INDIRECT SYSTEMS. The natural air circulation system operates on the same principle as the direct-indirect, and differs in construction only in that the radiators, which are of a special type developed for this use, are placed outside, usually beneath the room, and the cold air from outside is led through ducts to the radiating surface and then rises to the rooms through other flues or ducts. Perhaps its only important advantage over the direct-indirect is that the heated air can be admitted to the rooms at any desired height above the floor or from any wall or floor surface. The additional ducts required make the expense of installation considerably higher than that of the direct-indirect. FORCED CIRCULATION. Indirect heating systems with forced circulation are variously known as hot blast heating, plenum, fan blast, fan coil or mechanical warm air systems. These are but different names for what is essentially the same thing and in them all we find a blower or fan added to the usual complement of equipment in an indirect gravity system. Whatever advantages may be ascribed to the systems previously described (the fan-furnace possibly excepted), all are deficient in supplying the positive ventilation assured where the blower is added. No system of natural circulation can give absolute assurance of adequate ventilation in an auditorium or in any room occupied by a large number of people. Many devices in the way of attachments and auxiliaries have been developed to perfect the results obtained with this system. These auxiliaries include air washers, humidifiers, cooling coils, and automatic thermostatic and humidity control equipment. Where proper air washers or purifiers are operated in connection with this system, it has been found possible to recirculate air from the building, rather than take it from outside, and still obtain perfectly satisfactory ventilation. The quality of this recirculated and washed air is superior to that often obtained from outside where dust and dirt may abound in the source of supply, as in the case where the inlet is taken from a crowded city street. The practice of recirculation will frequently reduce the cost of coal from 30 to 50% but where the source of supply is outside the cost of fuel must necessarily be high. Electric power for driving the blower is desirable, but a low-pressure steam engine operating from the system's own boiler may be employed. Wherever basement rooms are used as school rooms, some form of fan circulation is necessary, if ventilation is to be positively secured, and in such buildings, of medium. or large size, it is recommended that a fan coil system be installed where the original cost and operating cost are not prohibitive. As sometimes designed, the use of automatic temperature control devices makes closed windows a necessity since the opening of windows in one room and the attendant drop in temperature will produce overheating in another room, and while a real source of trouble in some buildings, it is evidence of a defect in design rather than in principle. The regulation of humidity, where humidifiers are used, is a matter which must receive almost continuous intelligent attention. Thermostats likewise require frequent expert attention, if they are to be kept in working order. The real source of trouble with the forced circulation system of heating and ventilation arises through ignorance and lack of instruction of caretakers and teachers. With co-operation between teachers and plant operators, and intelligent supervision, this system must eventually prove superior to all others, but its installation is useless, and cannot be recommended, unless the school authorities are willing to pay a price for attendance commensurate with the high quality and first cost of equipment and the excellent results attending its skillful operation. COMBINATION OR SPLIT SYSTEM. The forced ventilating and heating system is sometimes varied by heating the rooms with direct radiation and supplying ventilating air at room temperature with a fan and tempering coils. This arrangement results in fuel economy to the extent that only the air furnished for ventilation need be heated. At times, when no ventilation is necessary, the fans and fan-coils may be shut down. Such a system will prove more expensive to install but is very generally regarded as an improvement over the full plenum system. PROVISIONS OF THE LAW. In analyzing the provisions of law, those provisions which, by their own specific statement, apply to but one type of heating, and those which unquestionably can be met by every form of heating, are omitted as having no bearing on the present questions. Neither is any attention given to those regulations which apply only to systems for use in old or temporary buildings. Following are the principal provisions of the Indiana law: a. Fresh air shall be taken from outside the building. b. Fresh air taken from outside shall be properly diffused without draughts through each school room during school session. of maintaining a temperature of 70° F. in all school rooms, halls, office rooms, laboratories and manual training rooms in all kinds of weather. f. Heaters of all kinds shall be capable of maintaining a relative humidity of not less than 40%. COMPARATIVE COMPLIANCE WITH THE LAW. In the following table, showing the relative capability of the various systems to meet the requirements, the numbers. in the first column of the table refer to the provisions of the law given above under the same number. The indications of the letters are as follows: A. Full compliance. The analysis is purely arbitrary and is based on the performance to be expected, with reasonable care, in normal operation in all kinds of weather. NOTE: In systems where the fresh air taken from the outside serves the double purpose of ventilating and heatcarrying medium, the provision requiring proper diffusion without draughts is interpreted, in the following tabulation, to include distribution of heat in room. The short-comings of the two direct systems are considered so great as not to warrant their serious consideration. A study of the various columns will indicate an increasing qualification from left to right. The requirements emphasized by the law are such as to permit a better showing by the direct-indirect heater and c. Each school room shall be supplied the furnaces than would actually be with foul air flues. d. Foul air flues supplied shall be of ample size to withdraw the foul air from each room at a minimum rate of 1800 cu. ft. per hour for each 225 cu. ft. of said school room space, regardless of outside atmospheric conditions. e. Heaters of all kinds shall be capable found in practice. No requirements are made for many of the points of operation in which these devices are most faulty. In so far as the provision of the statutes are concerned the direct-indirect radiator system is just as satisfactory as the indirect radiator system with gravity circulation. The fan-furnace equip 1. While the law requires a heating capacity sufficient to maintain an indoor temperature of 70° F. in all kinds of weather, it does not specify any maximum temperature limits; and this condition exists in spite of the fact that the injurious effects of overheating constitute the only undebated issue regarding ventilation. 2. No provision is made as to the point, or number of points, at which room temperature shall be observed during school hours. Thus, no specific plan is laid down for ascertaining that the temperatures are proper and uniform throughout the room. 3. It is compulsory that air be taken from outside, but no provision is made against the use of air containing excessive dust particles or other impurities. 4. While carbon dioxide in the air in quantities likely to occur in a school room, cannot be considered injurious to health, its presence in occupied rooms. 5. While the excellent qualities of the fan-coil system are recognized, it is readily admitted that conditions are not such as to warrant its use in all cases. Many times its cost renders it entirely prohibitive. In such cases the direct-indirect system offers the best substitute, considering both cost and results, since this system yields results practically equal to those obtained with the gravity indirect and, in general, possesses more mendable features than any other, excepting only the fan-coil system. com 6. Evidence is lacking to show that as a permanent equipment for schoolhouse heating and ventilating, a well designed and constructed system of a theoretically poorer type will render better satisfaction than a poorly constructed, cheap system of a theoretically better type. Furthermore, a mediocre type of apparatus, when given careful, intelligent attention, will yield more satisfactory results than can be attained with the finest system in the hands of an ignorant and inexperienced operator. 7. Finally, selection of all equipment and attendants should be made with the idea of safeguarding the expenditure of public money and the loss of property, and, more especially, the health and lives of school children. The findings of the report have received the endorsement of a number of prominent engineers, including Professor J. D. Hoffman, of the University of Nebraska; Professor John R. Allen, of the University of Michigan; Professor A. C. Willard, of the Illinois State University; S. A. Challman, commissioner of school buildings of the University of Minnesota; Professor J. L. Mowry, of the University of Minnesota; and Professor Frank C. Wagner, of the Rose Polytechnic Institute. Radiation Requirements under Varying Temperature Conditions With Chart Giving Graphic Solutions for Use in Weather Above and Below Zero. By EDWARD B. JOHNSON, C.E. There are a number of preliminary considerations which present themselves when undertaking the design of a steam heating system and among them none is more important than the relation of outdoor and indoor temperatures and the amount of radiation installed. When socalled standard conditions are to be met, 70° F. room temperature with zero weather outdoors, no difficulty will be experienced and this covers the majority of cases. But when either one of both of these temperature limits are varied, we are liable to get into difficulties. Varying the lower limit only should cause little trouble if it be remembered that in this case the amount of radiation is proportional to the temperature rise desired. Thus, assuming 0° to 70° F. as requiring 100% of radiation, 20° to 70° would require 71.4%.: 1.43 the radiation for each room and the desired result is obtained. But when the upper limit (room temperature) is varied, we immediately add to the problem the variable factor of radiator efficiency, due to the changing temperature of the air surrounding the radiating surface. The term efficiency, as here used, refers to the variation in the amount of heat radiated to the surrounding air from the heated surface as the room temperature varies. This amount is approximately proportional to the difference in temperature between the steam (or hot water) in the radiator and the room. Table 1, Column 2 gives these amounts for every 5°. Column 3 gives the pounds of steam condensed per hour, and Column 4, the relative tax on the boiler. This table shows that a radiator standing in, say, 50° still air produces a tax on the boiler equal to about 18% more than it would had the surrounding air been at 70°, a very important fact, as extra boiler capacity to this amount should here be provided. Assuming that each square foot of radiation standing in still air at 70° condenses 4 lb. of steam per hour and steam at 220° F., this would equal 9654241 B.T.U. per hour. This is generally taken as 240 B.T.U. The difference between the room and steam is 220° — 70° 150° and the B.T.U. radiated per square foot per degree difference of temperature is 240 150 1.60. This is called the heat emission factor. C. A. Fuller, in his article on "Heat From Radiators at Different Temperatures," (THE HEATING AND VENTILATING MAGAZINE for March, 1915), states that this heat emission factor is not absolutely a constant, but varies with the temperature difference between the steam and the room, increasing as the difference increases and, of course, diminishing with a smaller difference by an amount equal to 1/5% for every degree above or under 150° difference. Consider these conditions: Steam 240° F., room temperature 50° F. 240 50 190°. This is 40° in excess of 150° difference. 40 x 1/5=8%. As 190° is greater than 150° standard difference, 8% is to be added to 1.6, making the heat emission factor, under these circumstances, 1.73. Then 1.73190329 B.T.U., the heat transmission per square foot of heating surface per hour under the above conditions. Considerable difference, as compared with 240 B.T.U. for standard conditions. The third condition mentioned, when room and outdoor temperatures are both varied, will not be considered here, as it is taken up later. All of this is very fine for the mathematically-inclined, but most engineers prefer to arrive at results with the minimum of figures, thus saving time and tending to eliminate possible errors. Therefore, a table or curve which will give the result graphically of a complicated problem is almost universally acceptable. Fig. 1 has been drawn to assist in the easy solution of the above problems and probably of others which the reader may discover. Let us consider the first case previously discussed, when the outside temperature is varied. Upon the right side. of the curve will be found outdoor temperatures from-30° to 40° F. At the top are room temperatures from 40° to 130°. The curves represent varying percentages of radiation installed, the 100% curve indicating conditions when the correct amount is installed to heat the room to 70° in zero weather, as will be seen by its passing through the intersection of these temperatures. The 80% curve then would indicate the relation of outdoor and room temperatures when 80% of the correct radiation has been installed. To answer our problem find the desired outdoor temperature at the right and follow this horizontal line to its intersection with the 70° vertical line TABLE I-STEAM RADIATORS-HEAT TRANSMISSION PER SQUARE FOOT SURFACE WITH STEAM AT 220° F. |