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214 CHAPTER 17 1962 Guide And Data Book TIME - KMUTE3 Rg. 1 .... Car Temperature Variations for Open Rood and City Traffic nm-icwro Rg. 2 .... Typical PuJl*down Curves the industry experience during the past several years. Some of these will be presented in the following paragraphs. Capacity Requirements Cooling capacity is often prescribed in terms of cool-down, steady driving, or both, i.e., sufficient capacity to cool down to SO F in 10 min after being parked and dosed in 110 F ambient. The ability of the unit to cool down the temperature in a car from its maximum temperatures when parked to reasonably comfortable conditions is of vital importance to die car owner because it occurs so frequently and the temperatures in a dosed car are so very high. The need for frequent, rapid cool down is one of the major differences between car air condition ing and residential air conditioning requirements. When parked in the sun on a 110 F day, the interior of an auto mobile will reach temperatures as high as 135 F. Even if the windows are opened to flush the car out with 110 F air, the air temperature is far from comfortable and the seats and other interior surfaces are still at 135 F or more. The unit must not only have adequate capacity to rapidly pull the air tem perature down to a comfort levd, but also reduce the tem perature of the seats, door panels, ceiling, floors and instru ment panel, all totaling a mass of at least 700 lb. To cool this quantity of material from 135 F to 75 F in 15 min would re quire a capacity of 28,000 Btuh assuming an average specific heat of 0.2 Btu per (lb) (F deg). During this period the unit would also require sufficient additional capacity to hlH.nw> the steady state cooling load. By comparison, an American make compact car has a load of about 7800 Btuh. Since this sensible capacity requirement is based on 110 F outdoor, and 75 F indoor temperatures, the sensible heat transfer factor of the typical car would be 300 Btu per (hour) (Fdeg). Atasensible ratio of 0.8 at total steady* state load, including 20 percent latent heat, would be 12,750 Btub. The result of several cool-down tests on some representative cars and units are plotted in Fig. 2. The cars were soaked in the hot room for 1 hr at 110 F with full solar load (125 watts per sq ft) with windows closed. The car was then started and run at an engine speed equivalent to 25 mph and with air ap proaching the car at the same speed. Temperatures were meas ured at ankle, lap and breath levels, and averaged to give the temperature plotted. Fig. 3, shows an actual test run made in Yuma, Arizona, compared to a simulated hot room test. Note that the correlation is within 2 percent over most of the cool down curve. The steady state conditions at 25 mph after 60 min of operation are also shown and correlate quite well. Most auto air-conditioning systems available today have capacities in the range of 10,000 to 16,000 Btuh when operating at the above steady-state conditions. Speed Range The air-conditioning system must function properly over car speeds from idle to 120 mph. This extremely wide speed range of 12 to 1 considerably complicates the design problems At steady-state conditions, as well as during cool-down, the rate that outdoor air infiltrates into the car plus the amount of insulation in the panels, roof and floor are very significant factors affecting cooling capacity needs. Use of heat absorbing glass to reduce the direct solar radiation transmission from 67 percent for clear glass to 45 percent is also effective in reducing the cooling load. The steady-state cooling requirements can be evaluated at 25 xnph, maintuning 75 p inside the car with an outside sunny ambient of 110 F, 25 percent relative humid ity. The cooling load of a typical automobile would be broken down into various major sources as described in the table be low; Solar radiation (roof, walls, glass) * 3000 Btuh Normal heat gain through glass = 1200 Normal beat gain through walls, roof, floor =* 4250 Air leakage into car (estimated as 30 cfm at 25 mph) -- 950 Passengers (2), plus driver (sensible beat only) = 800 Total 10,200 Btuh Rg. 3 .... Comparison of Road and Hot Room Tests Surface Transportation Air Conditioning 215 in accomplishing adequate capacity at the low speed end, md reliability and endurance life at the high speeds. Life It is not unreasonable to expect the air-conditioning system to last the life of the car without major repairs. This is normally assumed to be 80,000 to 100,000 miles or 4 to 5 years. During this time some normal servicing and occasional addition of refrigerant is expected, but this should be confined primarily to good preventative maintenance practice. Air Distribution The method for distributing the cool air into the condi tioned space is one of the most important and probably one of the most controversial factors in automobile air-conditioning system design. The fact that the cooling load in relation to the volume of the conditioned space is so great places auto airconditioning distribution systems in a group of their own. The body volume of a middle class 4-door sedan is about 140 cu ft. The volume of a residential or office space that can be air con ditioned with approximately the same capacity would be in the neighborhood of 3000 cu ft. The usual four occupants in the caroccupy about 12 cu ft or 10.5 percent of the total space, they extend from the floor to within 4 or 5 in. of the ceil ing. Ten oecupants in the above room would take up 1 percent ofthe space, and their heads would be 2 to 4 ft from the ceiling. The free space in a car available for circulating the cool air within the passenger compartment without direct impinge ment on the pasengers is very limited. Another air distribution problem arises from the fact that -direct sunlight through some portion of the glass that sur rounds the top half of the car shines directly on two of the four passengers. The front floor temperatures are usually higher than the other panels. The cooling must be more concentrated in these hotter areas. The type of distribution desired during the cool-down period is much different than at normal relatively steady state condi tions after the car has reached the comfort temperature. Dur ing cool-down the air-off temperature is quite high and with the inside of the car being very warm, spot cooling or direct impingement on the occupants is desirable. When the car and air-off temperatures level out, direct impingement of the cold air is very undesirable. An ideal distribution system would: 1. Supply adequate air (approximately 250 cfm) to the car Interior without excessive drafts or direct cold air blasts on the passengers. . 2. Permit controlled amounts of cooling to the various areas of the ear as the rides, front floor area, rear seat area, and breath level. 3. Accomplish good distribution without objectionable noise from grilles, blowers, etc., with a minimum power requirement. 4. Provide for all recirculated air during the cool-down period. The tolerable air motion is considerably higher in an auto mobile due to more direct radiation through the windows from the direct 6un and from hot surrounding areas. It must be controllable, however, in the event that cooling is required when the radiation effects are low. The air velocity normally can be in tire order of 200 to 300 fpm without discomfort. When the air-conditioning air system is common with the with tire heating system, provisions must be made for satis factory beating as well as cooling. A combination air distribu tion system that, has been used effectively has provisions for directing air up and down for cooling, and down for heating. Temperature and Humidity Control Along with air distribution, suitable comfort air condition ing also requires temperature control and, to some extent. humidity control. The controls must maintain a minimum temperature variation in all parts of the conditioned space with controls that are as simple and low cost as possible. A description of some of the various control methods used today are drynspuri in the control section under Design Require ments of Specific Components. Again the number of variables in load distribution and rate of load changes while driving complicate the problem of maintaining close temperature con trol. The rate of temperature change at the thermostat sensing element b much faster than the rate in most residential or commercial air conditioned areas. The wide variations in unit capacity make it necessary to provide methods of coil freezeup protection. Ventilation The air infiltration into the car when it is in motion provides a varying amount of outdoor air depending upon the car speed and leakage. This air is unconditioned as it enters the car and mixes with the air in the conditioned space. Additional quantities of outdoor air ore often desirable to control odors. The factory installed systems have provisions for introducing precooled ventilating air into the car after the car has pulled down to a comfortable temperature. The field installed sys tems depend upon normal infiltration and at times a slightly open window to ventilate the car. Filters Filters are generally not used on auto air conditioners to day, although earlier factory installed systems did include them. The limited space for adequate filter area and inacces sible locations have made filters quite unpractical for auto air conditioners. Elimination of filters from these systems is not too serious since most of the dirt is made up of dust and small greaseless particles which wash off the evaporator coil with the condensate. lint and long fiber materials or grease vapors that would cling and clog the coil are' not prevalent in auto motive applications. Space Limitations Currently air conditioning b installed on only 5-10 percent of the automobiles as they leave the factory. For this reason the air conditioner holds a fairly low priority on available space. At the present time the evaporator, compressor and condenser are each in separate locations, and even with them split up in this manner each section has only a very limited space. The condenser must be set between the radiator and grille, a space normally used for the horns. The compressor b belt driven from the engine and therefore b in the same proximity with the generator, water pump, power steering, air ride compressor, and engine fan. All these are working in the usual narrow space between the engine and radiator. The. space for a front-end evaporator section b limited by the air cleaners, power brake mechanism, wiper motors, glove com partment, transmission housing, etc. Evaporators located un der the rear deck in the trunk,are limited the least for space but the long refrigerant lines become objectionable. Effect of Air Conditioning on Engine Performance and Operation The engine cooling system must be of vital concern when de signing and applying air conditioning to an automobile. The radiators, fans and pumps are carefully developed to cover the extreme range of engine cooling requirements. For economic reasons they must be designed closely with little reserve capac ity. In every air conditioning system today the condenser b