The fact that the refrigeration stored up during the hours of operation of the plant for use during the hours of rest is in the form of a coating of ice on the coils, instead of brine kept in constant circulation, further increases the necessity for a lower back pressure; first, because ice is a poorer conductor of heat than brine; second, because the congealing-tank surface, as well as that of the expansion pipes, is often insulated with ice; and third, because the stagnant brine, or, what is even worse, ice in the case of the congealing-tank system, absorbs and gives up heat less readily than the moving brine. As it is mechanically impracticable to make the thin, light congealing tanks indefinitely continuous, as is done in the case of ammonia or brine-cooling coils, the ammonia-expansion coils installed in such tanks must be made shorter. This necessitates the use of an increased number of return bends or fittings for the installation of the system as a whole and accordingly tends to increase the initial cost of the expansion piping for both material and labor.
Electrically Driven Plants While the use of electric power undoubtedly reduces the duties of the attendant, safety and the necessity of adjusting expansion valves by hand require more or less constant attention, and the slight additional attention required by the steam or combustion engine is usually too slight to make up for the usual comparatively high cost of electric power. The application of reliable safety devices, however, which protect the plant in case of abnormal pressure resulting from failure of water supply or the accidental closing of the wrong valves, together with a reliable automatic expansion valve, eliminates these two most important duties of the attendant. When the compressors are of the inclosed-crank self-oiling type, and electric motors are employed, very little attention need be paid to lubrication, and the item of attendance becomes very small indeed.
<Callout type="important" title="Key Safety Feature">The use of reliable safety devices is crucial in automatic refrigeration systems. These devices protect against abnormal pressures caused by water supply failures or valve malfunctions.</Callout>
Semi-Automatic Systems The semi-automatic system usually employed to meet such conditions is illustrated diagrammatically in Fig. 10. It will be noted that this system is essentially the same as the congealing-tank system illustrated in Fig. 9, except for the addition of an automatic expansion valve V and some suitable safety device P V for preventing the occurrence of abnormal pressures.
Completely Automatic Systems Still more elaborate systems have been devised to the end of eliminating all attendance. "Completely automatic" systems divide themselves into two classes according to the two possible cycles of cause and effect on which the systems may be operated.
Key Takeaways
- The use of ice instead of brine in commercial refrigeration systems requires a lower back pressure due to its poor heat conductivity.
- Electric power reduces the need for constant human attention but introduces safety concerns that must be addressed with reliable devices.
- Semi-automatic and completely automatic systems were developed to minimize human intervention, each employing different methods to control temperature.
Practical Tips
- Ensure proper insulation around expansion pipes and congealing tanks to maintain efficiency in ice-based refrigeration systems.
- Implement robust safety measures such as reliable pressure valves and water supply monitoring for electrically driven refrigeration plants.
- Consider using semi-automatic or completely automatic systems to reduce operational costs and minimize human error.
Warnings & Risks
- The use of electric power can lead to higher initial costs compared to steam engines, despite reducing the need for constant attention.
- Failure to properly adjust expansion valves in electrically driven plants can result in significant safety risks and system inefficiencies.
Modern Application
While the specific technologies described here are outdated, the principles of automatic refrigeration systems remain relevant today. Modern refrigerators still rely on similar concepts of temperature control, safety mechanisms, and automation to minimize human intervention while ensuring efficiency and reliability.
Frequently Asked Questions
Q: What is a key difference between ice-based and brine-based commercial refrigeration systems?
Ice-based systems require a lower back pressure due to the poorer heat conductivity of ice compared to brine, making them less efficient but potentially more practical in certain scenarios.
Q: Why are safety devices critical in electrically driven refrigeration plants?
Safety devices protect against abnormal pressures caused by water supply failures or valve malfunctions, which can be dangerous and lead to system inefficiencies if not addressed properly.
Q: What distinguishes semi-automatic systems from completely automatic systems in commercial refrigeration?
Semi-automatic systems typically require some manual intervention for safety adjustments, while completely automatic systems aim to eliminate all human attendance through advanced control mechanisms.