Refrigeration – Automatic control – Time or program actuator
Reexamination Certificate
2001-06-11
2003-06-17
Wayner, William (Department: 3744)
Refrigeration
Automatic control
Time or program actuator
C062S227000, C236S10100B, C340S589000
Reexamination Certificate
active
06578373
ABSTRACT:
BACKGROUND
In refrigerating systems, a positive displacement compressor such as a piston type compressor is typically used to compress refrigerant gas from a lower suction pressure value to a much higher discharge pressure. Such compressors include a crankcase as a storage location for the lubricating oil. A suction conduit connects an evaporator or cooling coil to the compressor inlet.
Positive displacement type compressors can easily be damaged should liquid refrigerant rather than vapor refrigerant be drawn either into its crankcase or its compression chamber. The term floodback is employed to denote and describe the condition when liquid refrigerant flows through the suction conduit and enters the crankcase or cylinders of a compressor. “Slugging” is the term used to describe the condition when so much incompressible liquid refrigerant enters the compressor cylinders or other types of compression chambers that audible noise of chattering or hammering occurs. In larger compressors, instantaneous compressor damage or destruction can occur under slugging conditions.
Even if insufficient liquid refrigerant enters the running compressor to cause slugging, compressor damage can occur if the lubricating oil in the crankcase is diluted by liquid refrigerant entering the crankcase or oil sump, thereby reducing its viscosity and lubricity and generating excessive wear of the lubricated parts such as main and connecting rod bearings and cylinder walls and piston rings. While this kind of damage may not be immediately observable, it always leads to shortened compressor life and progressively noisier operation. The debris of excessive wear can circulate around the system with the refrigerant plugging driers and filters and reducing system performance through increased pressure drop through these partially plugged components.
Even with the compressor ‘off’, migration of gaseous refrigerant to an unusually cold crankcase there, mixing with the oil, can cause both immediate and long term compressor destruction when the compressor starts up or runs under this condition. The dual destructive sequence begins when the compressor starts, sharply reducing the pressure within the crankcase and thereby causing the excessively refrigerant laden lubricant to foam and to be drawn into the compressor cylinders causing immediate slugging. Even if slugging does not occur, the reduced lubricity of the diluted oil promotes excessive wear and early compressor failure.
During compressor operation, the floodback condition of liquid refrigerant flowing to the compressor can be caused by incorrect expansion valve setting or expansion valve or restrictor malfunction or by loss or excessive reduction of evaporator load. Expansion valves are devices that control or restrict refrigerant liquid flow into the evaporator to just the amount or rate of flow that the evaporator can evaporate. Reductions in evaporator load can be caused by plugged filters in the evaporator airstream or frost clogging the evaporator face or failure of the evaporator fan/s to operate.
In the past various controls and piping artifacts have been used to try to ensure that the refrigerant being compressed is always in gaseous form, that is, that no floodback to the compressor occurs. One such refrigerant flow control is a thermostatic expansion valve that senses the superheat condition at the evaporator outlet and adjusts refrigerant flow in response. Piping artifacts employed to help ensure that only gas flows to the compressor include suction accumulators and suction traps; these are vessels that catch and retain liquid refrigerant while allowing refrigerant vapor to continue to flow to the compressor.
Service personnel on the site can detect floodback to the compressor while it occurs by employing rough indicia such as frost formation on the suction line, though this indication must be skillfully used since suction frost can be formed by dry gas (no liquid) that is simply colder than 32 F.
Therefore mechanics developed their own tools for deciding whether there was liquid refrigerant within a suction line. They would grip the bare line and judge how fast their hand chilled. We now know the high heat transfer coefficient between liquid refrigerant and the pipe makes the hand holding the pipe feel cool, and appear to cool, more rapidly than a similar pipe carrying only cold vapor with no liquid refrigerant in it.
Other mechanics would wet their finger tips and touch the pipe, thereby deciding on the presence or absence of liquid in the pipe by whether or not the wet finger froze to the pipe. Other mechanics would listen to the sound of the operating compressor to help them decide whether the compressor was attempting to compress a mixture of refrigerant vapor and liquid or vapor only.
However, even normal systems have a substantial potential for ‘noise’, that is, ‘hunting’ or cyclic or random variations in suction line temperature and pressure. These random changes could be caused by normal fluctuations in thermal expansion valves or by sharp load changes generated by momentary fans-off condition or other normal but transient events. Therefore early efforts to utilize suction line temperatures as predictors of floodback were ineffective and it became ‘common knowledge’ that protective measures based on suction line temperatures were unreliable.
None of these sensing methods was amenable to automatic mechanical or electrical sensing whereby a floodback condition hazardous to the life of the compressor could be observed and the compressor turned off or an alarm signaled.
Superheat measurement at the suction inlet of the compressor can indicate the presence of liquid refrigerant. However, to be effective superheat measurements must be made skillfully and with the proper equipment correctly applied.
Superheat is accurately measured only by measuring the pressure of the suction gas, determining the saturation temperature from a pressure/saturation temperature conversion chart, generally by interpolation, or by evaluation of an equation that simulates the pressure-temperature curve for that refrigerant. Then the calculated saturation temperature is subtracted from the observed suction line temperature, the difference being the superheat.
Among service personnel it is not well known that the commonly used Bourdon pressure gages that compare line pressure with atmospheric pressure are subject to variations from altitude and weather that can affect the expected saturation temperature, especially in freezer systems. Sufficiently accurate pressures can be observed employing ‘absolute’ pressure detectors. Such pressure detectors employ the pressure in a highly evacuated chamber as the reference against which the suction pressure is measured, instead of atmospheric pressure. While the difference between zero superheat, indicating a floodback condition, and a small positive superheat, a safe operating condition, is difficult to accurately determine accurately because of the need to interpolate temperature values from the ubiquitous service ‘pressure-temperature’ chart, accurate corresponding saturation temperatures can readily and quickly be generated from digitized stored data or from stored equations.
PRIOR ART
Superheat Gage with Plug-in Data Module
Barbier Patent Number 5,627,770
This is simply a superheat indicating device. It does not disclose any provision for detecting floodback as such. Further, the specification points out at col.2 139-44 that saturated temperatures are almost always employed rather than actual temperatures. It does not disclose or suggest any rate function.
Method and Apparatus for Calculating Super Heat in an Air Conditioning System
Aloise Patent Number 5,666,815
Aloise teaches the apparatus and method for storing the vapor pressure/temperature models for a number of refrigerants in the integral microprocessor, switch selecting the appropriate refrigerant, observing the desired system temperature and pressure, calculating the saturated temperature for the refrigerant selected, and subtracting the calculated temperature from
Kramer Daniel E
Wayner William
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