Coffee and chicory

by P. L. Simmonds

In these applications of natural and artificial heat to the curing of coffee, the heat is conveyed by the air through the whole depth of coffee in such a manner that each bean feels its influence, whilst the watery products elicited by the heat are at the same time, and by the same means, carried off. It cannot be doubted that these applications are far more effectual than any of the modes hitherto in use; in some cases stoves were employed in the apartment containing the coffee, but it is obvious that their influence could not extend beyond the surface of the mass, and that, if the apartment was closed, there was no provision for carrying off the air that had become loaded with moisture due to its temperature; whilst, if the apartment was open, so as to afford a free draught of air, the greater portion of the heat given out by the stove was carried out before the heated air could act on the coffee. In other cases, heating pipes of various kinds were used below the floors on which the coffee was placed. This arrangement, however, has the effect of injuring the coffee, by steaming it; no provision being made for carrying off the excess of hot watery vapour which accumulates within the mass, but, on the other hand, the natural processes of decomposition are assisted and promoted.

The construction of Mr. Clerihew’s heating apparatus is simple, and a moderate supply of fuel has a considerable effect in raising the temperature of cold damp air before it is brought into contact with the coffee through which it is drawn by the aid of the fans. This heated air becomes diffused throughout the whole of the chamber, which extends beneath the ground-floor in such a manner that no portion of the coffee which is on that floor can be free from its influence.

The fans at Rathoongodde are much more powerful than those in common use, the peculiarity in the shape of the blade giving them a great advantage as air-moving machines, in so far as the indraught is concerned, whilst one-half of the periphery being open a ready exit is afforded for the discharge of air. In the ordinary fan, if a smoking match is applied to any part of the indraught opening, the air will be seen to flow towards a neutral point in the centre of the fan, following a spiral direction, and thence in the periphery of the fan.

In Mr. Clerihew’s modification of the blade each film of air, so to speak, flows into the fan directly, until it impinges on the curvilinear part of the blade, and from that point is thrown at a right angle towards the periphery. The column of air being thus less distorted in its progress, there is not only a greater quantity discharged, but much less power is consumed in effecting that discharge—in the common fan it is evident, from the circumstance of the air flowing to an apex, that a great amount of power is wasted in producing the increased velocity with which a column of air equal in volume to the two ingress openings of the fan must pass so contracted an area before it is discharged; hence it is that the fan, as an air-moving machine, has been considered unequal to the screw.

The enclosed space of the coffee-curing house at Rathoongodde has an area in the cross section of 100 superficial feet, it is 70 feet long, and a pair of fans are placed at one end. Repeated experiments have shown that, when the fans make 100 revolutions per minute, a cloud of smoke travels to them from the centre of the enclosed space (a distance of 35 feet) in precisely 15 seconds, hence we have 100 × 35 = 3500 cubic feet of air discharged in a quarter of a minute, or 14,000 cubic feet per minute; a screw of nearly seven feet in diameter would be required to discharge the same amount of air, and the cost of it in England is 84 guineas, whilst the pair of fans made and fitted up at Rathoongodde cost under 9l.

In the centre of the enclosed space, with a depth of four feet of coffee in the upper floor, the flame of a candle is blown to a right angle when the whole power of the fans is put on that floor; near to the fans it is extinguished, the air moving forward with a uniformly accelerated velocity from the farther end towards the fans, owing to the constant accessions made by the air entering the enclosed space throughout its whole length.

It has already been mentioned that the only entrance of air into the air-chamber beneath the ground-floor is by the opening in which the stove is placed, consequently, when this opening is closed by a damper, it is obvious that the whole power of the fans is exerted on the mass of coffee which is being cured on the upper floor, and that the division of this power may be regulated at will by more or less obstructing the entrance of air to the air-chamber by the damper. The upper floor is not supposed to be an air-tight apartment, but as the chief entrance of air is by the two doors in the end, its influx may be so far obstructed by closing them as to throw the greater part of the power of the fans on the coffee which is on the ground-floor, when this is required. Again, since it is obvious that, in wet weather, when the atmosphere is fully saturated with moisture proportionate to its temperature, it becomes a desideratum to introduce a portion of the artificially heated air into the vacant space which is over the coffee in the upper floor, so that the air which passes down through that coffee may have an absorbing tendency; this is accomplished by shutting the doors of the upper floor and throwing open the top-covering of the fan. By this means one-half of the air which is drawn from the stove is thrown in above the coffee in the upper floor, whilst the other half is discharged altogether. This infusion of heated air would on many occasions be attended with benefit, but the advantage will naturally depend on the comparative state of dryness of the coffee on the two floors.

In having recourse to these practical modifications some little judgment and observation are of more service than precept. It will be found, for instance, that if the coffee in the upper floor approaches the dry stage, it is better in wet weather to shut the doors of that floor as well as the tops of the fans, so that only a small flittering of air sufficient to ward off the first stages of decomposition may pass through that coffee, whilst the wet coffee below has the full benefit of a more rapid circulation of absorbent air.

Attention may now be directed to the practical results which these arrangements have afforded in the curing of coffee.

The coffee in the upper floor, as the crop advanced, gradually increased in depth until it stood at four feet all over the floor. When at this depth, with the fans making 100 revolutions per minute, the flow of air was quite sensible to the hand placed on the surface of the coffee, and was rendered apparent by the smoke from a match following the direction of the air; at the same time the rarefaction of the air within the enclosed space was so very slight as barely to be appreciable by a very delicate mountain barometer, though it had the effect of causing the door to shut with a slam; thus showing that a slight rarefaction of the air is sufficient to disturb the balance of atmospheric pressure, even when acting through a medium of coffee of considerable depth. The current of air thus established continued to flow without interruption until the fans were stopped.