Multiple Riser Pipes for Falling Film Evaporator Pressure Drop Control
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Solution Overview
Problem
Falling film HVAC systems using low pressure refrigerants face significant frictional pressure drops due to reduced pressure differential, limiting flow control and efficiency when the condenser and evaporator are arranged side-by-side, as the momentum of the two-phase refrigerant mixture is insufficient to maintain constant flow rates.
Innovation Solution
The system incorporates a plurality of riser pipes connecting the condenser output pipe to the evaporator input pipe, with varying cross-sectional areas, allowing selective use of riser pipes based on system load to manage pressure drops and ensure consistent refrigerant flow, reducing the length of piping subject to frictional pressure drops.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the vertical pipe of the riser is sized to ensure constant flow rate into the evaporator for all flow conditions, then the momentum of the two-phase refrigerant mixture is sufficient to maintain flow, but the frictional pressure drops become very large at large flow rates
Solution Approach 1:
The riser is divided into multiple parallel vertical pipes instead of a single large pipe. This segmentation reduces the frictional pressure drop in each individual pipe while maintaining the total flow capacity. The refrigerant flow is distributed across multiple paths, reducing the velocity and friction in each pipe segment.
Solution Approach 2:
The solution transitions from a single-dimension flow path (one large riser pipe) to a multi-dimensional network of parallel pipes. This dimensional change allows the system to achieve both sufficient momentum for reliable flow distribution and reduced frictional losses through the parallel configuration.
2Loss of energy
If multiple riser pipes are used to reduce frictional pressure drops, then the pressure differential and flow rates are optimized, but the piping complexity increases
Solution Approach 1:
The multiple riser pipes serve universal functions: they all transport two-phase refrigerant from the common outlet pipe to the evaporator, provide redundant flow paths, and distribute refrigerant evenly. This multi-functionality justifies the increased piping complexity by achieving multiple objectives simultaneously.
Solution Approach 2:
The system changes the parameter of pipe configuration from single to multiple parallel pipes, optimizing the balance between frictional pressure drops and flow distribution. This parameter change allows the system to maintain efficient operation across varying loads while managing pressure differential effectively.
3Volume of moving object
If the condenser and evaporator are arranged side-by-side with evaporator inlet higher than condenser outlet, then the system layout is compact, but the two-phase refrigerant mixture must be routed through a vertical riser against gravity
Solution Approach 1:
The momentum of the high-velocity two-phase refrigerant mixture exiting the metering device provides a counteracting force against gravity in the vertical riser. The kinetic energy of the refrigerant flow compensates for the gravitational force, enabling the refrigerant to rise to the higher evaporator inlet while maintaining compact side-by-side arrangement.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration optimizes refrigerant flow by balancing pressure differential and flow rates, maintaining efficient operation across varying loads while minimizing refrigerant charge and piping complexity, especially during part-load conditions.
Implementation Method 1
the two-phase refrigerant mixture will have to be routed through a two-phase riser into the evaporator
Implementation Method 2
the flow through the components is driven by gravity
Implementation Method 3
the pressure differential due to lift is about 25% of that of a typical medium pressure refrigerant
Implementation Method 4
the liquid refrigerant is fed in through the top of the evaporator and falls over the tubes, where it is evaporated
Implementation Method 5
In a falling film evaporator system, the liquid refrigerant is fed in through the top of the evaporator and falls over the tubes
Data Source
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AI summary
A heating, ventilation and air conditioning (HVAC) system includes a condenser (18) flowing a flow of refrigerant therethrough and to an output pipe (56) and a falling film evaporator (12) in flow communication with the condenser and having an evaporator input pipe (58) located vertically higher than the output pipe. A plurality of riser pipes (60) connect the output pipe to the evaporator input pipe. The flow of refrigerant flows through selected riser pipes of the plurality of riser pipes as required by a load on the HVAC system.