Abstract: The herein disclosed multipass corrosion-proof preheater comprises a plurality of passes. The passes are formed by bundles of heat-transfer tubes for the passage of heating gases and interconnected in series, with respect to air supply, by means of connecting air lines. Air is supplied to the tubefield of the heat-transfer tubes via air supply line accommodating a device designed for high-temperature preheat of part of the air supplied to the tubefield of the heat-transfer tubes of the first pass. The air preheater further comprises a bypass air line serving to communicate the air line with the connecting air line via mixer mounted in the latter. In addition, the air preheater is provided with a device for low-temperature preheat of air, mounted in the air line upstream of the device when viewed in the direction of air flow. The bypass air line is connected to the air line between the devices for low- and high temperature air preheat.

Abstract: Air enters each pass of a two-pass air heater in the same direction with respect to gas flows. Bypass air conduits connect each tube bundle of a first tier to the tube bundle of a second tier of adjacent gas ducts and are located between inlet and outlet air conduits and have common walls therewith.The bypass air conduits connect the tube bundles of all the gas ducts in a single closed (ring) circuit. It is possible to set the gas ducts either along the radius on a single circumference or in a joined-up arrangement with the gas ducts divided into two halves by a vertical partition.In an alternative embodiment, the bypass air conduits connect the tube bundles of all the gas ducts in an open circuit, the tube bundles of both tiers of the extreme gas ducts are half as wide and are arranged to form a single pass with respect to the air flow.The thermal efficiency of the air heater is the same as that of a Z-criss-cross type air heater.

Abstract: In an air heater, an initial group of passes on the airside is arranged in the form of a package wherein the passes consisting of banks of tubes are connected in parallel with the uptake gases.The passes of the package are located predominantly at the same horizontal level, are made of tubes of the same length and run in a zone of the same gas temperatures which is dangerous insofar as corrosion can be provoked therein.The width of the packaged passes progressively increases directly with the increase in the rates of air flow through the passes, and a feeding air duct admitting the last inflow of cold air is connected downstream of the last and widest pass to the connecting air duct serving the rest of the passes of the air heater.In an apparatus like this one, assembled from standard components, minimum overall dimensions and low weight are combined with high thermal efficiency.