Spindle Screw Pump Technology for Medical Cooling

Spindle screw pumps feature a simple construction that offers high reliability, performance and efficiency for liquid cooling systems. Comprised of a single rotor element that is powered, while two others are idle, spindle screw pumps move fluid axially without turbulence, eliminating foaming that would otherwise occur in viscous fluids. The idle rotors are rotated by liquid pressure, essentially generating a fluid bearing, or hydrodynamic film, that provides radial support similar to journal bearings. To get more news about NETZSCH Pump Accessories, you can visit hw-screwpump.com official website.
Spindle screw pumps move fluids of higher viscosity without losing flow rate, and pressure changes have little impact on spindle screw pumps This is important in a rotating application, such as a CT gantry system used in a CT scanner, as the pump is able to maintain constant flow and pressure while under high stress caused by high g-forces.
One of the most critical components in a liquid cooling system is the pump. Pumps are in constant operation when the unit is turned on and typically have the shortest mean time between failure (MTBF) of all components due to friction wear of bearings, pump blades and seals. If poorly chosen, the pump will fail prematurely and the liquid cooling system will fail and cause the end instrument to stop working. This can be a problem for expensive medical, laser or semiconductor equipment where hundreds of thousands of dollars could be lost per day. Compared to centrifugal pumps, spindle screw pumps provide higher reliability, performance and efficiency for liquid cooling systems.

Types of Pumps
Many different types of pumps are available for liquid cooling systems, which can make it difficult choosing the appropriate pump. Each configuration has its own advantage and disadvantage when compared to each another. Positive-displacement pumps like gear pumps, rotary pumps, vane pumps or centrifugal pumps are the most common types of pumps available for medical, industrial laser or semiconductor use. They are cost-effective but can have shorter operating lifetimes and tend to be noisier. Typical operating life is about 9,000 hours.
Designing a liquid cooling system requires specifying the pump head and motor, which is typically sold separately for larger flow rates. Pump performance can be impacted by the motor size, as a poor performing pump head will require a stronger motor. As a result, the motor will be larger, heavier, louder and consume more power than necessary. Another problem that arises from this is the additional heat generated by the larger motor will transfer to the pump and coolant. This will drop the cooling capacity and make the liquid cooling system work harder to compensate for the additional heat transfer losses. Most motors run on AC power due the lower cost of eliminating a universal power supply. Single-phase AC motors are well established and are typically cheaper than three phase AC motors, but are also bigger and less efficient.

Failure Modes
Internal mechanical components limit the pump’s operating lifetime because of friction wear. Customers have to replace the pump after reaching a defined MTBF, which is typically 12K hours. This will result in higher service and maintenance costs which are often over looked at time of purchase.
To get the best performance in terms of pressure and flow rate, the tolerances of the mechanical components have to be very precise. For example, the gear pump is a high precision machine with extremely tight fits and tolerances. At a minimum, metal to metal contact is a given for moving parts inside the pump. This can create problems of high friction and abrasion. To reduce friction wear particles contained inside the coolant require a low mesh strainer. This will prevent large particle sizes from entering the narrow clearance tolerances and damage the mechanical components inside the pump. Cavitation, the sudden formation of low pressure bubbles, can also reduce lifetime, generate higher noise and lower pressure drop.

Other Issues
Most pump types do not self-prime, i.e. push coolant through pump from initial start without being gravity fed by coolant. Therefore, the pump needs to be located below the tank reservoir. Most pump types also pulsate when pushing coolant through a liquid circuit, which can be unusable for higher end applications. Pulsations will cause disturbances to maintaining peak performance and also increases vibration, which reduces operational life of a high-end system. To get the best performance and efficiency of a pump, tight tolerances of internal moving parts are required. The tighter the tolerance the better the performance, but this also increases noise and motor performance requirements due to the increase in friction. The tighter clearance of the working parts inside a gear pump are what enable it to efficiently pump coolants in high pressure environments. Low viscosity coolants such as water with glycol or other solvents have more of a tendency to “slip” through these tight clearances due to the higher-pressure discharge side of the pump back to the lower-pressure suction side of the pump. The phenomenon of slip causes a reduction in flow rate and pump efficiency.

Slip is a characteristic of positive displacement pumps and is defined as the quantity of fluid that leaks through internal clearances of a pump per unit of time. It is dependent upon the internal clearances, the differential pressure, the characteristics of the fluid handled, and, in some cases, the speed.