278. It is common practice to use the aspiration and natural draft systems together, and in many cases it would be difficult to fix the line of demarcation between them. One system naturally supplements the other, and the combination is usually an advantageous one.

There are many instances, also, in which the exhaust system is used in combination with a pressure or plenum system, both being operated by fans. Under all ordinary circumstances, this combination is not to be recommended. It does not afford any particular advantage, and there are several substantial objections to it. The cost for flues, fans, power, motors, installation, and attendance is greater than in a simple pressure system of equal aggregate capacity.

The exhaust and pressure fans, in a combined system, handle the same air in succession; consequently, one of them is sufficient to furnish the required volume. In order to do the same work with a single fan, it is necessary only to increase the pressure to the total of that produced by the two. This may be done by increasing the peripheral speed of the fan wheel, either by enlarging the diameter, or increasing the speed of rotation-usually both. The larger single fan will cost much less than the two smaller ones, and will require less care and attention to maintain it in good order.

The single fan will require only about 65 to 75 per cent.

of the power consumed by the other two, the saving being due to the smaller waste by friction. The motor or engine will also cost less than two motors required for the other fans. There is also a considerable advantage in favor of the single fan in the matter of space occupied, and cost of installation.