1404 



GREENHOUSE 



terns where some method of placing the water under 

 pressure is used. See, also, under Forcing-houses, p. 1402. 

 As the various flowers and vegetables grown under 

 glass require different temperatures, the piping of 

 greenhouses has to be varied accordingly. Thus, 

 although it may vary from 3 to 5 for different varie- 

 ties of the same species, our common plants require 

 the following night temperatures: violets and lettuce, 

 45 to 55; radishes and carnations, 50 to 55; roses 

 and tomatoes, 60 to 63; cucumbers and stove plants, 

 70. 



Boilers. 



For small ranges, whether steam or hot water is used 

 for heating, the best boilers are those constructed of 

 cast-iron as they will be found more durable than those 

 in which wrought-iron or steel is used. By using either 

 vertical or horizontal sections, it is possible to build 

 up boilers of considerable size, but, especially if to be 

 used for steam heating, it will be preferable to use 

 wrought-iron or steel boilers if they have a capacity 

 of more than 2,000 square feet of radiation. Except 

 for those of extremely large size, the ordinary tubular 

 boilers will be found adapted both for steam and hot- 

 water heating, although when used for hot water they 

 will be more effectual if the entire shell is filled with 

 tubes, as there is no occasion for leaving a steam space 

 at the top of the boiler. Such boilers are of low cost, 

 economical and durable. 



There are also on the market several forms of wrought 

 tubular boilers which are giving good results for heat- 

 ing greenhouses with hot water. For ranges of the 

 A largest size, 



where forced 

 draft is used, 

 water-tube boil- 

 ers are extremely 

 powerful and 

 very satisfac- 

 tory. When in- 

 stalling a heat- 

 ing-plant, it will 

 be safest to use 

 two or more boilers rather than one large one of the 

 same capacity, as when there is only a single boiler 

 serious losses may result if repairs to the boiler become 

 necessary in extremely cold weather, which might be 

 lessened or entirely prevented when there are two or 

 more boilers in the battery, and it is possible to cut out 

 the one which has become damaged. Especially in 

 mild weather during the spring and fall, the firing will 

 be more economical when it is possible to use a boiler 

 just large enough to heat the houses, rather than one 

 which is several times larger than is necessary at that 

 time, as would be the case when only one boiler is used. 

 The durability of the boiler and the economy of 

 heating will be greatly increased when the heating 

 capacity is considerably larger than is really necessary, 

 as when the firing is forced in extremely cold weather 

 it will not only result in a loss both in fuel and labor, 

 but will shorten the life of the boiler. 



The size of hot-water boilers is usually expressed 

 in terms of radiation, or the number of square feet of 

 heating surface it can supply economically. In a given 

 boiler there is a fixed ratio between the size of the 

 grate and the area of the fire surface of the boiler, but 

 this will depend very largely upon its construction and 

 efficiency of the fire surface, as well as upon the size of 

 the boiler. In the case of small hot-water boilers the 

 ratio between the grate and fire surface is often as 

 small as 1 to 15, while it may be as much as 1 to 35 in 

 larger ones, and even more when the boilers have fre- 

 quent attention and hard coal is used. One reason for 

 using a relatively large grate in small boilers is because 

 it makes it possible to leave the fire for eight or ten 

 hours without care or attention, while for large boilers 



1761. Horizontal tubular boiler for 

 hot water. 



and where a night fireman is employed, the ratio 

 between the grate and fire surface may be much greater. 



The capacity of steam boilers is usually rated in 

 horse-power, and it is considered that for each horse- 

 power a boiler will heat 100 square feet of radiation; 

 an average of 15 square feet of fire surface is con- 

 sidered equal to one horse-power, it being customary to 

 estimate that 10 or 12 feet in a large boiler will equal 

 one horse-power, while in a very small one as much as 

 18 feet would be required. Thus, in medium-sized 

 boilers an area of 10 square feet of grate will answer for 

 250 square feet of fire surface and this will be sufficient 

 for about 1,700 square feet of radiating surface when 

 steam is used; and as 75 to 100 per cent more radiation 

 will be required when hot water is used, a boiler of the 

 above size will answer for 2,800 to 3,400 square feet of 

 hot-water radiation. In the case of small boilers that 

 will not have attention at night, it is usually advis- 

 able to reduce the above estimates about 25 per cent, 

 and when a boiler is required for 1,000 square feet 

 of radiation, we should select one that is rated at 1,250 

 square feet. 



Home-made coil boilers are sometimes constructed 

 for hot-water heating since the cost will generally be 

 considerably less than for tubular boilers. As a rule, 

 however, they will be found less durable and lacking 

 in efficiency as compared with the better class of green- 

 house boilers now on the market. For making such 

 boilers, 2-inch wrought-iron pipe in lengths of 4 to 6 

 feet is used. Formerly 1-inch pipe was used for coil 

 boilers but it is comparatively thin, and, especially where 

 the threads were exposed it was quickly eaten through 

 so that it proved far from being as durable as the larger 

 sizes of pipe. There was also more trouble from the 

 boiling over of the water than when larger pipes were 

 used and if the boilers are constructed of 1-inch pipe 

 it is necessary either to have an elevated expansion tank 

 or run it as a closed system. In making a coil boiler, 

 the pipes are cut of the desired length and the ends are 

 connected either by return bends or by manifolds so 

 as to form a number of vertical coils, each containing 

 from six to ten pipes. The upper ends of the manifolds 

 are joined at the front end of the heater and connected 

 with the main flow-pipe; while the lower ends of the 

 rear manifolds are joined to the returns. As a rule, the 

 grate is of the same width as the coils and from one- 

 half to two-thirds as long. 



Although a box coil is much cheaper than a cast- 

 iron heater, when we have added the cost of the grate, 

 doors and other fittings, and of bricking it in, the 

 amount saved will not be large, and its use will often 

 be found less economical, especially as the coil boilers 

 are, as a rule, not more than one-half as lasting as cast- 

 iron boilers, most of which are complete in themselves 

 and require no brickwork or trimmings. 



Hot^water piping. 



When hot water first came into use for the heating 

 of greenhouses, 4-inch cast-iron pipes were used, but, 

 as the joints were packed with oakum, cement or iron 

 filings, they frequently gave trouble by leaking and it 

 was much more difficult to make changes or repairs 

 than in the present systems for which small, wrought- 

 iron pipes with screw joints are used. Owing to the 

 large amount of water in the cast-iron pipes, the circu- 

 lation was necessarily quite sluggish and it was not easy 

 to secure the high temperature in the water that can be 

 obtained with smaller pipes. Another objection to the 

 use of these large pipes is that it is not possible to carry 

 the flows overhead, while with smaller pipes one may 

 not only have the flows but some or all of the return- 

 pipes above the level of the benches. By elevating the 

 pipes above the level of the boiler, the rapidity of the 

 circulation and the temperature of the water in the 

 pipes can be considerably increased. 



In case a number of houses are to be supplied from 



