require occupants to engage in active work or require heavy protective clothing, both of which would require lower design temperatures [29]. The primary sources are computers, printers, and copiers. Typical rates of heat dissipation by people are given in Table 5. Note that the latent heat constitutes about one-third of the total heat dissipated during resting, but rises to almost two-thirds of the level during heavy physical work.

Also, about 30 percent of the sensible heat is lost by convection and the remaining 70 percent by radiation [27]. The radiative sensible heat, on the other hand, is first absorbed by the surrounding surfaces and then released gradually with some delay. Heat given off by people usually constitutes a significant fraction of the sensible and latent heat gain of a building, and may dominate the cooling load in high occupancy buidlings.

The rate of heat gain from people in table 5. The design-cooling load of a building is determined assuming full occupancy.

In the absence of better data, the number of occupants can be estimated on the basis of one occupant per 1 m2 in auditoriums, 2. The rate of heat gain at any given moment can be quite different from the heat equivalent of power supplied instantaneously to those lights [26]. Only part of the energy from lights is in the form of convective heat, which is picked up instantaneously by the air-conditioning apparatus [26]. The remaining portion is in the form of radiation that affects the conditioned space once it has been absorbed and re-released by the walls, floors, furniture, etc.

This absorbed energy contributes to space cooling load only after a time lag, so part of this energy is reradiating after the lights have been switched off [26]. The primary source of heat from lighting comes from the light-emitting elements, or lamps, although significant additional heat may be generated from associated components in the light fixtures housing such lamps.

The ratio of the lighting wattage in use to the total wattage installed is called the use factor, and it must be considered when determining the heat gain due to lighting at a given time since installed lighting does not give off heat unless it is on. For commercial applications such as stores, the use factor would generally be unity [27]. Improper installation or design of lighting systems can have apparent HVAC implications. Many lighting fixtures serve as return air ducts, an integral part of the heating, ventilating and air conditioning system.

There are several considerations in HVAC systems that can drastically affect a building operation expenses as well as occupant comfort. Reduced lighting causes a corresponding reduction in the cooling load for the air conditioning equipment, especially in the interior zones, where outdoor conditions have little influence. Demand for winter space heating may increase incrementally with reduced building lighting.

This decreases the savings from the light reduction program by the amount of energy that must be added to offset the loss of heat. For example, in a terminal reheat system, a change in lighting could require as much additional energy to reheat the duct air as is saved by reducing the lighting [35].

The reheat requirement, however, can be minimized by raising the cool supply air temperature so comfort conditions in the room with the maximum cooling load are satisfied without reheating the air going to other rooms.

In the variable air volume system, a reduced cooling load would reduce the amount of cool air that is distributed through the building. This reduction may present an opportunity to replace the supply fan motors with smaller motors, saving additional energy. An HVAC expert is necessary to evaluate the retrofit savings potential [35]. For a fan, for example, part of the power consumed by the motor is transmitted to the fan to drive it, while the rest is converted to heat because of the inefficiency of the motor.

The fan transmits the energy to the air molecules and increases their kinetic energy. But this energy is also converted to heat as the fast- moving molecules are slowed down by other molecules and stopped as a result of friction [27]. Therefore, we can say that the entire energy consumed by the motor of the fan in a room is eventually converted to heat in that room.

The power rating Wmotor on the label of a motor represents the power that the motor will supply under full load conditions. But a motor operates at part load, sometimes at as low as 30 to 40 percent, and thus it consumes and delivers much less power than the label indicates.

Also, there is inefficiency associated with the conversion of electrical energy to rotational mechanical energy. Therefore, it is not a good idea to oversize the motors since oversized motors operate at a low load factor and thus lower efficiency.

Another factor that affects the amount of heat generated by a motor is how long a motor operates. Motors with very low usage factors of dock doors can be ignored in calculations.

If electric motor load is an appreciable portion of cooling load, the motor efficiency should be obtained from the manufacturer. The tremendous variety of appliances, applications, usage schedules, and installations, makes estimates very subjective.

Electric typewriters, calculators, checkwriters, teletype units, posting machines, etc. Table 5. Computer rooms housing mainframe or minicomputer equipment must be considered individually.

Computer manufacturers have data pertaining to various individual components. In addition, computer schedules, near-term future planning, etc. While the trend in hardware development is toward less heat release on a component basis, the associated miniaturization tends to offset such unitary reduction by a higher concentration of equipment [27].

Temperature tb may range widely from the conditioned space. Actual temperatures in adjoining spaces should be measured when possible. Infiltration, exfiltration, and natural ventilation airflows are caused by pressure differences due to wind, indoor-outdoor temperature differences, and appliance operation [26]. Outdoor air must be introduced to ventilate conditioned spaces.

Local codes and ordinances frequently specify ventilation requirements for industrial installations. Generally, outdoor air for ventilation is introduced at the air-conditioning apparatus rather than directly into the conditioned space [26]. Fenestration components include: 1 glazing material, either glass or plastic; 2 framing; 3 external shading devices; 4 internal shading devices, and 5 integral between-glass shading systems.

The total instantaneous rate of heat gain through a glazing material can be obtained from the heat balance between a unit area of fenestration and its thermal environment.

Solar radiation that is transmitted indoors is partially absorbed and partially reflected each time it strikes a surface, but all of it is eventually absorbed as sensible heat by the furniture, walls, people, and so forth. Therefore, the solar energy transmitted inside a building represents a heat gain for the building.

Also, the solar radiation absorbed by the glass is subsequently transferred to the indoors and outdoors by convection and radiation. The sum of the transmitted solar radiation and portion of the absorbed radiation that flows indoors constitutes the solar heat gain of the building [27].

The heat gains through sunlit double-strength sheet glass are designated as solar heat gain factors SHGF. The Solar heat gain factors are based on terrestrial measurements [26] which represent solar intensity. These data do not give the maximum value of solar intensity that can occur in a year, but rather are representative of conditions on average cloudless days.

The data for the month of August is being used because it is observed that Peak design cooling loads occur in August. Also, the times that have been considered for estimation of cooling load are AM, Noon, PM and PM, which are the usual daylight solar times in which peak loads occur. For dates, times and latitudes other than those considered, linear interpolation can be used. It differs primarily as a function of the mass and nature of wall or roof construction, since those elements affect the rate of conductive transfer through the composite assembly to the interior surface [26].

Sol-air temperature is the temperature of the outdoor air that, in the absence of all radiation changes, gives the same rate of heat entry into the surface as would the combination of incident solar radiation, radiant energy exchange with the sky and other outdoor surroundings, and convective heat exchange with the outdoor air [26].

The sol-air temperatures for July 21, are given in table 5. The values correspond to a solar time of as it is known from observation peak load occurs in late afternoon. For walls and roofs the heat gains are estimated using the table 5. For determining the thermal resistance for the surfaces, the R-values of the materials of the surfaces of roofs and walls are used as discussed in the development of second module in chapter 4.

Orientation of Sol-air temp. For estimating heat gains the procedure is similar to the estimation of heat losses except that in place of Outdoor temperature, the sol-air temperature is used in the temperature difference term. In the present system developed Heat gain from miscellaneous sources is not dealt with.

These parameters require analyzing system performance as a sequence if individual psychrometric processes [26]. An economizer system is a mixed air control system that utilizes outdoor air as the first stage of cooling to reduce energy usage. Most commercial buildings generally have a cooling requirement even during mild and cold weather conditions, because of the internal loads. A cooling system with an economizer can use cool outside-air to satisfy all or part of the cooling demand.

This reduces the cooling energy required by the system. A properly designed economizer will have no impact on the heating energy used by the building. Economizers use controllable dampers to increase the amount of outside-air intake into the building when the outside-air is cool and the building requires cooling.

In addition to the controllable outside-air dampers, there are several other key components in an economizer system: return-air dampers, exhaust air dampers, economizer controller, temperature controller, and minimum position limiter. In addition, the relief- or exhaust-air dampers are required to prevent the building from being over-pressurized when large amounts of outside-air are introduced.

This controller arbitrates when to use outside-air for cooling and how much to use. It is required to provide the correct amount of outside air, while preventing the economizer from inadvertently increasing heating or cooling loads by introducing more outside-air than required. It is implemented by sensing and controlling how cold the mixed-air or supply air temperature gets.

It overrides the economizer controller, limiting the amount of cold outside air, to prevent coil freezing and uncomfortably cold drafts in conditioned spaces during cold weather. Minimum Position limiter- When outside-air conditions are not favorable for economizing, the outside-air damper system is positioned to provide the minimum outside-air intake required to meet the fresh air ventilation requirements for occupants. The control strategies employed in economizer systems are differential dry- bulb temperature based, differential enthalpy-based, high-limit dry-bulb temperature- based, and high-limit enthalpy-based.

With differential control strategies, the outside- air condition is compared with the return-air condition. As long as the outside-air condition is more favorable for example, with dry-bulb temperature control the outside-air dry-bulb temperature is less than the return-air temperature , outside-air is used to meet all or part of the cooling demand.

If the outside-air alone cannot satisfy the cooling demand, mechanical cooling is used to provide the remainder of the cooling load. With high-limit control strategies, the outside-air condition is compared to a single set point or fixed set point usually referred to as a high limit. If the outside air condition is below the set point, then outside-air is used to meet all or part of the cooling demand; the remainder of the cooling load is provided by mechanical cooling.

Dry bulb economizers only control the outside air dampers based on temperature. If it is a cool but rainy day, the outside air will be brought in and extra cooling capacity will be required to dehumidify it. Hence, dry bulb temperature economizers would be suitable for dry and arid climates. Enthalpy economizers take temperature and humidity into account.

With enthalpy control, humid air below a conventional dry bulb setpoint is locked out. Cooling costs are lowered in most climates when using enthalpy instead of dry bulb temperature with the economizer [43]. There are two enthalpy control strategies available: single and differential dual sensor enthalpy control. The single enthalpy control uses one enthalpy sensor located in the outdoor air in any orientation that exposes it to freely circulating air and protects it from rain, snow and direct sunlight.

The enthalpy sensor replaces the dry bulb high limit used in a standard economizer. Instead of switching the mixed air control loop from outdoor air-dry bulb temperature, on a call for cooling from the controller or space thermostat the economizer logic module compares the outdoor enthalpy to a preselected setpoint.

The value of the setpoint is illustrated on the psychrometric chart in the fig 5. The setpoint selected will vary based on climate, activities in the controlled area and the type of mechanical equipment used to provide cooling.

An installer can choose a more aggressive setpoint A for more free cooling or a conservative setpoint D for less free cooling. Each setting corresponds to an enthalpy curve with A equalling the highest enthalpy changeover and D being the lowest enthalpy. The output of the controller can be used to switch the mixed air and back as required for maximum efficiency. The psychrometric chart shows effects of the various economizer logic setpoints listed in table 5. Air to the left of the Curve is brought in from outdoors to be used for cooling.

Outdoor air to the right of the curves is not used. For differential enthalpy the setpoint knob is turned past D setting and the lower of return or outside air is brought into the building. F B 70 deg. F C 67 deg. F D 63 deg. This is also referred to as differential enthalpy. The air with lower enthalpy, outdoor or return, is selected to be brought into the conditioning section of the air handler.

The setpoint on the controller is turned to D whenever differential enthalpy is used [43]. This is a very efficient method of controlling outdoor air usage since the return and outside air is continuos and automatic year-round. It eliminates operator error by eliminating seasonal changeover that is frequently overlooked. Though it may appear wasteful to cool outdoor air at a higher dry bulb temperature than return air, the fact is that the amount of mechanical cooling required to dehumidify air often exceeds the amount required to lower the dry bulb temperature.

In buildings where there is moisture-generating activity this type of control sequence can result in substantial savings in cooling costs. Figure 5. At low outdoor air temperatures, relatively little air is needed whereas at mild temperatures, a great deal of outdoor air is needed to meet the cooling load. Of course, at still higher temperatures, outdoor air can no longer be used for cooling, and the mechanical cooling system will operate.

Also, a part of the return is used. This is done to provide sufficient airflow circulation within building spaces.

The amount of outdoor air needed, by itself, is insufficient to maintain comfortable air circulation under all conditions [28]. For estimating the energy savings there are certain technical details which have to be noted usually from the name plate of the cooling system.

The energy savings are obtained based on the enthalpy of the outside air. Department of Energy Weather data can be used to generate enthalpy readings for each hour per month based on the outdoor temperature and the location where economizer usage is to be tested.

The enthalpy readings for each hour per month are given in Table 5. The enthalpy is a function of the dry bulb temperatures and the humidity. The higher the dry bulb temperatures and humidity levels the larger the enthalpy value.

The enthalpy can also be determined directly from a Psychometric Chart. In order to do this, the user needs either the dry bulb temperature and the humidity or both the wet bulb temperatures and the dry bulb temperatures. The energy efficiency ratio EER is the ratio of heating capacity Btu per hour to the electric input rate watts. EER thus has the units of Btu per watt per hour. For obtaining the enthalpy readings we can use the weather data supplied by the DOE for other major locations and extract the enthalpy readings based on standard guidelines provided by the Department of energy [3].

This part of the Chapter is to make the User run through the Expert system for the estimation of Cooling Load. After the estimation of solar heat gain, the expert system proceeds to calculate the heat gains from the walls and roof based on the Sol-air temperature. Since, the form of input is same as that in chapter 4 the screens are not presented in this chapter and the next part of consultation is shown in fig. The User has to select the number and type of occupancy during the consultation.

By type of occupancy, it is meant the degree of activity inside the space or the building that can contribute to the heat gain. As different spaces would involve different activity and the energy released during different activities is different, the heat gain is influenced by the degree of activity. The heat gain due to various kinds of office equipment is selected and heat estimated by the System.

Various kinds of office appliances used are listed by the expert system and the user can select accordingly. Also, the internal heat gain due to motor driven equipment and lights is also estimated by the system.

In this part of the consultation, it is important to know certain parameters like the time of operation of equipment and lights. The form of input and the screens are the same for conduction heat gain through glass and infiltration heat gains as in the Heat load estimation. Also, a minimum total of 5-ton Cooling Capacity will be required to retrofit an Economizer. The Economizer input and the energy savings in MMBtu is furnished by the Expert system which can be used for estimating the cost savings based on the cost of electricity.

It can be seen from the figure that with the increase in the Outdoor temperature, the cooling load The Graph shown also gives an idea about using outside air cool air. During the cooling season, one can get rid of heat gains and postpone the onset of cooling by opening the windows or increasing the ventilation which is also termed as operating in economizer mode.

The conditioner is needed only when the outdoor temperature goes beyond the threshold Tmax. F Figure 5. Further, the functioning of the system necessary to estimate the cooling load is presented. It can be concluded that in the Cooling Load analysis, transmission heat gains contribute to the maximum heat gain.

The motor driven equipment and the internal heat gain sources are the next major part of the cooling load. Determination of Cooling load is important for efficient use of Cooling systems. The Use of economizers in most climates brings about good amount of savings, as the cooling system does not operate in the economizer mode. Validation is the process of determining that the system actually fulfills the purpose for which it was intended.

That is, substantiating a system or model performs with an acceptable level of accuracy. Initially, validation efforts were informal and highly individualized often characterized as a “craftsman approach” which exercised program code against a small set of ad hoc test cases. As modeling efforts grew from rather small projects to more complex endeavors, validation complexities increased.

Later, more rigorous validation techniques backed by statistical tests were developed. Today, validation is an important component of expert system research and development. Many of the validation techniques currently employed by expert system modelers owe their origin to early simulation and conventional software developers. Validation answers the question “is it the right system? The scope of the specifications is rarely precise, and it is practically impossible to test a system under all the rare events possible.

Therefore, it is impossible to have an absolute guarantee that a program satisfies its specification, only a degree of confidence that a program is valid can be obtained.

That is, to what extent is the user satisfied with the technical details provided in the system? To what extent are the utilities provided useful? In order to perform the technical validation of the present expert system designed for the estimation of heating and cooling loads results obtained by the expert system are compared with results from two other methods.

The results obtained from an assignment given to the MAE class, students majoring in Mechanical engineering is considered. ASEAM models energy use in both commercial and residential buildings. The program simulates heating and cooling requirements, calculates the results of energy efficiency measures. Figure 6. Area of walls : sq. Window area : sq. When the above data is input to the expert system, the output is obtained as shown in figure 6.

Also, the outdoor design temperature in the assignment is F whereas it is 13 deg. F in the expert system. The Latent heat component in the infiltration appears to be a much higher contribution to the total heat load in the estimation. The cooling load value from the expert system differs from the values from the other two methods because the expert system considers the latent component of heat gain due to infiltration. The outdoor temperature in this case is 95 deg. F whereas a value of F was used in the class.

It can be concluded that the system developed gives output comparable to the results obtained by the ASEAM software and hand calculations. Table 6. The results obtained by the expert system are higher than the other two estimations because more detailed considerations have been made in the design of the expert system.

The Space heat and cooling loads estimated by the expert system are compared with the results obtained from the students of class who used the ASEAM software. The loads estimated by hand calculations are also given. From the validation it is learnt that the HVAC analysis done using the expert system has been robust enough to get appropriate results. It can be concluded that the expert system definitely proves to be an available, dependable and detailed technical tool to evaluate heat recovery and estimate the HVAC loads.

Turner, W. Goldstick , R. Thumann, A. Kennedy, W. Nagarajan, S. Pabba, R. Veena, R. Jacques, J.