Pumping Liquid CO²
AxFlow looks at pumping CO² for cold rooms and freezers
The changing face of food distribution and retailing is having a knock-on effect throughout the many industries that supply building services and associated equipment. As retail outlets in the food sector get bigger and turnaround of frozen and perishable goods becomes faster, it is becoming more common for stores to have extensive cold rooms and freezer facilities. Consequently, essential items of equipment originally developed for industrial plant have to be adapted or redesigned for the retail environment.
Where freezer cabinets and cold store facilities are required in retail outlets, there is the rather delicate matter of pumping CO². This highly versatile material is fast becoming popular for supermarkets because it is ideal for refrigeration and for maintaining perfect atmospheric conditions during the transportation of food products. However, it is 1.5 times as heavy as air, is almost odour-free and potentially life-threatening should air contain more than 10% be inhaled. Because of the danger of gas leakage to the atmosphere, the low viscosity of CO² in liquid form and the requirement for it to be pumped at high pressures due to its weight, means that the pumps used must provide total containment.
When pumping CO² in a liquid form users must be aware of the difficulties that this compound can present. Firstly, it is a low viscosity product so with a conventional centrifugal pump leakage can occur across the faces of the mechanical seal resulting in gas escaping into the atmosphere. Secondly, ice forms on the pump components and this will cause the mechanical seals to freeze resulting in pump failure. For these reasons the option should be to select a seal-less or canned pump. Even when completely encased in ice, the canned pump will continue to operate.
The double containment design free of a shaft sealing device makes the canned motor pump one hundred percent leak free. Even in the unlikely event of a stator liner rupturing there is no leakage to atmosphere. Furthermore, the absence of mechanical seals and the employment of maintenance free slide bearings, as opposed to roller bearings, ensure optimised MTBF (Mean Time Between Failure).
Without there being any shaft penetration, torque is transmitted electro-magnetically whereby rotation is generated by the stationary motor stator winding. Both parts are inherently separated by a thin stator liner as the sealing element, this typically being manufactured from a Hastelloy material for minimum electrical losses during operation.
These factors make the canned motor or seal-less pump ideally suited for the safe handling of potentially lethal liquids. Based on international environmental laws, as well as its own intentions to reduce emissions, the pump industry is not only seeking permanent improvements in production, performance and maintenance, but also control and reductions in risks both the employees and the environment.
Hermetic Pumps, specialises in the design and manufacture of hermetically sealed centrifugal pumps for difficult pumping applications. Responding to the changing requirements of the refrigeration industry, Hermetic now offers three pumps specifically for CO² duties all with nominal pressures up to 40 bar. The CNF, CAM and CAMR models are of mono-bloc design (pump and motor share a common shaft) and as such are quieter, more compact and easier to install than conventional pumps. They do not require a base plate, couple or coupling guard, nor is there a need for alignment of motor and pump to ensure totally leak-free operation.
The CAM and CAMR 40 bar models are built with two to five impeller stages to suit the application and offer a capacity of up to 50m3/h and a head of up to 130m. The CAMR series is a special version designed for compact plants with small collecting vessels. This enables space saving by mounting the pump directly under the vessel. Furthermore, it permits gas to escape through the suction port, allowing shorter re-starting times. The Hermetic CNF series differs in that it is a single stage pump with an additional impeller, enabling pumping of fluids with a steep vapour pressure diagram.
Typical applications for these three pumps include cold storage depots, supermarkets, slaughter-houses, breweries, dairies, cool ships and rail tankers. Units can be supplied with a constant flow regulator designed for installation at the refrigerant outlet. With this regulator, it is possible to accurately control the flow rate and enable pumps to function safely in a range not normally available to pumps using discharge orifices. Furthermore, operation close to the unrestricted pump flow curve can be achieved without exceeding the calibrated quantity of flow.
Hermetic is one international; pump manufacturer that is adapting its long proven CO² handling experience and making it available to retail outlets that see the business potential in increasing their refrigeration infrastructure and require pumps that meet their specific requirements.