R-744 and R-717 in industrial refrigeration plants Challenges and tips for pump selection

At the pump manufacturer HERMETIC from Gundelfingen, the trend towards the use of R-717 and R744 in large refrigeration systems, which was forecast against the background of the new F-gas regulation, is confirmed. These refrigerants have no or very low global warming potential and are not harmful to the environment if small amounts escape. However, R-717 and R-744 also pose hazards to humans. HERMETIC explains the associated technical challenges for the refrigerant pumps and gives tips for pump selection. R-717 as a refrigerant is characterized not only by its environmental compatibility but also by its high level of economy. The reason for this is its good thermodynamic properties. Main disadvantage: R-717 has a pungent smell and is toxic to humans. Breathing ammonia as a highly concentrated gas can be fatal. For this reason, the use of R-717 places the highest demands on the safety and tightness of the system and components. R-744 is also dangerous of released in very large volume. The main difficulty with CO₂ as a refrigerant lies in the higher operating pressures. The need for thicker tubing and more pressure-tolerant components increases the costs of materials and construction.

CENTRIFUGAL PUMPS PREFERRED PUMP TYPE
Centrifugal pumps are preferably used as refrigerant pumps. In a conventional centrifugal pump, the liquid in the impeller is thrown outwards by centrifugal forces. In the following housing or the diffuser, this liquid speed is converted into pressure. This creates an almost pulsation-free flow. This creates a negative pressure at the impeller inlet. For this reason, it must be ensured that there is sufficient supply pressure, and that liquid is continuously provided. The weak point of this construction is the exit of the shaft from the housing, which must be sealed. The used stuffing boxes or mechanical seals can freeze and break at low temperatures. In addition radial and axial bearings are subject to wear due to their design. Error analyzes show: over 50 % of pump failures are caused by leakage at the mechanical seals / stuffing boxes and 16 % by ball bearing failures. Hermetically sealed canned motor pumps are primarily suitable for avoiding these reasons for failure.