Part 13 (1/2)
FEED WATER HEATING AND METHODS OF FEEDING
Before water fed into a boiler can be converted into stea to the pressure within the boiler Steae pressure has a terees Fahrenheit If water is fed to the boiler at 60 degrees Fahrenheit, each pound must have 311 B t u added to it to increase its terees, which increase must take place before the water can be converted into steam As it requires 11678 B t u to raise one pound of water frorees and to convert it into stearees required sirees will be approximately 27 per cent of the total If, therefore, the terees before it is introduced into a boiler by the utilization of heat from some source that would otherwise be wasted, there will be a saving in the fuel required of 311 11678 = 27 per cent, and there will be a net saving, provided the cost ofthis saving is less than the value of the heat thus saved
The saving in the fuel due to the heating of feed water by means of heat that would otherwise be wasted may be co per cent = --------------- (1) H + 32 - t_{i}
where, t = te, t_{i} = te, and H = total heat above 32 degrees per pound of steam at the boiler pressure Values of H may be found in Table 23 Table 17 has been co under the conditions assue pressure
TABLE 17
SAVING IN FUEL, IN PER CENT, BY HEATING FEED WATER GAUGE PRESSURE 180 POUNDS
+-----------+-----------------------------------------+ | Initial | Final Terees Fahrenheit | |Temperature|-----+-----+-----+-----+-----+-----+-----| | Fahrenheit| 120 | 140 | 160 | 180 | 200 | 250 | 300 | +-----------+-----+-----+-----+-----+-----+-----+-----+ | 32 | 735| 902|1069|1236|1404|1820|2238| | 35 | 712| 879|1046|1214|1382|1800|2218| | 40 | 672| 841|1009|1177|1345|1765|2186| | 45 | 633| 802| 971|1140|1308|1730|2152| | 50 | 593| 763| 932|1102|1272|1695|2119| | 55 | 553| 724| 894|1064|1234|1660|2086| | 60 | 513| 684| 855|1027|1197|1624|2052| | 65 | 472| 644| 816| 987|1159|1588|2018| | 70 | 431| 604| 777| 948|1121|1552|1983| | 75 | 390| 564| 736| 909|1082|1516|1948| | 80 | 348| 522| 696| 870|1044|1479|1913| | 85 | 306| 480| 655| 830|1005|1441|1878| | 90 | 263| 439| 614| 789| 965|1404|1843| | 95 | 220| 397| 573| 749| 925|1366|1807| | 100 | 177| 354| 531| 708| 885|1328|1770| | 110 | 89| 268| 447| 625| 804|1250|1697| | 120 | 00| 180| 361| 541| 721|1171|1622| | 130 | | 91| 273| 455| 637|1091|1546| | 140 | | 00| 184| 367| 551|1009|1468| | 150 | | | 93| 278| 463| 926|1389| | 160 | | | 00| 187| 374| 841|1309| | 170 | | | | 94| 283| 755|1227| | 180 | | | | 00| 191| 667|1143| | 190 | | | | | 96| 577|1058| | 200 | | | | | 00| 486| 971| | 210 | | | | | | 392| 882| +-----------+-----+-----+-----+-----+-----+-----+-----+
Besides the saving in fuel effected by the use of feed water heaters, other advantages are secured The time required for the conversion of water into steam is diminished and the steam capacity of the boiler thereby increased Further, the feeding of cold water into a boiler has a tendency toward the setting up of temperature strains, which are diminished in proportion as the temperature of the feed approaches that of the stea feed water is that in certain types of heaters a large portion of the scale for the boiler, with a consequent saving in cleaning and losses through decreased efficiency and capacity
In general, feed water heaters may be divided into closed heaters, open heaters and economizers; the first two depend for their heat upon exhaust, or in some cases live steam, while the last class utilizes the heat of the waste flue gases to secure the same result The question of the type of apparatus to be installed is dependent upon the conditions attached to each individual case
In closed heaters the feed water and the exhaust steam do not come into actual contact with each other Either the steah tubes surrounded by the other medium, as the heater is of the steam-tube or water-tube type A closed heater is best suited for water free froes
Cleaning such heaters is costly and the efficiency drops off rapidly as scale forines work interines In this class of work the frequent coolings between operating periods and the sudden heatings when operation commences will tend to loosen the tubes or even pull them apart For this reason, an open heater, or econoiveapparatus
Open heaters are best suited for waters containing scale-for matter
Much of the temporary hardness may be precipitated in the heater and the sediment easily reent for precipitating permanent hardness in the combined heat and chemical treatment of feed water The so-called live stea raised to the boiling temperature and the carbonates and a portion of the sulphates being precipitated The disadvantage of this class of apparatus is that some of the sulphates remain in solution to be precipitated as scale when concentrated in the boiler Sufficient concentration to have such an effect, however,down
Econon of the boiler is such that the ases of co effected by the use of the economizer, and it is sometimes possible to raise the teh pressure stea a in some cases to as much as 20 per cent The fuel used bears directly on the question of the advisability of an economizer installation, for when oil is the fuel a boiler efficiency of 80 per cent or over is frequently realized, an efficiency which would leave a sh the addition of an economizer
From the standpoint of space requiree in that they are bulky and require a considerable increase over space occupied by a heater of the exhaust type They also require additional brickwork or a , which increases the cost
Soh an economizer make its adaptability questionable because of the draft conditions When figuring the net return on economizer investment, all of these factors must be considered
When the feed water is such that scale will quickly encrust the econo an excessive portion of the ti it into an economizer to oing, it is clearly indicated that it is impossible tofeed water in any of the three types Each case must be worked out independently and a decision can be reached only after an exhaustive study of all the conditions affecting the case, including the tirowth of the plant When, as a result of such study, the possiblethe problem have been determined, the solution of the best apparatus can bepossible by each ainst its first cost, depreciation,of Water--The choice offeed water into a boiler lies between an injector and a pump In most plants, an injector would not be economical, as the water fed by such means must be cold, a fact which makes impossible the use of a heater before the water enters the injector Such a heater ht be installed between the injector and the boiler but as heat is added to the water in the injector, the heater could not properly fulfill its function
TABLE 18
COMPARISON OF PUMPS AND INJECTORS _________________________________________________________________________ | | | | | Method of Supplying | | | | Feed-water to Boiler | Relative a of fuel over| | Temperature of feed-water as | coal required per | the amount required| | delivered to the pump or to | unit of tirees Fahren- | amount for a direct-| fed by a direct- | | heit Rate of evaporation of | acting pu purees | heater | | per pound of coal frorees Fahrenheit | being taken as unity| | |______________________________|_____________________|____________________| | | | | | Direct-acting Purees without | | | | a heater | 1000 | 0 | | | | | | Injector feeding water at | | | | 150 degrees without a heater | 985 | 15 | | Injector feeding through a | | | | heater in which the water is | || | heated frorees | 938 | 62 | | | | | | Direct-acting Puh a heater in | | | | which it is heated frorees | 879 | 121 | | | | | | Geared Puh a heater in which it | | | | is heated frorees | 868 | 132 | |______________________________|_____________________|____________________|
The injector, considered only in the light of a combined heater and pump, is claimed to have a thermal efficiency of 100 per cent, since all of the heat in the steam used is returned to the boiler with the water
This clai the water to a boiler and the heat in the exhaust froh a thermal efficiency as the injector The pue that it uses so iven quantity of water into the boiler that it h the use of the exhaust fro the feed, which exhaust, if an injector were used, would be wasted, as has been pointed out
In locomotive practice, injectors are used because there is no exhaust stea utilized in producing a forced draft, and because of space requirements In power plant work, however, puh injectors are so