allows the operation of the plant to proceed without interruption. When employing this method for weighing the refrigerating liquid, it is necessary that the pipes connecting with the weighing tanks be sufficiently long to insure flexibility to the system. The liquid level* should never be allowed to rise to the pipes M and iV, as any liquid other than that vertically over the driuns will not be weighed correctly.
<Callout type="important" title="Critical Rule">Ensure the liquid level does not exceed certain pipe levels to maintain accuracy.</Callout>
Tonnage Computed from Quantity of Refrigerant The number of units of cooling effect available in the evaporation of one pound of ammonia under standard conditions has been found by equation [7] to be 465.5. The amount of ammonia required per ton is accordingly 200 pounds per minute, equivalent to 25.778 pounds per hour, or 618.7 pounds per twenty-four hours. If, for example, it is found by test that 3,000 pounds of liquid ammonia per hour pass through a refrigerating system, the compression limit of which is a 19x38-inch double-acting compressor running at 45 revolutions per minute under standard conditions, the cooling effect produced is found to be 3000 / 25.778 = 116.37 tons.
Actual Displacement Efficiency of Compressor Obviously, the efficiency of the compressor can be approximated by dividing the probable cooling effect, as determined by calculation based on the properties of the liquid, by the tonnage computed from the apparent displacement per minute in cubic feet as calculated from the dimensions of the compressor. Expressed as an equation this becomes Actual Cooling Effect 116.37 ^ Actual Efficiency = —— = 80-4% Apparent Cooling Effect 144.66
<Callout type="warning" title="Safety Hazard">Excessive clearance in compressors can reduce efficiency and cooling capacity.</Callout>
Approximate Displacement Efficiency of Compressor In the majority of cases the tonnage capacity of the system is required with reasonably close accuracy, but it is often impractical to weigh the ammonia. In such cases a somewhat less accurate estimate of the efficiency of the compressor can be made with the assistance of an indicator. For all practical purposes the weight of ammonia gas may be considered proportional to its absolute pressure, and within narrow limits the amount of refrigeration represented by a cubic foot of ammonia gas will likewise be proportional to its absolute pressure.
Example: The apparent number of cubic feet of ammonia gas discharged per minute by a 19x38-inch double-acting compressor running at 45 revolutions per minute has already been found to be 561.15. If it is found, by the indicator-diagram method just described, that the displacement efficiency is 76.2 per cent, the actual number of cubic feet discharged will be . 762 X561 . 15 = 427 . 59 cubic feet which, multiplied by the number of British thermal units of refrigeration represented in each cubic foot of ammonia gas actually displaced gives the total cooling effect of the machine expressed in British thermal units per minute. This quantity divided by 200 reduced the capacity to tons per 24 hours.
<Callout type="tip" title="Pro Technique">Use indicator diagrams to estimate compressor efficiency for more accurate tonnage calculations.</Callout>
Example: The apparent number of cubic feet of ammonia gas discharged per minute by a 19x38-inch double-acting compressor running at 45 revolutions per minute has already been found to be 561.15. If it is found, by the indicator-diagram method just described, that the displacement efficiency is 76.2 per cent, the actual number of cubic feet discharged will be . 762 X561 . 15 = 427 . 59 cubic feet which, multiplied by the number of British thermal units of refrigeration represented in each cubic foot of ammonia gas actually displaced gives the total cooling effect of the machine expressed in British thermal units per minute. This quantity divided by 200 reduced the capacity to tons per 24 hours.
COBiPUTATION OP CAPACITY To expedite the figuring of capacities, not only under standard but also under other conditions, the accompanying tables of constants have been derived. To determine the tonnage capacity of a double-acting compressor of any size operating at any piston speed and under various conditions of head and back pressure it is necessary only to substitute appropriate values from these tables in the following equations.
<Callout type="gear" title="Specific Equipment">Double-acting compressors with indicator diagrams are essential for accurate capacity calculations.</Callout>
Key Takeaways
- Calculate the cooling effect of ammonia using its properties and standard conditions.
- Estimate compressor efficiency through indicator diagrams for more accurate capacity calculations.
- Use tables to determine tonnage capacity under various operating conditions.
Practical Tips
- Always ensure that liquid levels do not exceed certain pipe levels to maintain accuracy in weighing refrigerating liquids.
- Use indicator diagrams to estimate the efficiency of compressors, providing a more precise method for calculating cooling capacities.
- Double-check your calculations by cross-referencing with tables and historical methods.
Warnings & Risks
- Excessive clearance in compressors can significantly reduce their efficiency and overall cooling capacity.
- Failure to properly weigh refrigerating liquids can lead to inaccurate tonnage calculations, affecting the performance of the refrigeration system.
Modern Application
While the specific techniques for calculating refrigeration machine capacities have evolved with modern technology, understanding these historical methods provides a foundational knowledge that is still relevant. Modern survival scenarios often require quick and accurate assessments in resource management, making this knowledge invaluable. Additionally, the principles of efficiency and accuracy remain crucial in any system design or maintenance.
Frequently Asked Questions
Q: How can I ensure the liquid level does not exceed certain pipe levels?
Ensure that the pipes connecting with the weighing tanks are sufficiently long to provide flexibility to the system. The liquid level should never rise above specific points M and N, as any liquid other than that vertically over these driuns will not be weighed correctly.
Q: What is the significance of using indicator diagrams for compressor efficiency?
Indicator diagrams help estimate the efficiency of compressors by showing the actual back pressure in the suction pipe and comparing it to the line of absolute vacuum. This method provides a more accurate way to calculate cooling capacities.
Q: Can you explain how to use tables for determining tonnage capacity?
Tables provide constants that can be used to determine the tonnage capacity of a compressor under various operating conditions by substituting appropriate values into specific equations, as described in the chapter.