Coolants and Refrigerants
In the refrigeration business, we focus on two areas of mediums used for heat transport. The coolants who are the “secondary refrigerants” like brines and glycol. You find them in low temperature applications and especially for the use in more complex piping networks. Most of the time regular centrifugal pumps are in use for this area.
For colder applications [-40°C; frozen food processing; freezing; deep temp. storage], the refrigerant fluids generally employed are ecologically harmless “primary refrigerants” such as CO2 or ammonia.
This article is about the selection of these primary refrigerants related pumps.
Because these liquids [CO2 and ammonia] are dangerous to people if leaked to atmosphere, pumps have to be sealed in a special way. One way is the use of a “canned” motor-pump, the other by using a magnetically coupled pump.
Two main types of pump – Canned Motor
Canned motor-pumps have been very popular of recent times because of their safety and the higher cost of “mag-drive” pumps. There are also several high quality manufacturers of these pumps. The canned motor pump has a hermetically sealed casing around the motor, which is directly coupled to the pump. There is therefore no need for mechanical seals and the chances of a leak is very minimal. The only draw-back of this design is that pumped fluid needs to pass through the motor chamber to keep the motor cool, therefore using part of the motor’s energy to provide this function.
Two main types of pump – MAG DRIVE
Of recent times, the cost of magnetically coupled pumps has reduced, and German pump manufacturer, EDUR, has produced a line of refrigerant pumps using this principle. Having a magnetic coupling means the pumps are hermetically sealed without having to use specialised motors or use the pumped media for motor cooling. This has several benefits.
Firstly, off the shelf, mass produced TEFC motors can be utilised in lieu of specialist motors used in the canned motor-pumps. And because of the spatial separation between motor and pump, heat transfer into the medium is reduced or avoided, benefiting the energy balance of the system. Finally, and most importantly, because energy is not being used to pump fluid over the motor, EDUR pumps are more efficient – a benefit when pumps are running 24 hours per day, 365 days per year.
Any pump used in refrigerant systems needs to have a low NPSHr because the pumped fluid will always be close to its “boiling” point. It is also best if the selected pump is able to handle certain levels of gas content.
Both canned motor-pumps and EDUR magnetically coupled pumps have low NPSHr, but because of their “diffuser devices”, EDUR pumps handle some evaporation of refrigerants reliably.