Selecting and Specifying Pumps
The pump is the heart of any fluid loop. Like any engineering decision, the specification and selection of a pump is an experience in tradeoffs.
Small coolers and chillers have few moving parts. Other than low cost fans, the components usually are designed in-house by chiller and cooler manufacturers or self-selected from charts. Because pumps must be sourced from a vendor, they must be specified and selected in the early design stages -- whether the design is an on/off cooling loop or part of a machine that will be mass produced.
Many chiller and cooler manufacturers advertise the pump contained in their machine, boasting about its robustness and performance. Clearly, if manufacturers give this much consideration to their pumps, a bit of background knowledge is helpful when evaluating a chiller or cooler for purchase or when specifying the requirements for a built-in temperature control system.
Like any engineering decision, the specification and selection of a pump is an experience in tradeoffs. Low cost vs. long life. Quiet operation vs. high pressure capability. No one can make a proper selection without first understanding the needs of the system or application. Once the general layout is complete though, selecting the heart of the system need not be complicated.
A look at the general classifications of small recirculation pumps will provide a better understanding of the range of units available. The two main categories of pumps used in chillers and coolers are positive-displacement and centrifugal. Within each of those general categories, there are several designs, each suited to particular applications.
Gear. Gear pumps have long been a favorite when high pressure is the most important aspect of the application. Gear pumps generate up to 100 psi and beyond in standard designs. They also are relatively compact and can have ports aligned in the same plane, meaning the in and out liquid ports face the same direction. This can be helpful if the pump is not drawing fluid directly from an overhead reservoir.
Gear pumps typically are metallic though nonmetallic designs in small sizes are available. Due to their metallic wetted parts, they can be relatively inexpensive unless they need to handle deionized water or a high purity fluid.
Sliding Vane. Actually a type of centrifugal design, these pumps are classified as positive-displacement because they act like gear pumps in most ways. The sliding vanes can operate well at high speeds, but the vanes wear and can shed particles. This is something to remember if the cooling unit will be passing the pumped fluid over any sort of optics or through small voids that particulate can contaminate.
Multiple Diaphragm. Small, multiple diaphragm pumps commonly are used in drinking water and similar applications. They are inexpensive and generate respectable pressures -- up to and beyond 50 psi is common. They also are compact and, due to the mini-diaphragm design, can be made "seal-less" inexpensively. Usually, wetted parts are nonmetallic, which is a plus in most cooling applications. They can also be run dry without damage.
The multiple diaphragm pumps presently marketed do not have long design lives -- 8,000 to 15,000 hours usually is the longest life of the standard models. This means they cannot be expected to operate continuously for more than a year or two. Repair is possible but not practical for equipment makers.
Single Stage. Magnetic-drive centrifugal pumps are nearly the de facto standard for coolers and chillers that do not require more than 30 psi. They are inexpensive, quiet and operate at less than 50 dBA with design lives in excess of 25,000 hours. They can operate for five to seven years either in intermittent or continuous duty. Many designs allow flexibility in mounting with the most practical being suction up directly under a small reservoir.
Due to the relatively low pressure capability, their application flexibility is limited. Because single-stage centrifugal pumps rely on the pumped fluid for lubrication, running them dry is a roll of the dice. Most manufacturers' operation manuals suggest this will damage the pump. Size also can be an issue compared to other pump designs.
Dual/Multiple Stage. Multiple-stage centrifugal pumps have many of the benefits of the single-stage unit. How-ever, they are neither inexpensive nor compact and offer few benefits compared to positive-displacement pumps when high pressures are required.
Regenerative Turbine. Often grouped with positive-displacement pumps, the regenerative turbine has its roots in centrifugal pump design. A quick glance at the pump's impeller shows the difference lies in the paddle-wheel look and tight fluid spaces in the front of the pump. The pump actually tumbles the fluid many times as it is carried along inside the impeller. This regenerates pressure, boosting the pressure to 60 psi and beyond.
Although not a new type of pump, manufacturers of regenerative turbine pumps are only now offering nonmetallic and compact, sealless designs. If costs can be lowered, the turbine pump has great promise in chiller and small cooling equipment. The compact size, high pressure generation, relatively quiet operation (they whine at high frequency) and long design lives offer nearly all of the key design features positive-displacement and centrifugal pumps boast.
Methods of Sealing Pumps
All chiller/cooler pumps have an electric motor of some type. This motor needs to transfer its power and torque to the pump. Historically, this was done by a direct coupling of the motor's shaft to the impeller or gears of the pump. This direct coupling made it difficult to keep the fluid in the pump-end and away from the motor and environment. Keeping the fluid in the pump and coupling to the motor became a major consideration in pump design with our society's ever-growing concern about the environment.
When selecting a pump, most suppliers will either ask about the sealing method preferred or suggest a sealless design. The most commonly used methods in small pump design include mechanical seals, magnetic drive and canned motor.
Mechanical Seals. Mechanical seals replaced packing in pumps in the early 1900s. Basically, a mechanical seal is a static O-ring seal on the pump shaft and motor shaft that meet at two flat surfaces. Liquid evaporates as it travels between the seal faces and the pump effectively is "sealed." In addition to the vapor and occasional fluid weeping, seals are prone to failure in many circumstances. Most pump manufacturers have moved away from mechanical seals for industrial process pumps, and their usage is becoming rare.
Magnetic Drive. Magnetic drive pumps use magnetic force to transfer the torque of the motor to the pump's shaft. This allows the liquid end of the pump to be completely leak-free. The magnetic couplings vary somewhat, but all of the designs rely on a static seal of the liquid end and a drive magnet on the motor shaft driving a magnet inside the pump. That magnet usually is fully encapsulated in nonmetallic pumps, and the best manufacturers balance drive magnets to eliminate vibration.
Canned Motor. A mature design commonly used in large chemical pumps, the canned motor pump design is now being employed by many manufacturers of small pumps for new reasons. Canned motor pumps are similar to magnetic-drive designs with one major difference. Rather than couple the two shafts of the pump and motor together, the canned motor makes the rotor of the motor part of the pump. Usually the rotor is a magnet affixed to, or actually part of, the pump's impeller. This makes the total package extremely small and eliminates a set of bearings.
Before You Select a Pump
To ensure proper pump selection, consider several items prior to selecting a pump:
Fluid to be handled (especially if the fluid is deionized water or Fluorinert, both of which have special needs).
Estimated fluid pressure to be overcome.
Maximum temperature the pump will see. Is the pump on the hot or cooled side of the heat transfer device?
Required fluid flow rate to manage the heat load.
Space available for the pump and location of reservoir.
Required lifespan in k-hours.
Voltage available to power the pump. Include any special notes such as 50 Hz operation required.
Required approvals (UL, CE, CSA, medical leakage current certification, etc.).
John Goreham is product development manager at Iwaki Walchem Corp., Holliston, Mass. For more information, call (508) 429-1440 or visit www.iwakiwalchem.com