International Appliance Manufacturing 2008 - (Page 16) application. When determining this, be sure to take into account the maximum allowable pressure or vacuum tolerated by all devices in the system. In this case, look especially at connecting lines and hoses to be sure they are specified properly. Define the airflow requirement at the maximum working point. It is very important that the airflow requirement be tied to the highest pressure or vacuum required, as standard airflow ratings for pumps are usually “wide open” or “full flow” – meaning at zero pressure and zero vacuum. Manufacturers usually have performance curves showing pressures or vacuum that can be delivered at various flows. Most applications do not require the maximum flow, pressure or vacuum at all times, so also define a typical working point and provide some idea of how frequently and for how long peak or maximum performance is required. This information can help prevent pump over-sizing and potentially reduce cost. If stop and restart under pressure or vacuum is a requirement, be sure to state this. 2. Motor Requirements Define your power source. If direct current (DC), specify the voltage and the source – for example, rectified AC, battery, solar, DC generator. If alternating current (AC) define the voltage as well as the frequency (generally 60 Hz in North America and 50 Hz in Europe and many other places in the world). List any power consumption and current draw limitations, and whether thermal overload protection is required. If the motor is to run at a constant speed, define how precisely must this speed be maintained. If variable flow through an adjustable-speed motor and controller is required, be sure to state that. Finally, determine the duty cycle of the application. Define as either “continuous” or, if intermittent, indicate a pattern of “minutes on, minutes off.” 3. Unit Envelope List height, width and length maximums that can be accommodated. Also, determine how much free air space will exist around the unit. Knowing this will help the pump manufacturer to know whether there is sufficient cooling air, or whether additional fans or other cooling mechanisms will be required. Pump mounting must also be considered. Mounting systems can be as simple or complex as needed to meet the specific needs of an application. When noise and vibration are important considerations, careful attention must be paid to both the mounting system and piping or hose connections to ensure that vibrations are not transmitted to surrounding structure inadvertently. There is inherent vibration in all types of reciprocating pumps due to torque pulsations that result from the motor speeding up and slowing down on each revolution in response to the build up of pressure in each cylinder and the subsequent intake of air at atmospheric pressure. Care has to be taken to isolate the compressor so that minimal vibration is transmitted to the surrounding structures and enclosures to minimize noise and vibration of the device or system in which a WOB-L pump is used. In larger pumps, an isolation system will typically involve elastomeric members. These provide some damping and snubbing capability to prevent impacting of metal parts during shipping and handling or under startup and shutdown conditions. In smaller pumps, standard elastomeric mounts are often used to provide a measure of isolation and to withstand shock loads. Often overlooked are vibration transmission paths created by piping or hose connections to the pump. These need to be flexible enough to provide isolation, just as the mounting system does. In small OEM pumps where cost is critical, isolation systems may consist of simply an elastomer squeezed between recesses on each side of the pump housing and the inner surfaces of the enclosure in which the pump is mounted, thus avoiding the need for fasteners. The elastomeric members must be carefully designed so that they are soft enough to provide adequate isolation and yet strong and stiff enough to withstand shock loading from handling and 16 International Appliance Manufacturing 2008
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