Sealless Pumps Reap Energy Savings

Seal-less magnetic pumps, or mag-drive pumps, have become the preferred option for a variety of conditions in the ammonia refrigeration industry, mainly because they take the heat generated by the pump’s electric motor out of the cooling process.

Sealless magnetic-drive pumps (MDP) are equipped with totally enclosed fan-cooled (TEFC) motors, offering versatility for a variety of conditions in ammonia and carbon-dioxide refrigeration processes. Because the MDP avoids adding motor heat to the system’s cooling load, the result can be significant energy savings. The other type of sealless pump is a cannedmotor pump (CMP), which is totally enclosed liquid-cooled (TELC).

“The electrical motor on our magdrive pump is fan-cooled by ambient air temperature, so we’ve removed the internal motor heat from a cooling process,” says Joe Warrender, general manager of Warrender, Ltd, which has specialized in manufacturing sealless magnetic pumps for more than 30 years. “That is the primary source of heat, and adds to the overall heat load. Canned motor pumps must cool the electric motor windings with process liquid,” he said.

“Another source of heat is the resistance to the magnetic field through static metallic barriers, with MDP rear casing or CMP isolation shell,” Warrender said. “As opposed to CMP’s electromagnetic field, the MDP has a permanent magnetic field in the drive coupling. Magnetic hysteresis losses from either type of field is reduced exponentially by lowering the rotational speeds.”

A MDP rear casing with twice the wall thickness of a CMP, operating at 1,800 rpm, has comparable magnetic energy losses or resistance as the canned motor shell with the same metallurgy, operating at 3,600 rpm. Therefore, both canned-motor pumps and magnetic pumps generate eddy currents.

However, the heat load of TELC canned-motor pump windings is based on motor stator heat, independent of pump speed. For example, the CMP “reverse flow” by-pass line is necessary to manage liquid vaporization through the flow circuit. The CMP internal motor heat induction is more than five times that of the corresponding MDP magnetic drive coupling.

Continuous-duty recirculation pumps (vs. intermittent transfer) represent a more significant impact on energy consumption. Motor full-loadamp ratings are one indication. However, the heat rise through a pump’s internal flow and cooling circuit is the primary determining factor.

There are four heat induction sources common to both MDP and CMP systems: pump hydraulic efficiency and internal recirculation, which varies with operating point and pump efficiency; eddy current losses, factored by speed, grade of alloy and thickness; motor efficiency; and internal bushing friction, factored by speed.

Heat induction sources unique to CMP include motor stator winding heat and motor armature/rotor slippage.

Magnetic-drive pumps utilize standardized motors that are accessible without decommissioning and evacuating the pump of ammonia. Thus, the MDP pump and motor offer the ability for field repair. Lower operating speeds provide stable NPSH (Net Positive Suction Head), without flow restrictive inducers, with lower wear coefficients, and extended pump life cycles. “When operating a CMP at 3,600 rpm, NPSHa (Net Positive Suction Head Available) can be marginal [between the system suction head pressure and the pump’s Net Positive Suction Head Required],” Warrender said. “Operating with marginal NPSHa can affect pump life and potentially lead to vaporization and cavitation within the pump.”

Operating at higher speeds may compromise the liquid film on the wearing surfaces. Compressed gases such as NH3 and CO2 have low viscosity and specific heat values, and can vaporize through the pump lubrication and cooling circuit, increasing wear on the shaft sleeves, journal and thrust-bearing surfaces. Additionally, bearing-wear coefficients drop exponentially with speed.

CMP internal motor windings continue heating the liquid even after the pump stops. That means it may be necessary to operate continuously to avoid vapor-locking the pump, thus consuming excess energy. “We’ve developed a sealless transfer pump that will overcome the pressure in the highpressure receiver and transfer directly from a control pressure receiver or lower pressure vessel,” Warrender said.

Ammonia transfer pumps have long caused issues for conventional pumps due to the risks of extremely low NPSH, potential entrained vapor and frequent on/off cycling. Pump life is severely diminished with low NPSH, while on/off cycling can lead to flashing and vapor locking.

Steep pump performance curves, with higher rise to shut-off, accommodates varying head conditions for improved reliability. The rise to shutoff is the determined operating point before flow ceases and the pump is damaged – in other words, how much head pressure is generated before the pump reaches a shut-off condition.

“NH3 vapor pressure and corresponding head conditions can vary with ambient temperature,” Warrender said. “Our mag-drive transfer pumps can generate very high head pressure that compensates for variations in discharge heads due to temperature fluctuation. We have two-stage and three-stage pumps. The three-stage pump can deliver liquid from the sidewalk to the sky deck of the John Hancock Tower in Chicago, which is 1,100 feet up.”