Part 39 (2/2)
The factors deter the economic load for the seneral the statements made relative to the question are equally applicable
The econo at which a boiler plant should be run is dependent solely upon the load to be carried by that individual plant and the nature of such load The economical load for each individual plant can be determined only from the careful study of each individual set of conditions or by actual trial
The controlling factor in the cost of the plant, regardless of the nature of the load, is the capacity to carry the maximum peak load that may be thrown on the plant under any conditions
While load conditions, do, as stated, vary in every individual plant, in a broad sense all loads rouped in three classes: 1st, the approximately constant 24-hour load; 2nd, the steady 10 or 12-hour load usually with a noonday period of no load; 3rd, the 24-hour variable load, found in central station practice The economical load at which the boiler roups:
1st For a constant load, 24 hours in the day, it will be found in most cases that, when all features are considered, the iven amount of steam can be produced the most cheaply will be considerably over the rated horse power of the boiler
How much above the rated capacity this ely upon the cost of coal at the plant, but under ordinary conditions, the point of maximum economy will probably be found to be somewhere between 25 and 50 per cent above the rated capacity of the boilers The capital investh increased thermal efficiency and the labor account, which increases with the nuiven proper consideration
When the question is considered in connection with a plant already installed, the conditions are different from where a new plant is conteh boilers to operate at low rates of capacity, the capital investe, and it will be found that the most economical load at which boilers may be operated will be lower than where a new plant is under consideration
2nd For a load of 10 or 12 hours a day, either an approximately steady load or one in which there is a peak, where the boilers have been banked over night, the capacity at which they her than for uniforinal investiven aer return through the higher overload, and this will hold true to a point where the added return more than offsets the decrease in actual boiler efficiency Here again the deter factors of what is the econoainst the thermal efficiency With a load of this character, there is another factor whichload This is from the viewpoint of spare boilers That such added capacity in the way of spares is necessary is unquestionable
Since they must be installed, therefore, their presence leads to a fixed charge and it is probable that for the plant, as a whole, the economical load will be somewhat lower than if the boilers were considered only as spares That is, it ular equipment at all ti and repairs, thus reducing the load on the individual boilers and increasing the efficiency Under such conditions, the added boiler units can be considered as spares only during such time as some of the boilers are not in operation
Due to the operating difficulties that her overloads, it will ordinarily be found that the s at which to run boilers for such load conditions will be between 150 and 175 per cent of rating Here again the maximum capacity at which the boilers may be run for the best plant economy is limited by the point at which the efficiency drops belohat is warranted in view of the first cost of the apparatus
3rd The 24-hour variable load This is a class of load carried by the central power station, a load constant only in the sense that there are no periods of no load and which varies widely with different portions of the 24 hours With such a load it is particularly difficult to make any assertion as to the point of maximum economy that will hold for any station, as this point is more than with any other class of load dependent upon the factors entering into the operation of each individual plant
Thea load of this description vary probably more than with any other kind of load, dependent upon fuel, labor, type of stoker, flexibility of coeneral, under ordinary conditions such as appear in city central power station here the maximum peaks occur but a few times a year, the plant should be made of such size as to enable it to carry these peaks at the in of course being allowed for insurance against interruption of service
With the boilers operating at this e sacrifice in boiler efficiency is allowable, provided that by such sacrifice the overload expected is secured
[Illustration: Portion of 4890 Horse-power Installation of Babcock & Wilcox Boilers at the Billings Sugar Co, Billings, Mont 694 Horse Power of these Boilers are Equipped with Babcock and Wilcox Chain Grate Stokers]
Soiven below:
Certain plant operating conditions make it advisable, from the standpoint of plant economy, to carry whatever load is on the plant at any time on only such boilers as will furnish the power required when operating at ratings of, say, 150 to 200 per cent That is, all boilers which are in service are operated at such ratings at all ti taken care of by the number of boilers on the line Banked boilers are cut in to take care of increasing loads and peaks and placed again on bank when the peak periods have passed It is probable that this enerally used
Other conditions of operation make it advisable to carry the load on a definite nuhtly below their rated capacity during periods of light or low loads and securing the overload capacity during peaks by operating the sas In this method there are no boilers kept on banked fires, the spares being spares in every sense of the word
A thirdso the plant as divided, one part to take care of what may be considered the constant plant load, the other to take care of the floating or variable load With such athe steady load is so proportioned that the boilers may be operated at the point ofraised to a eneral installation of any apparatus leading to such results The variable load will be carried on the re boilers of the plant under either of the s of all boilers in service and banking others, or a variable capacity froain taken to indicate the very general character of any statements made relative to the economical load for any plant and to emphasize the fact that each individual case must be considered independently, with the conditions of operations applicable thereto
With a thorough understanding of theof boiler efficiency and capacity and their relation to each other, it is possible to consider more specifically the selection of boilers
The foremost consideration is, without question, the adaptability of the design selected to the nature of the work to be done An installation which is only temporary in its nature would obviously not warrant the first cost that a permanent plant would If boilers are to carry an inter load or a reversing n would have to be selected that would not tend to prime with the fluctuations and sudden dehest possible efficiency with fuel of one description, would not of necessity give such efficiency with a different fuel A boiler of a certain design which ood for small plant practice would not, because of the lie installations A discussion of the relative value of designs can be carried on alh has been said to indicate that a given design will not serve satisfactorily under all conditions and that the adaptability to the service required will be dependent upon the fuel available, the class of labor procurable, the feed water that must be used, the nature of the plant's load, the size of the plant and the first cost warranted by the service the boiler is to fulfill
TABLE 60
ACTUAL EVAPORATION FOR DIFFERENT PRESSURES AND TEMPERATURES OF FEED WATER CORRESPONDING TO ONE HORSE POWER (34 POUNDS PER HOUR FROM AND AT 212 DEGREES FAHRENHEIT)
----------------------------------------------------------------------------------------------------------------------------------------- Tee--Pounds per Square Inch | Feed | | Degrees | | Fahrenheit | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 | 130 | 140 | 150 | 160 | 170 | 180 | 190 | 200 | 210 | 220 | 230 | 240 | 250 | -----------+-----------------------------------------------------------------------------------------------------------------------------| 32 |2841|2836|2829|2824|2820|2816|2813|2809|2807|2804|2802|2799|2797|2795|2794|2792|2790|2789|2787|2786|2783| 40 |2861|2854|2849|2844|2840|2835|2832|2829|2826|2823|2821|2818|2816|2814|2812|2811|2809|2807|2806|2805|2803| 50 |2885|2879|2873|2868|2864|2860|2856|2853|2850|2847|2845|2843|2840|2838|2836|2835|2833|2831|2830|2828|2827| 60 |2910|2904|2898|2893|2888|2884|2881|2877|2874|2872|2869|2867|2865|2862|2860|2859|2857|2855|2854|2852|2851| 70 |2936|2929|2923|2918|2914|2909|2906|2902|2899|2896|2894|2892|2889|2887|2885|2883|2882|2880|2878|2877|2876| 80 |2962|2955|2949|2944|2939|2935|2931|2927|2924|2922|2919|2917|2914|2912|2910|2908|2907|2905|2903|2902|2900| 90 |2988|2981|2975|2970|2965|2961|2957|2953|2950|2947|2945|2942|2940|2938|2936|2934|2932|2930|2929|2927|2925| 100 |3015|3008|3002|2996|2991|2987|2983|2980|2976|2973|2971|2968|2966|2963|2961|2960|2958|2956|2954|2953|2951| 110 |3042|3035|3029|3023|3018|3014|3010|3006|3003|3000|2997|2995|2992|2990|2988|2986|2984|2982|2981|2979|2977| 120 |3070|3063|3056|3051|3046|3041|3037|3033|3030|3027|3024|3022|3019|3017|3015|3013|3011|3009|3007|3006|3004| 130 |3099|3091|3084|3079|3073|3069|3065|3061|3057|3054|3052|3049|3047|3044|3042|3040|3038|3036|3035|3033|3031| 140 |3128|3120|3113|3107|3102|3097|3093|3089|3086|3083|3080|3077|3075|3072|3070|3068|3066|3064|3062|3061|3059| 150 |3158|3149|3142|3136|3131|3126|3122|3118|3114|3111|3108|3106|3103|3101|3098|3096|3094|3092|3091|3089|3087| 160 |3187|3179|3172|3166|3161|3156|3151|3147|3144|3140|3137|3135|3132|3129|3127|3125|3123|3121|3119|3118|3116| 170 |3218|3210|3202|3196|3191|3186|3181|3177|3173|3170|3167|3164|3162|3159|3157|3154|3152|3150|3149|3147|3146| 180 |3249|3241|3233|3227|3222|3216|3212|3208|3204|3200|3197|3195|3192|3189|3187|3184|3182|3180|3179|3177|3175| 190 |3281|3272|3265|3259|3253|3247|3243|3238|3235|3232|3229|3226|3223|3220|3217|3215|3213|3211|3209|3207|3205| 200 |3313|3305|3297|3291|3285|3279|3275|3270|3266|3263|3260|3257|3254|3251|3249|3246|3244|3242|3240|3238|3236| 210 |3347|3338|3330|3324|3318|3313|3308|3303|3299|3295|3292|3289|3286|3283|3281|3279|3276|3274|3272|3270|3268| -----------------------------------------------------------------------------------------------------------------------------------------
The proper consideration can be given to the adaptability of any boiler for the service in view only after a thorough understanding of the requireood steam boiler, with the application of what has been said on the proper operation to the special requirements of each case Of almost equal importance to the factors mentioned are the experience, the skill and responsibility of the n of boiler selected that is best adapted to the service required, the next step is the determination of the boiler power requirements