Boiling.—The time during which the boiling must be continued will depend upon several cir cumstances, such as the evaporative power of the copper and the amount to which the wort has to be concentrated. Generally the proportion evaporated during the boiling is about one-seventh; and there is a further loss by evaporation as the wort cools down from the boiling point. In determining the duration of the boiling, the time required tb obtain the necessary extract from the hops has to be considered. The stronger the hops, the longer boiling they require to obtain the full extract.
The quantity of hops added to the wort depends upon the quality of the beer being brewed and the time it is intended to be kept. The measured quantity of hops is sometimes merely thrown into the copper, and stirred into the wort; sometimes the hops are picked out and strewn on the surface, where they are allowed to remain for some time before being stirred in. The object of surface treatment is to allow the hops to be permeated by the rising steam, thus opening the pores before immersion in the wort. When boiling takes place ill an open copper, the layer of hops on the surface of the worts prevents contact with the atmosphere. In many breweries, where two or three mashes are made, the hops, after boiling with the wort from the first mash, are discharged with it to the hop-back, and are returned into the copper t,o be boiled with the second wort, and so on. This is the general practice at the London breweries, and in some the hop-backs are fitted with elevators, by which the hops can be transferred to the coppers. At Charrington's, a long Archimedean screw, similar to those used for transporting malt or grist, placed at an angle of about 30°, is used for raising the hops from one of the hop-backs to the copper. When hops are treated in this way, the moisture finally retained in them, which, unless the hops are allowed ample time for drainage, will amount to one barrel for every 60 lb. of hops, is only of the strength of the wort with which they were last heated, and is of comparatively little value. Another plan is to discharge the hops from the copper with the first wort, and allow them to remain in the hop back, the succeeding worts being merely poured over. This plan effects a gradual weakening of the liquor retained by the hops. Another method of preventing loss by the retention of wort, is to
subje,ct the wort to pressure. At Salt's brewery at Burton the practice is, with the best pale ales, to boil the hops with only the first wort. After discharge from the copper with the wort, they are removed from the hop-back and pressed, and are then available for another brewing. The hops are sometimes boiled with the first and second worts, and are then pressed, so as to thoroughly remove any wort held by them. At Allsopp's, Bass's, Salt's, and other large breweries, hop presses, worked by hydraulic power, are used, whilst in smaller establishments screw presses are employed. At Younger's brewery at Edinburgh, the wort is expelled from the hops in centrifugal drying machines.
Extended series of experiments on heating by tubes containing steam have been made, and very various results have been obtained by the several authorities. It will be sufficient for general purposes to be enabled to calculate the amount of surface necessary to boil or to evaporate one barrel of water in one hour by means of steam pipes, and as well to furnish similar data for steam heated boilers with double bottoms. The steam may be assumed as at 30 lb. pressure a square inch above atmospheric pressure.
A barrel of water weighs 360 lb., and to increase its temperature from 52° to 212° F., or 160°, it is necessary to impart 360 x 160 57,600 thermal units. As the latent heat of steam at atmospheric pressure is 966°•6, additional heat amounting to 360 x 966°•6 347.976 thermal units is necessary to convert a barrel of water into steam; or a total of 405,576 thermal units is required to evaporate one barrel of water from an initial temperature of 52° F.
Proceeding in this way, it has been calculated that the areas of steam-heated surface required to raise one barrel of water an hour from an initial temperature of 52° F. to the boiling point, is for Sq. ft. Sq. ft.
Copper steam-pipes 1* Copper with double bottom .. 2 Cast-iron boiler with double bottom .. 4* The areas of steam-heated surface required to evaporate one barrel of water an hour from au initial temperature of 100° (21'2° F.) are:— Sq. ft. Sq. ft.
Copper steam-pipes 24* Copper with double bottom .. .. 30 Cast-iron boiler with double bottom .. 731 no same data for 100 gallons are :— To boll. To evaporate.