Industrial Plant Design/System Planning/Determination of Loads
Determination of Loads
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General
Determination of the load is the Electrical Engineer's first problem and may be difficult to solve. The size and number of primary and secondary substations, the size, number, and arrangement of primary feeders, and the type of secondary distribution are largely dependent on the amount and nature of the load and its distribution.
The plant distribution system usually must be designed before all loads are known. This is at a time when the equipment layout itself is only in the formative stage. Equipment may be bought piecemeal during which time changes in machines are taking place either in number or size. Ideas are changed by the impact of what is commercially available, or manufacturers’ recommendations for improved models, better ways of securing a given result, or competitive conditions. Manufacturing processes are being changed as available equipment is fitted into the prospective production schedule.
Many plants are built to manufacture new products, which adds to the difficulty of establishing power requirements. Plant layouts are subject to considerable modification of the original scheme. Entire plant rearrangement may be necessary in the middle of a job; air and refrigerating compressors, fans, blowers, and pumps may come into the picture or shift position rapidly oil-fired annealing furnaces may become electrically heated as a result of laboratory tests that prove a controlled atmosphere necessary, thus adding hundreds if not thousands of kilowatts to the plant load.
Even after a plant is in operation, loads may change in size and location. New models, new products, and production methods call for continual change in the distribution system, bu. These changes can be minimized by careful planning.
Preliminary Loads
Preliminary estimates of loads is a problem deserving the closest study. These estimates may have to be used as the basis for major decisions. At this stage in the plant design, the Electrical Engineer often has available only a few building layout drawings or perhaps a plant map. The general locations of the major pieces of equipment will usually be roughly indicated and their power requirements may or may not be known. Starting with this information the Electrical Engineer must call on all his knowledge and experience as well as on that of other plant engineers and designers to enable him to arrive at an estimate which will stand up as the loads become better defined. In most cases, it is better to consider the lighting and power loads separately and combine them later to determine the demand in any one area, since present practice is usually to supply these loads from a load-center substation.
The factors most frequently used in determining distribution system loads are as follows:
- Demand Factor
- The ratio of the maximum demand on a system to the total connected load of the system. The maximum demand is usually the integrated maximum kilowatt demand over a 15 or 30-minute interval, rather than the instantaneous or peak demand.
- Diversity Factor
- The ratio of the sum of the individual maximum demands of the various parts of a system to the maximum demand of the whole system.
- Load Factor
- The ratio of the average load over a designated period of time to the peak load occurring in that period.
Information on the demand and the diversity factors for the various loads and groups of loads is needed to design the system. For example, the sum of the connected loads on a branch load circuit, multiplied by the demand factor of these loads, will give the maximum demand that the branch circuit must carry.
The sum of the maximum demands of the branch circuits associated with a sub-load center or panelboard divided by the diversity factor of those branch circuits will give the maximum demand at the sub-load center and on the circuit supplying it.
The sum of the maximum demands of the circuits radiating from a load center, divided by the diversity factor of those circuits, will give the maximum demand on the transformer at the load center. The sum of the maximum demands of the load-center transformers divided by the diversity factor of the transformer loads will give the maximum demand on their primary feeder. With the use of the proper demand and diversity factors as outlined above, the maximum demands on the various parts of the system from the branch load circuits to the power source can be determined.
Lighting Loads
Estimating the lighting is usually not difficult. For rough estimates, only the area of the building and the illumination level desired need be known. For more accurate estimating, the general type of construction must be known as well as mounting height and spacing and location of roof trusses and columns, so that the optimum arrangement can be checked against physical requirements. The intensity of illumination and the type of lighting (mercury, fluorescent, or incandescent) desired together with general construction features will make possible computation of load using formulas in lighting handbooks or data from fixture manufacturers.
For quick estimates of lighting with the most efficient fluorescent units, approximately three watts per square foot will provide 50-footcandle illumination. If incandescent lamps are used, the wattage will be approximately twice as much.
Information on outdoor lighting is readily available. Fence lighting can be estimated at 200 watts per 100 feet. "Yard lighting" is often adequate if a 200watt lamp is placed every 100 feet along the exterior walls of buildings. The total outdoor lighting will seldom exceed 25 percent of all lighting and it may be as little as 5 percent; hence, a rather nominal allowance in primary substation and feeder capacity will provide for considerable leeway for changes in the outdoor lighting which may develop late in the construction period.
Table 1.1 gives lighting requirements in various industries.
| Industry | Lighting in percent of the Total Connected Load (Percent) |
|---|---|
| Steel Foundries | 1 to 3 |
| Steel Rolling Mills, Oil Refining | 3 to 5 |
| Heavy Electric Equipment, Wire Drawing | 5 to 8 |
| Auto Equipment, Baking | 8 to 10 |
| Machine Parts | 10 to 15 |
| Auto Assembly and Parts | 15 to 25 |
The diversity factor of lighting load will be low and the demand factor of the lighting connected to any load center should be considered as 100 percent.
Power Loads
Estimating power loads is considerably more difficult that estimating the lighting load. Table 1. gives estimated load densities in representative industries.
| Type of Plant | Volt-Ampere Demand Light and Power (VA/sq.ft) |
|---|---|
| Airplane Factories | 15 to 25 |
| Beer, Sugar Factory and Refinery | 19 |
| Paper Mills | 14 |
| Textile Mills, Engine Builders | 12 |
| Cigarette Manufacturing | 11 |
| General Manufacturing, Chemicals, Electronic Equipment | 10 |
| Small Appliance Manufacturing, Machine Repair Shop | 7.5 |
| Lamp Manufacturing | 5 |
| Small Device Manufacturing | 3.5 |
Table 1.2 should be used for preliminary estimating only since the size of the plant and its processes will vary considerably within a given industry category.
Demand Factors
When the loads of individual machines or areas are known, it is necessary to combine them to obtain the maximum demand. The maximum demand determines the system capacity which must be provided to supply it as well as optimum system voltage, It is determined by applying demand and diversity factors to the connected load, The selection of demand and diversity factors, like load density, is based on known conditions, experience, and similar operations in existing plants. Table 1.3 gives the factors for the more common types of manufacturing loads and is the factor by which the connected load must be multiplied to obtain total plant demand assuming diversity factor of one, If the diversity factor is known, the demand thus obtained should be divided by the diversity factor to obtain the actual demand.
| Load | Estimating Demand Factors (Percent) |
|---|---|
| Arc Furnaces | 100 |
| Arc Welders | 30 |
| Induction Furnaces | 80 |
| Lighting | 100 |
| Motors | |
| 1. General Purpose, Machine Tool, Crane Elevators, Ventilation, Compressors, Pumps, Rolling Mills, etc. | 30 |
| 2 Small Appliance Manufacturing, Machine Repair Shop | 60 |
| 3. Continuous Operations Textile Mills, Chemical Plants, etc. | 90 |
| Resistance Ovens, Heaters and Furnaces | 80 |
| Resistance Welders | 20 |
As the table shows, the demand factor will vary considerably with different types of loads. For example, the demand factor of a group of motors driving a conveyor belt will approach 100 percent, while the demand factor of a group of hand tools in a small furniture factory or machine shop might be only 10 percent A diversity factor of unity is often used to provide ample system capacity, since the margin provided in this way is soon used by load growth. Although a diversity factor of unity represents the sum of the maximum demands of the individual load centers and of the equipment applied at the distribution voltage on the distribution system, the usual practice is to provide a system adequate for a maximum demand obtained with a diversity factor of unity or to provide even more than 100 percent capacity in the main system to take care of load centers which would be added in the future.
Power Requirement Per Bulk Product Weight
Any of a wide variety of plants may come under consideration in making a load survey. A knowledge of approximate KVA load necessary to produce unit weights of the material being made is a great help. This knowledge may come from past experience or available published material. It would be impossible to list all types of industries here, but requirements for some of the more important industries follow:
| Product | KilowattHours per Pound of Product |
|---|---|
| Gasoline | 0.0015 |
| Liquid Sulphur Dioxide | 0.002 |
| Glycerene | 0.007 |
| Ammonium Phosphate | 0.007 |
| Sulphuric Acid | 0.016 |
| Formaldehyde | 0.030 |
| Tri-Sodium Phosphate | 0.038 |
| Portland Cement | 0.050 |
| Ethylene Oxide | 0.070 |
| Alumina (ex. Bauxite) | 0.090 |
| Nitric Acid | 0.180 |
| Synthetic Ethyl Alcohol | 0.300 |
| Electric Steel | 0.330 |
| Carbon Disulphide | 0.450 |
| Benzene Hexachloride | 0.600 |
