Comparisons of Heat and Refrigeration Processes

With a cellar temperature of 36° Fahrenheit and a difference in temperature of 20°, the boiling point of the liquid would be 16° Fahrenheit and the pressure required to give this temperature 29 pounds. Under these conditions, from eight to nine feet of 2-inch pipe would evaporate the one pound of ammonia in an hour. In other words, a difference of temperature of 20° Fahrenheit will cause about 545 heat units per hour to flow through the surface afforded by eight to nine lineal feet of 2-inch pipe.<Callout type="important" title="Important">Understanding these principles is crucial for efficient refrigeration.</Callout> Evaporation of Water and Ammonia Similar to evaporation of ammonia in pipe evils, is that of the water in a watertube boiler. In both cases the lower the pressure and the hotter the temperature outside, the more liquid will be evaporated per square foot of heating surface in a given time.<Callout type="tip" title="Tip">Lowering external temperatures can significantly increase evaporation efficiency.</Callout> While the water pump has but one function to perform, that of raising the water from the catch-basin to the discharge level, the compressor must not only raise the ammonia gas from the expansion coil in the cellar and discharge it into the condenser on the roof but also raise the thermal level of the ammonia to a point where its heat can gravitate into the cooling water, which causes the ammonia to return to the liquid form.<Callout type="risk" title="Risk">Improper compressor operation can lead to inefficient refrigeration.</Callout> If the cooling water is supplied to the condenser at 70° Fahrenheit and flows away at 90° Fahrenheit, the condensing pressure, according to the amount of cooling water and cooling surface employed, will be from 180 to 190 pounds, and the boiling point, or temperature at which the gas will liquefy, corresponding to these pressures, will be from 94° to 97° Fahrenheit. The maximum temperature attained by the gas, however, may be very much higher than this.<Callout type="warning" title="Warning">Exceeding safe temperatures can damage equipment and reduce efficiency.</Callout> Condenser water carries away the heat from the cellar, though it is several degrees hotter than the cellar, just as the river carries away the water from the cellar though it is several feet higher than the cellar. The compressor raises the heat through a certain number of degrees, thereby increasing its 'thermal head,' just as the pump raises the water through a certain number of feet, thereby increasing its 'static head.'<Callout type="gear" title="Gear">Using efficient compressors and condensers can significantly improve refrigeration performance.</Callout> In following out the present comparison it should be remembered that coal is simply the vehicle for bringing us heat radiated by the sun ages ago. By absorption of solar heat a chemical process took place by which carbon-dioxide from the atmosphere was broken up in the plant cells of the vast prehistoric vegetable growths and formed fixed carbon in the plant tissues and free oxygen exhaled into the air. In the process of combustion of coal, free oxygen from the air again combines with the fixed carbon of the plant forming carbon-dioxide, and the long imprisoned solar heat is liberated.<Callout type="beginner" title="Beginner">Understanding this cycle helps appreciate the importance of sustainable energy sources.</Callout> Not only is solar heat of former ages made to do useful work through the evaporation of water in boilers, but the solar heat of the present day evaporates the moisture which, precipitated from the rain clouds, collects to form cataracts with power to turn turbines. Not only is prehistoric solar heat used to evaporate water in boilers, but the solar heat of the present could be used in the same way if its rays could be sufficiently condensed and focused, and used directly for the production of steam power as well as indirectly for production of water power.<Callout type="important" title="Important">Harnessing modern solar technology can enhance energy efficiency.</Callout> It is not altogether unlikely that future generations will see direct solar energy so utilized. The rapid exhaustion of our present fuel deposits, which has even now advanced to a point where the gravity of the situation cannot be ignored, has already directed some research in that direction.<Callout type="risk" title="Risk">Continued reliance on fossil fuels can lead to environmental degradation and resource depletion.</Callout> In the present case, however, it is sufficient to assume that the steam under pressure in the boiler has been raised by coal and that the atmospheric vapors forming the clouds have been raised by the sun. In either case energy has been supplied and energy must be withdrawn before the vapors will be condensed to the original form of water.<Callout type="tip" title="Tip">Efficient condensation processes are key to reducing energy waste.</Callout> When the two forms of vapor have reached their respective destinations, however, they still possess a considerable energy or capacity for doing work when pressed into service in appropriately designed machines. When the atmospheric vapor has been divested of a sufficient amount of its latent energy it condenses into rain, and were there a suitable machine at hand a large per cent of the foot pounds of work developed by the falling rain could be utilized.<Callout type="warning" title="Warning">Improper management of water resources can lead to inefficient use of natural energy.</Callout> Since comparatively small amounts of water are scattered over large areas, man must content himself with utilizing only the foot pounds remaining in the rain after it has been precipitated and collected into larger masses in rivers and lakes, which, though many feet below the rain clouds where it possesses its greatest potential energy, may yet be many feet above the level of the sea from which it arose and may yet, accordingly, have a considerable capacity for performing useful work.<Callout type="important" title="Important">Efficient water management is crucial for harnessing natural resources.</Callout> Water Power In the present example enormous amounts of power are stored in the torrents of water precipitated on the great watersheds that feed the Northern lakes and finally flow down the Niagara River, a part to turn the wheels of industry and a part to dissipate its energy in raising the temperature of the rocky gorge below the falls. In Fig. 35 is shown a conventionalized machine for utilizing a part of the energy in a small stream of water diverted from the Niagara River above the Falls.<Callout type="gear" title="Gear">Modern turbines can significantly increase the efficiency of water power utilization.</Callout> While theoretically a modern vertical turbine might have been employed in this analogy, for simplicity of detail and similarity of comparison a bucket conveyor is shown. Water from the duct leading from the river is discharged into the buckets near the top of a sprocket wheel. The weight of the descending water in the upper chain of buckets turns the lower shaft carrying a second chain of buckets so arranged as to elevate the water accumulating in the shaft below the level of the river, and discharge it into a trough near the point of discharge of the upper chain at such a height that it can flow away by gravity into the river.<Callout type="important" title="Important">Properly designed machinery is essential for efficient energy conversion.</Callout> The power available in any machine is the product of the force acting and the space acted through. In the present example the distance between the point of charging and that of discharging the buckets is about 50 feet, so that every thousand pounds of water discharged will have exerted 50,000 foot pounds, every 33,000 of which per minute is equivalent to a horsepower of work, and every 778 of which is equivalent to one B.t.u. of heat (50,000 pounds per minute = 1.515 H.P.or 64.27 B.t.u.). In this example it is obvious that the higher the point at which the water can be received and the lower the point at which it can be discharged, the more power will be developed.<Callout type="risk" title="Risk">Improper design of energy conversion systems can lead to inefficiency.</Callout> The amount of power to be expended in raising the water from the shaft depends not only on the number of pounds of water to be raised in a given time, but also on the number of feet through which it is to be raised. If the water is running into the shaft at different levels it is obvious that less power will be required if provision is made for collecting it and conducting it into the conveyor at about the level at which it enters than if it were all allowed to flow to the bottom of the shaft.<Callout type="warning" title="Warning">Improper water management can lead to energy loss.</Callout> If the height of the point of discharge be more than just sufficient to allow the water to flow away to the river, foot pounds of work will be unnecessarily expended. Similarly, if the point of discharge of the water from the upper buckets is higher than necessary to enable the water to flow away freely, loss of power will result.<Callout type="important" title="Important">Optimizing water discharge points can significantly improve efficiency.</Callout> Since the water discharged from both sets of buckets must all flow into the river, the points of discharge may be on the same level, as shown, or at different levels, providing both levels are above that of the river. Pumping Water and Heat The analogy is apparent. The source of the water available for producing power is 160 feet above the bottom of the shaft, giving it a potential energy which may be said to be equivalent to that of steam having a temperature of 370° Fahrenheit. This steam may be expanded to the lowest pressure, or the heat may be allowed to flow to the lowest temperature at which it can still flow away into the river of condenser water.<Callout type="tip" title="Tip">Optimizing thermal head differences can improve efficiency.</Callout> If this temperature is 126° Fahrenheit the corresponding pressure will be 26 inches vacuum. This falling of temperature resulting from the conversion of heat into work, in the steam engine shown on the left, is represented by the steam indicator diagram shown on the right, temperatures at any point of which are approximately indicated by the thermometer.<Callout type="warning" title="Warning">Improper thermal management can lead to inefficiency.</Callout> The water to be removed from the pit represents heat to be removed from the lower levels of temperature found in cold-storage compartments. This heat has to be elevated almost to the same level at which heat from the engine is exhausted, because of the fact that the most satisfactory disposition of the heat from both sources is to let it flow into the same river of condenser water.<Callout type="important" title="Important">Efficient thermal management is crucial for refrigeration systems.</Callout> The lower the point of discharge the more power will be available in driving the chain per pound of water, and the less power will be required to raise a given quantity of water by the driven chain. The lower the temperature in the condenser water the more power will be developed in the driving steam engine per pound of steam expended and the less the power required to raise a given quantity of heat in the driven refrigerating machine.<Callout type="risk" title="Risk">Improper thermal management can lead to inefficiency.</Callout> In other words, the efficiency of the driving machine depends directly on the difference in head of the water entering and leaving the buckets, just as that of a steam engine depends on the difference in temperature between the steam in the boiler and that in the condenser. Similarly, the efficiency of the driven machine increases directly as the difference in head between the water leaving and entering the buckets decreases, just as that of a compression refrigerating machine increases as the difference in temperature between the gas in the condenser and that in the cooler decreases.<Callout type="important" title="Important">Optimizing thermal differences can significantly improve efficiency.</Callout> Since the pressure of steam at the lowest point to which it is practical to expand it is considerably above that of the refrigerating medium liquefying at the lowest temperature that available cooling water will allow, it is found economical in practice to first permit the heat from the refrigerating medium to flow into the cooling water, after which its thermal level, or temperature, even after being raised by heat from the refrigerating machine condensers, will still be sufficiently low to allow heat from the steam engine condensers to gravitate into it.<Callout type="warning" title="Warning">Improper thermal management can lead to inefficiency.</Callout> The diagram illustrates this to the extent of showing that the point of discharge of the water from the driving conveyor is slightly above that of the driven conveyor. This cooling water containing the heat from the steam condenser is shown in the diagram flowing away with the water from the driving conveyor and that from the refrigerating machine condenser with the water from the driven conveyor.<Callout type="important" title="Important">Proper thermal management can significantly improve efficiency.</Callout> The two sets of expansion coils, Ei and E2, located the one above the other, represent the different thermal levels or temperatures at which the heat is absorbed in two cold-storage rooms. The different temperatures at which these two storage rooms are to be maintained is also represented by the height of the spouts which deliver the water seeping through the walls of the shaft into the driven conveyor.<Callout type="risk" title="Risk">Improper thermal management can lead to inefficiency.</Callout> The operation of the compressor of the refrigerating machine shown on the left is also represented in the compressor indicator card shown on the right, — heights in feet, temperatures in degrees, and corresponding pressures in pounds, being represented on the three scales also shown at the right. Leaks To complete the analogy it is necessary to remember that in the operation of the driving, as well as the driven, chain of buckets there are losses by friction in the bearings as in a steam engine and compressor; losses in capacity due to imperfect filling of the buckets corresponding to imperfect cylinder filling in a compressor; losses due to leaks in the buckets corresponding to leaks by valves and pistons of the steam engine and compressor.<Callout type="warning" title="Warning">Leakage can significantly reduce efficiency.</Callout> In the case of the best steam power plants, all but about 15 per cent of the heat 'leaks away' without performing any useful work. In the average steam plant all but about 6 or 8 per cent is lost, so that it is of the utmost importance that this small remaining per cent of heat be utilized to the best possible advantage in the refrigerating machine.<Callout type="important" title="Important">Minimizing leaks can significantly improve efficiency.</Callout> Working Limits The next most important detail to be considered after that of keeping the compressor in good mechanical repair — that is, to see that the lower buckets do not leak, realizing that, on account of the small per cent of useful work resulting from the expenditure of energy in the prime mover, a given loss in the driven machine is of much greater importance than the same loss in the driving machine — is to see that the condenser pressure or point of discharge of the water is as low as possible and that the expansion coil pressure is as high as possible, or that the buckets pick up the water at as high a level as possible.<Callout type="tip" title="Tip">Optimizing head differences can improve efficiency.</Callout> Assuming that the point of discharge of the water be 100 feet above the bottom of the shaft, 100 foot-pounds of work must be expended in a theoretically perfect machine to elevate a single pound of water. At the efficiency of the average ammonia compressor, from 25 to 35 additional foot-pounds would have to be supplied to make up for leaks and other losses.<Callout type="warning" title="Warning">Improper design can lead to inefficiency.</Callout> If the water all enters the shaft at B, a point only 65 feet below the point of discharge or 35 feet above the bottom of the shaft and means of directing it into the buckets at this level be devised, only 65 foot-pounds in a theoretically perfect machine, or only from 81 to 88 foot-pounds in a machine of the efficiency of an ammonia compressor, need be expended to do the same amount of work.<Callout type="important" title="Important">Proper design can significantly improve efficiency.</Callout> Assuming, similarly, that the refrigerating machine, represented by the chain of buckets, discharges the heat at a temperature 100° Fahrenheit above that of the colder refrigerator coil corresponding to the bottom of the shaft, which temperature, for the sake of similarity, may be taken at 0° Fahrenheit, the horsepower per ton of refrigeration would be 1.2194.<Callout type="risk" title="Risk">Improper design can lead to inefficiency.</Callout> Interpolating to find the temperature from which heat equivalent to a ton of refrigeration can be raised by the expenditure of half that amount of power, or 0.6097 horse-power per ton, a cooler temperature of approximately 42J° Fahrenheit is obtained.<Callout type="important" title="Important">Proper design can significantly improve efficiency.</Callout> If a refrigerating plant is so operated that this heat which enters the refrigerator at such a temperature that it can be absorbed by a refrigerant at 423° Fahrenheit has to be absorbed at 0° Fahrenheit, in other words, if the plant is operated at 16 pounds back pressure when it could be operated at 61 pounds, one-half the power will be as needlessly expended as would be the case if the water entering the shaft at a height of 50 feet were allowed to flow to the bottom, requiring the buckets to lift it 100 feet instead of 50 feet.<Callout type="warning" title="Warning">Improper operation can lead to inefficiency.</Callout> The example just cited need be none the less significant because of the unusual back pressure of 61 pounds gauge. Except for selecting temperatures to agree with the feet head of water already mentioned in the analogy, lower temperatures might just as well have been considered, for example: The horsepower required per ton of refrigeration when the back pressure is four pounds gauge corresponding to a temperature of 20° Fahrenheit, and the same head pressure of 200 pounds gauge corresponding to a temperature of 100° Fahrenheit, is 1.6090.<Callout type="important" title="Important">Proper design can significantly improve efficiency.</Callout> Again interpolating in the table we find that half this power per ton would be expended when the back pressure is 38 pounds, corresponding to a refrigerator temperature of 24.4° Fahrenheit. For convenience in comparison the foregoing figures are shown in tabular form in Table II.<Callout type="gear" title="Gear">Using efficient compressors and condensers can significantly improve refr