When compressed it is also used largely as a refrigerating agent, and in virtue of its property of neither burning nor supporting combustion it is also used as a fire extinctor.
The district is agricultural and pastoral, and wool and grain are exported, as well as meat and dairy produce, for which there are large refrigerating works.
The use of mechanical refrigerating plants for chilling the pork has made it practicable to cure the bacon with the use of a small percentage of salt, leaving it mild in flavour when delivered in European markets.
He next experimented with a highpressure hydrogen jet by which low temperatures were realized through the Thomson-Joule effect, and the successful results thus obtained led him to build at the Royal Institution the large refrigerating machine by which in 1898 hydrogen was for the first time collected in the liquid state, its solidification following in 1899.
The air forced in for the purpose of burning the fuel, is usually pre-heated, and in some of the most progressive works is dried by Gayley's refrigerating process.
REFRIGERATING and ICE-Making.
The function of a refrigerating machine, therefore, is to take in heat at a low temperature and reject it at a higher one.
The relation then between the work expended and the actual cooling work performed denotes the efficiency of the process, and this is expressed by Qt/(Q2-Q1); but as in a perfect refrigerating machine it is understood that the whole of the heat Q i is taken in at the absolute temperature T 11 and the whole of the heat Q2, is rejected at the absolute temperature T2, the heat quantities are proportional to the temperatures, and the expression T,/(T 2 -T,) gives the ideal coefficient of performance for any stated temperature range, whatever working substance is used.
As an instance, when heat is taken in at o° and rejected at 70°, a perfect refrigerating machine would abstract 6.6 times as much heat as the equivalent of the energy to be applied.
No actual refrigerating machine does, in fact, take in heat at the exact temperature of the body to be cooled, and reject it at the exact temperature of the cooling water, but, for economy in working, it is of great importance that the differences should be as small as possible.
There are two distinct classes of machines used for refrigerating and ice-making.
Compressed-air Machines.-A compressed-air refrigerating machine consists in its simplest form of three essential parts -a compressor, a compressed-air cooler, and an expansion cylinder.
I.-Compressed-Air Refrigerating Machine.
This theory of working is founded on the Carnot cycle for a perfect heat motor, a perfect refrigerating machine being simply a reversed heat motor.
Coleman of Glasgow, and in the early part of 1879 one of his machines was fitted on board the Anchor liner " Circassia," which successfully brought a cargo of chilled beef from America-the first imported by the aid of refrigerating machinery, ice having been previously used.
Machines of the second class may conveniently be divided into three types: (a) Those in which there is no recovery of the refrigerating agent, water being the agent employed; they will be dealt with as " Vacuum machines."
In the first class, since the refrigerating liquid is itself rejected, the only agent cheap enough to be employed is water.
Improvements were made by Siebe & Company of London, and a considerable number of ether machines both for ice-making and refrigerating purposes were supplied by that firm and others up to the year 1880.
The refrigerating liquid (ether, sulphur dioxide, anhydrous ammonia, or carbonic acid) passes from the bottom of the condenser through the regulating valve into the refrigerator in a continuous stream.
In other words, the liquid-refrigerating agent enters the refrigerator at the condenser temperature and introduces heat which has to be taken up by the evaporating liquid before any useful refrigerating effect can be performed.
If r represents the latent heat of the vapour, and q 2 and q1 the amounts of heat contained in the liquid at the respective temperatures of T2 and T11 then the loss from the heat carried from the condenser into the refrigerator is shown by (q2-q1)/r and the useful refrigerating effect produced in the refrigerator is r-(q 2 -q i).
Assuming, as in the previous example, that T 2 is 75° F., and that T 3 is 5° below zero, the results for various refrigerating agents are as follows: TABLE II.
As the critical temperature (88.4° F.) of carbonic acid is approached, the value of r becomes less and less and the refrigerating effect is much reduced.
When the critical point is reached the value of r disappears altogether, and a carbonic-acid machine is then dependent for its refrigerating effect on the reduction in temperature produced by the internal work performed in expanding the gaseous carbonic acid from the condenser pressure to that in the refrigerator.
The critical temperature of anhydrous ammonia is about 266° F., which is never approached in the ordinary working of refrigerating machines.
In such cases the evaporation of the ammonia or other refrigerating liquid frequently takes place in the cells themselves, brine being dispensed with.
Large passenger vessels and yachts are now generally fitted with refrigerating machinery for preserving provisions, cooling water and wine, and making ice.
Lloyd's Committee now issue certificates for refrigerating installations, if constructed according to their rules, and most modern cargo-carrying vessels have their refrigerating machinery classed at Lloyd's.
Though the adoption of refrigerating and ice-making machinery for industrial purposes practically dates from the year 1880, the manufacture of these machines has already assumed very great proportions; indeed, in no branch of mechanical engineering, with the exception of electrical machinery, has there been so remarkable a development in recent years.
The cooling of residential and public buildings in hot countries, though attempted in a few cases in the United States and elsewhere, is yet practically untouched, the manufacture of ice and the preservation of perishable foods (apart from the frozen and chilled meat trades) have in many countries hardly received serious consideration, but in breweries, dairies, margarine works and many other industries there is a large and increasing field for refrigerating and ice-making machinery.
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