About the authorHand Expansion Valves – 4
In the final entry on this series, the application of hand expansion valves (HEV) to liquid overfeed systems will be discussed. The application of HEV’s in this type of system somewhat differs from those of the prior discussions. When selecting HEV’s for the evaporators where liquid overfeed is being utilized the hand expansion valves take on a secondary function, which may not be obvious at first glance.
The use of hand expansion valves in a mechanically pumped liquid overfeed system are similar to control-pressure receiver (CPR) systems. The HEV is controlling the flow of liquid into the evaporator. As with the CPR system the valve is selected for the operating pressure differential (liquid feed pressure – evaporating pressure) and the required mass flow to achieve the rated performance of the evaporator.
The one major difference with this application from the others is that a refrigerant pump is used to circulate the liquid ammonia to the evaporators. This adds another layer of complexity to the overall system, since the pump performance can be defined by improperly adjusted (or improperly selected) hand expansion valves. It is important to remember that while the refrigerant pump may be selected for a specific flow and head requirement, any operating condition that changes may also impact an associated device.
In liquid overfeed systems the hand expansion valve also provides a secondary function of balancing liquid refrigerant flow to each evaporator. If the HEV is adjusted too far open (whether due to selection or adjustment) the pump head can be drastically lower. This can force a re-balance of the pump such that the pump flow actually increases. This is sometimes called riding the pump operating curve. When this occurs, the Net Positive Suction Head Required (NPSHR) increases. This can create periodic issues with cavitation and be difficult to find without being aware of the interaction between pump curves and hand expansion valves.
This commonly occurs when the refrigerant pumps are selected with very conservative operating conditions, lack of effective commissioning, and not being aware of the dependencies of the interactions of the associated components and equipment.
In closing this series, the discussion has been on the application and selection of hand expansion valves. Like most application scenarios it is common to see the same component applied in many different ways. The fundamental tasks of the system designer is to understand the requirements of the application to ensure adequate performance is achieved. It is also important to determine the full range of operating conditions that may impact the performance of the selected component or associated equipment. If the full range of operating conditions has not been carefully evaluated, various seasonal weather conditions may dictate periodic adjustments of the component.
If you have questions, please add your comments and I’ll try to address them. Thanks for visiting the blog.
Tags: analysis of refrigeration systems, cavitation, controlled pressure systems, facility operations, hand expansion valves, liquid overfeed, operating issues, System design
