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Book - School of Science and Technology

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356 VentilationCowlBelt driveHinged cover(a)BaseRo<strong>of</strong> sheet(b)Figure 13.8 Fan powered extract units: (a) centrifugal; (b) propellorA difficulty arises however in providing a satisfactory means <strong>of</strong> admitting the bulk <strong>of</strong>the air required to balance the extract volume, <strong>and</strong> <strong>of</strong> heating it in winter. Fresh air inletsbehind ranges <strong>of</strong> gilled tube or some other form <strong>of</strong> natural convector, as in Figure 13.7,need regular cleaning <strong>and</strong> have only a limited application.Figure 13.9 Vertical jet-discharge ro<strong>of</strong> unit

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