HORTICULTURE 



December 25, 1915 



To the Editor of Horticulture: 



I wish to call your attention to a very 

 common error which works to the detri- 

 ment of boiler users (greenhouse men), 

 and of well meaning and honest boiler 

 manufacturers and salesmen. The fallacy 

 I refer to is this: "A square foot of radia- 

 tion is a square foot of radiation wherever 

 it is." No statement could be more incor- 

 rect in its obvious meaning. 



Most boilers are rated for utmost capaci- 

 ties under best conditions, i. e., with chim- 

 neys of proper height and area, good coal, 

 frequent firing, etc. Many boilers are 

 rated for capacities entirely beyond them 

 under any conditions. 



Greenhouse boiler manufacturers only, 

 rate their boilers in lineal feet of pipe for 

 greenhouse heating (generally SVi and 2- 

 inch pipe). 



Any boiler will take care of a greater 

 amount of radiation in a dwelling house 

 than in a greenhouse, and a salesman who 

 attempts to sell a boiler for j^reennouse 

 service on its dwelling house rating either 

 • does not understand what he is doing, or 

 is trying to deceive. 



I am sending to you herewith a paper 

 written for a purpose other than green- 

 house heating, but it may be of interest 

 to greenhouse people. 



Yours very truly, 



FRED J. ELDER. 



"Conditions Under Which a Boiler 



Should Fulfill Its Ratings." 



A Talk by F. .T. Elder. Mgr. Boston Office. 

 Lord & Bumham (Company. Given at 

 the Burnham Boiler Get-T"gi'tlicr Pon- 

 vention, April 23-24, 1914. 



It has become a custom to rate heat- 

 ing boilers in square feet of east iron 

 radiation of standard height (38 in.) 

 and two columns wide, to specify 

 that the radiation be proportioned to 

 heat the building to 70 degrees, and 

 that the boiler be connected to a flue 

 of sufficient area to provide volume 

 and sufficient height to produce the 

 required velocity or intensity of draft. 



Ratings are also based on a stand- 

 ard of two pounds pressure for steam 

 boilers, and a temperature of 180 de- 

 grees P. in radiating surfaces for 

 water boilers. It is also called to the 

 catalog reader's attention that lie must 

 include in his total radiation figure all 

 •exposed pipe surface of mains, risers, 

 connections, etc., plus 25 per cent., and 

 it is explained that pipe surface is ap- 

 proximately 25 per cent more efficient 

 for heat transmission than standard 

 cast iron radiation. It is further 

 stated that 25 per cent, should be added 

 to the total amount of direct-indirect 

 radiation, and 50 per cent, to the 

 amount of indirect radiation, that 

 proper allowance be made for the ab- 

 sorption of heat by fire pot coils, and 

 that greenhouses, factories, garages, 

 storage buildings, etc., which are 

 heated to low temperatures, will con- 

 dense more steam per square foot, or 

 require more heat units per square 

 foot of radiation than in living rooms 

 heated to 70 degrees. 



Radiation now means any one ot 

 three things. It means first to the 

 fitter and the buyer of a Ijoiler a 

 quantity of radiating or transmitting 

 surface, a number of cast iron radia- 

 ting units or radiators. Radiation 

 particularly means the heat pro- 

 jected in rays, or straight lines from 

 a heated surface, that is, radiant heat. 

 Radiation is now considered in its 

 broader sense, that is, as including 



the whole transmission of heat from a 

 heating unit. It covers the radiant 

 heat that projected in rays, the trans- 

 mission by convexion, or rising In 

 fluid masses, or through flues, and the 

 transmission by conduction, or heat- 

 ing by contact with the hot surfaco. 



Air can be heated only by conduc- 

 tion, that is, by direct contact with hot 

 surfaces, or substances. Pure dry air 

 cannot be heated by radiant heat. 

 Rays of heat pass through air unless 

 intercepted by particles of dust or 

 vapor. These, and other substances 

 and surfaces which may intercept the 

 rays, will in turn heat the quiet sur- 

 rounding air by conduction. This 

 property possessed by air is called 

 diathermacy. The same property is 

 possessed by glass through which sun 

 rays pass without warming the glass. 

 Hot surfaces are surrounded by films 

 or fluid masses of hot air, which cling 

 to the surfaces, and it is evident that 

 an easy circulation or passage of air 

 over the surfaces by convexion cur- 

 rents is desirable in order to constant- 

 ly remove the film of heat, and for 

 this reason extended surface radiation, 

 such as pin and corrugated or ridged 

 radiators, are not as efficient in quiet 

 air as the more common types. The 

 fluid masses of heat cannot be disen- 

 gaged from the rough surfaces with- 

 out swifter currents of air than are 

 possible by convexion only, as with 

 direct radiation. Extended surface 

 radiators are, however, most satisfac- 

 tory in indirect heating where air ve- 

 locities are greater. 



Radiators are rated or measured 

 according to the number of square 

 feet of surface with which air may 

 come in contact. The efficiency of a 

 radiator or its coeflicient of transmis- 

 sion is the number of heat units 

 transmitted per hour througn one 

 square foot of emitting surface for 

 each degree of difference in temperat- 

 ure of the radiating surface and that 

 of the air surrounding. Efficiency de- 

 pends on the form, shape, and condi- 

 tion. As explained above, extended 

 surface radiators would have a low 

 efficiency in quiet air. Single column 

 radiators give off more heat per square 

 foot than two column radiators, De- 

 cause there is an easier circulation of 

 air around the single column radia- 

 tors, and the heat film, as it has been 

 called, is more easily disengaged from 

 the radiator's surfaces. With three, 

 four and five column radiation, effi- 

 ciency is correspondingly lower because 

 the heating is pocketed or trapped, 

 and because every column interferes 

 more or less with the radiant heat 

 from every other column. 



Efficiency of radiators depends also 

 on their height, A low radiator is 

 more efficient than a high one. Air 

 is drawn to the bottom of a radiator 

 or induced by convexion currents to 

 rise beside and against the radiator's 

 hot surfaces, and it is obvious that the 



Size Grate. Lin. ft. 4" 



Outside Diam. Pipe 

 24 X 24 1200 



higher a current of air rises against 

 or the longer it passes over a heated 

 surface, the higher its temperature be- 

 comes, and its ability to absorb heat 

 decreases at the same rate. Low radi- 

 ators give off more heat foot by foot 

 than high ones. The efficiency ot a 

 radiator is affected by Its location or 

 position; a radiator should not be too 

 close to a wall, or otherwise pocketed 

 so as to prevent a free circulation of 

 air around it. Radiators, whether of 

 cast iron or in the form of pipe coils, 

 are much reduced in efficiency when 

 placed high on walls, or hung from 

 ceilings. It is about as hard to get 

 warm air to circulate below the level 

 of the heating device as it is to get 

 hot water to do so inside of radiators 

 and piping. 



The efficiency of horizontal pipe sur- 

 face is greater than that of any other 

 form of radiator, and pipe coils placed 

 flat, or side by side in horizontal runs, 

 as in greenhouse practice, have the 

 highest possible efficiency. Different 

 authorities have given coefficients of 

 transmission ranging from 1.4 to 1.8 

 for cast iron radiators of common 

 type, and efficiencies of 2.2 to 3+ for 

 horizontal pipe. 



To determine the amount of heat 

 given off by a steam radiator in a 

 specified time, it is only necessary to 

 weigh the water condensed in that 

 time and to multiply the quantity in 

 pounds by 965, which is the number 

 of thermal units given up by the con- 

 densation of one pound of steam at 

 zero gauge pressure or transforming 

 one pound of steam at 212 degrees to 

 water the same temperature. The 

 product w-ill be the number or heat 

 units, or quantity of heat given off 

 by the radiator in the time named. 



From the foregoing, it is obvious 

 that very little or very much heat 

 may be emitted from a square foot ot 

 radiation, and that this all depends 

 briefly; on the form, shape, character, 

 position and location of the radiator. 

 It also depends even more on the 

 temperature of the air surrounding. 



In greenhouse and garage heating 

 and in other such special cases where 

 the temperatures required are below 

 70 degrees, it should be remembered 

 that each square foot must emit In 

 B. T. U.'s the product of its coefficient 

 of transmission times the difference 

 between the temperature of the radiat- 

 ing surface and that of the air sur- 

 rounding it. 



The lower the temperature required, 

 the greater will be the difference, the 

 higher the product, and the greater 

 the heat transmission. 



In looking through a boiler catalog 

 recently, I noticed that greenhouse 

 boiler ratings were given for various 

 temperatures and for the various 

 quantities of greenhouse glass sur- 

 face, and the following are the ratings 

 of a particular boiler as set forth in 

 that catalog: — 



