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Motors are designed with certain speed-torque characteristics to match speed-torque requirements of various loads. A motor must be able to develop enough torque to start, accelerate, and operate a load at rated speed. The National Electrical Manufacturers Association (NEMA) has established class designations for motors on the basis of motors' starting-torque and accelerating loads. The four standard NEMA designs are NEMA A, NEMA B, NEMA C, and NEMA D. NEMA A motors usually are used for applications that require extremely high efficiency and extremely high full-load speed. NEMA A-design motors are special and are not used very often.
We have developed an explosionproof electric unit heater system to operate in ambient temperatures down to -30F. (-34C.) [-70F. (-57C.) with modification]. The self-contained system includes a blower, heating elements, controls and provisions for focusing the warm airstream on provisions for focusing the warm airstream on personnel up to 20 feet (6.1 meters) away. personnel up to 20 feet (6.1 meters) away. Ratings up to 91 KW (310,000 BTU/HR) will provide air temperature rises as high as 164F. provide air temperature rises as high as 164F. (91C.).
Comfort heating on the deck of a rig operating in arctic and subarctic climates presents a unique set of engineering challenges. Traditionally steam unit heaters, consisting of a steam coil and explosionproof blower mounted in a cabinet, have been used. However, this requires a source of steam, usually a separate boiler, to feed the unit heaters. Both the boiler and the lines running to the heaters are high maintenance items, especially in harsh environments where they tend to freeze up. If insulation is not properly replaced after repair, freezeup will properly replaced after repair, freezeup will soon reoccur.
To avoid the problems associated with a steam system, we were asked to develop an electric explosionproof unit heater system for use on a large semi-submersible rig that was placed in service early last year. The design was to meet the following criteria:
1. Safe operation in a methane gas atmosphere and prompt, safe shutdown when hydrogen sulfide is detected.
2. Resistance to salt spray corrosion.
3. Qualification to U.S. Coast Guard requirements which involves approval by either the Underwriters Laboratories or Factory Mutual Laboratories.
4. Airflow of at least 2000 standard cubic feet of air per minute (944 dM3/S).
5. A capacity of 91 KW (310,000 BTU/HR) at 575 volts, 3 phase, which produces an air temperature rise of 148F. (82C.).
6. Suitablility for operation in an ambient temperature of -30F. (-34C.).
7. Focusing the airstream to achieve 50F. (28C.) minimum temperature rise 20 feet (6.1 meters) away from the unit outlet.
8. Rotation of the entire unit in a horizontal plane to any one of eight positions, and/or tilting it at 15, 30 or 45 to the horizontal.
9. Individually replaceable heating elements.
10. A protecive housing around the entire assembly to reduce the chances of accidental damage.
To operate safely in an environment containing combustible gases, electrical equipment must satisfy two basic conditions:
First, any electric device that might produce a spark, such as a magnetic contactor, fuse or disconnect switch, must be housed in an explosion proof enclosure that will contain an explosion without rupturing. (The British term "flameproof" is equivalent to the U.S. term "explosionproof.") Cast aluminum enclosures are commonly available for such service. To gain access to the electrical components, these enclosures are normally furnished with bolted covers. The mating surfaces of the cover and box are machined flat to within 250 micro inches (635 micro centimeters) and must be at least 3/4 inch (1.91 centimeters) wide so that the flame resulting from an internal explosion will be cooled before it reaches the outside atmosphere.
The prime mover (PM) rotates the gear-reducer gears through a V-belt drive. The two most common PMs are electric motors and internal combustion (IC) engines. These considerations, as well as other factors, have been discussed in numerous publications. The characteristics of these engines are summarized here, and the detailed comparisons and field experiences have been published elsewhere. These test data should be requested and furnished to the purchaser from the manufacturer. The data should include the manufacturer's curves showing the torque, maximum brake HP, and the rated-brake HP vs. engine speed. These are important to know the speed range in which the engine would be able to operate. A general guide for installation and maintenance of gas engines is API RP 7C-11F,  which covers all three types of engines and includes a troubleshooting section.
Electrical grounding can be classified in either system grounding and equipment grounding. Requirements for system grounding are covered in detail in the Natl. System grounding includes grounding of the power supply neutral so that the circuit protective devices will remove a faulty circuit from the system quickly and effectively. To protect personnel from electric shock, all enclosures that house electrical devices that might become energized because of unintentional contact with energized electrical conductors should be effectively grounded. If the enclosures are not grounded properly, unsafe voltages could exist, which could be fatal to the operating personnel.