Enclosure Applications

Motor enclosure types for compressor service under shelters are applied as follows:

Division 1 Areas a. Force ventilated b. Totally enclosed inert gas or instrument air-filled

Division 2 and Nonhazardous Areas a. Drip -proof b. Modified drip-proof (air intake at bottom)

The force-ventilated type is preferred when a Division 1 classification is necessary. The standard drip-proof enclosure is preferred in Division 2 or in safe locations. For an outdoor installation, serious consideration should be given to using NEMA Type 2 weather-protected enclosures rather than the completely enclosed types.

In the force-ventilated or drip-proof types, the cooling air passes directly over the motor winding insulation. As a rule of thumb, a motor requires about 4,000 cfm of cooling air per thousand horsepower. Therefore, the possibilities of airborne dust and dirt collecting on the winding must be considered carefully. Filters in the air intake will lessen the hazards of this condition. Not much can be done about moisture drawn in with the cooling air, but careful selection of motor location inside the shelter can minimize the amount of wind-blown rain striking the motor.

On first examination, the totally enclosed inert gas or air-filled enclosure with a closed ventilating circuit using a gas-to-water heat exchanger would seem the best answer for all locations. However, the cost of this enclosure runs at least 75% more than the drip-proof enclosure. Also, the auxiliary requirements of this type enclosure must be considered. It requires an external cooling water supply of good quality and high relia bility. Where salt water or fresh water with corrosive impurities is used, double-tube heat exchangers are necessary. A cooling water failure alarm and subsequent automatic shutdown feature and enclosure moisture detector and alarm must also be provided. In addition, an external supply of inert gas or instrument air for leakage makeup is required. All factors considered, this type enclosure requires a fairly extensive auxiliary installation. The service factor for this enclosure is normally 1.0.

The force-ventilated enclosure, in addition to the disadvantage of con tact between the cooling air and the winding, requires external ducting and a pressuring blower. Standby blower arrangements are necessary because the motor is not self-ventilating on loss of the pressurizing blower. The service factor for this enclosure is normally 1.0.

The drip-proof enclosure offers a ventilating system that is not depen dent on external auxiliaries. Its installed cost is the lowest of all the enclosure types. Standard drip-proof ratings above 500 rpm may have a 1 15

service factor. The winding insulation is exposed to all the hazards associated with direct contact with cooling air. Also, the higher noise levels of this type may be objectionable, if noise level is a design consideration.

Recognizing all the factors mentioned, the drip-proof type has been the favored enclosure for service in shelters classified Division 2 or in safe areas because it offers adequate protection at the lowest installed cost (see Figure 7-6). In most applications, air-intake filters and screens have been provided. In one instance, a modified drip-proof enclosure was installed with provision for future filter installation.

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