by Francesco Dioguardi, DH Industries BV, email@example.com; Martijn Donkers, DH Industries BV, M.Donkers@dh-industries.com; and Harrie Vermeulen, DH Industries USA Inc., firstname.lastname@example.orgDH Industries designs and builds closed loop cryogenic cooling systems, which often require a pump as a driving force. These pumps, and their efficiencies, will increase the heat load of the system. As cryogenic cold, especially if provided by cryocoolers, is expensive and typically limited in supply, it is important that these pumps be as efficient as possible.
Typically, cryogenic pumps are not optimized for small flows, high efficiencies and low heat in-leak. This is due to the nature of the industry: Large quantities of cryogenic fluids (nitrogen, oxygen, LNG, etc.) often need to be transferred in a short period of time—as in the filling of tanks—and inefficiencies are not a major issue because the cryogen is intended to be used in gas phase anyhow. Because there were no pumps with the right characteristics available on the market, CryoZone developed their own liquid gas pump, based on the design concept of their successful CryoFans.
High efficiency liquid gas flow for closed loop cooling systems
The liquid gas circulation pump has been optimized for closed loop cooling systems. These pumps are used to circulate a liquid gas in order to transport cooling power from a cold source into and through an application. Examples are superconducting cables and thermal shields in vacuum chambers or other large devices. The required cooling power usually comes from a cryogenic system using a cooling machine or bulk liquid. In either of these cases there is a limited budget of cooling power, which makes the efficiency of the pump of the utmost importance. The efficiency of CryoZone’s liquid gas pumps ranges from 30 to more than 50 percent, depending on the setup of the closed loop system. Important factors are the required flow versus the pressure drop over the system; by balancing these at the system design phase, the total setup can be optimized.
Design concept of the liquid gas pump
CryoZone’s liquid gas pumps are based on the generic design concept of its successful CryoFans, which resolves similar issues. Like all centrifugal pumps, the main parts are the impeller and the volute. Their dimensional shape determines the functionality and efficiency of the pump. The impeller is driven by a high rpm air-cooled electric motor to create the flow. All of the rotating components are integrated into a single housing with the motor installed inside the pressurized gas volume, but outside of the cryogenics.
In order to create this thermal barrier between the motor at ambient temperature and the cold impeller, the latter is mounted on a long stainless steel shaft in a cantilever setup with a high precision balanced shaft and impeller. The only bearings are therefore located in the motor operating at normal temperatures, so the usual reliability issues with cryogenic cold bearings are eliminated. This setup ensures a mean time between maintenance of over 30,000 hours. Because the motor is at system pressure, this solution does not require a rotating seal, minimizing the possibility of leakage. The only feed-through is for the electrical connectors (see Figure 1 for a schematic drawing).
Once a sufficiently low temperature has been reached, the system can be switched to liquid mode and the pump will be set to run slower for use with liquid. The motor of the pump is driven using a VDF, so its pumping capacity can be regulated as required. The pump has a capacity of approximately 30-60 liter/hr, a pressure head of 15-40 meter (~1-5 bar for liquid nitrogen) and can be used for most cryogenic liquids. Designs for oxygen and LNG are under development.
Only a central bore with a flange pattern is required to integrate the liquid pump concept into a system cryostat. This pattern can be machined in the cryostat flange or in an ISO-K or CF blind flange. The pressure housing with pump volute is mounted inside the cryostat and the circuit lines are connected. The motor with shaft and impeller can now be mounted from the outside. This means that the complete impeller unit can be removed without breaking the vacuum of the application. Normally the pump is supplied as a loose item that needs to be integrated in the customer’s application (cryostat).
Typical applications in which this type of pump can be used include high temperature superconductor cable cooling, space simulation chamber cooling, LNG spray pumps, cryogenic liquid densification and cryogenic liquid transfer where the cryogen has to remain liquid.
To illustrate the application of the CryoZone pump and the importance of its efficiency, let’s assume a closed loop cryogenic cooling system using sub-cooled liquid nitrogen at 70K and 5 bar, for instance to cool a high temperature superconductivity cable or a space simulation chamber. With a heat load of the application of 3.5 kW and a delta T across the application of 2.5K, the required flow will be around 50 liters/min.
This required flow will induce a pressure drop over the connecting VJ lines and application (heat exchanger). The flow time’s pressure drop results in watts of labor that the pump will have to put into the liquid. The pump gets the blame for these heat losses, but in reality it is the geometrical design of the application that causes these losses—i.e., the friction of the system.
It is, of course, easy to select a large pump that can overcome this, but this will come at high cooling power losses. To reduce these, the flow losses in the application should be considered and kept to a minimum in the design stage.
This is why the CryoZone liquid pump will create only about 33 m of pressure head, with an efficiency near 50 percent. For displacement pumps, 2.7 bar is very low and seems useless; however, the CryoZone pump is intentionally created this way. If the CryoZone pump seems too small, the application most likely is not designed efficiently for flow.
At the given 50 liters/min and 2.7 bar, the labor to move the LN2 is 225 W. This is a result of the application, not the pump.
Pump efficiency is about 50 percent, and its static heat losses are around 20 W. Therefore, total heat load into the liquid is only 470 W.
If the application were to be designed for 10 bar and 100 liters/min. because a conventional pump can overcome this dP anyway, the labor into the liquid would be eight times greater. This would result in a total loss of more than 3.5 kW, even assuming that the conventional pump has the same low static loss and high efficiency as the CryoZone pump.
So, the system with the CryoZone pump and the optimized piping for dP would require a cooling capacity (cryocoolers) of approximately 4,000 W, versus a system with a conventional pump, which might need as much as 8,000 W. As it is well known that cryogenic cold is very expensive, the slight additional cost for an optimized system and efficient pump will outweigh the cost of this extra cooling capacity. www.d-h-industries.us