To heat the air in the supply ventilation units, heaters (air heaters) are used.

Fig.8. Water heater

High-temperature water or steam is used as a heat carrier for heaters. When the heating liquid is water, the heaters are marked as HW, when steam is marked as HS.

Heaters designed to work with a steam coolant are manufactured single – pass, with a water coolant-both single-pass and multi-pass. In single-pass heaters, the coolant passes through the entire bundle of tubes simultaneously from one collector to another, and in multi-pass collectors are separated by internal partitions that repeatedly change the direction of movement of the coolant, which increases the speed of water movement through the tubes and, as a consequence, increases the heat transfer of the heater. The connection of fittings in single-pass heaters is diagonal, and in multi-pass ones it is one-sided (Fig. 9).

Fig. 9. The flow diagram of the coolant in the heaters:

a – single–pass; b – multi-pass

Fins in the form of plates (plate heaters) or wound steel tape (spiral-wound heaters) are placed on the tubes to increase the contact area with the air passing through the heater. The outer fins are galvanized to reduce corrosion and better contact of the fins with the tubes. Fittings for the coolant are welded into the collectors, and to protect the fins from damage from the side, side shields are welded between the collectors. Flanges are used to connect the heater to adjacent elements of the ventilation system.

The arrangement of the tubes with the heating liquid can be consistent in the direction of air movement (corridor), staggered and offset along the corridor (most effectively). The tubes themselves can be either round or flat-oval in cross-section.

The best thermal engineering indicators have spiral-rolled bimetallic three- and four-row heaters for both steam (single-pass) and water (multi-pass) heater. The tubes for the heat carrier in these heaters are steel, the fins are rolled from aluminum.

Heaters are often grouped into several pieces, both with a parallel installation through the air, and with a sequential or combined one. If the heat carrier is steam, then the heaters are installed with a vertical arrangement of tubes and steam supply to the upper branch pipe; if the heat carrier is water, then the position of the tubes should be horizontal, which ensures the removal of air when filling the heaters with water and draining water from it when the system stops working.

Electric heaters are also available (Fig.10).

Fig.10. Electric heater

Electric heaters consist of a steel casing with tubular heating elements: 1.6 or 2.5 kW each. To increase the heating surface area, the heating elements have ribs with a diameter of 42 mm. Electric heaters can work both in manual and automatic mode, maintaining a constant temperature of the air at the outlet or in the room.

The duct heater is used to heat the supply (outdoor) air in the ducts (conventional circular section). In central ventilation systems, duct heaters are used as auxiliary heaters, and in decentralized ones – as the main air heaters.

The heater body is made of galvanized steel. Air heating is carried out by heating elements. It is mandatory to have protective and regulating thermostats, which provides the product with high safety and the ability to function in automatic mode at the same time.

Channel heaters are equipped with two thermostats for overheating protection: heat-protective with automatic restart (operating temperature +50 °C) and fire-fighting with manual restart (operating temperature +110 °C). Channel heaters are designed for a minimum air flow velocity of 1.5 m/s and a maximum operating temperature of 40 °C of the outgoing air.

Duct air coolers (Fig. 11) are designed for cooling and dehumidification of supply, recirculating air or a mixture thereof in ventilation and air conditioning systems of industrial, public or residential buildings.

Fig. 11. Air cooler with ducted water and freon coolers

Water or non-freezing mixtures can be used as a refrigerant in ducted water coolers. The maximum allowable liquid pressure in them is 1.6 Mpa.

Freons are used as a refrigerant in ducted freon coolers. Upon delivery, the heat exchangers are filled with inert gas, which must be removed during connection to the refrigeration circuit.

The design of the cooler is a housing made of galvanized steel, inside which a heat exchanger, a drop trap and a pallet are installed.

The heat exchanger is made of copper tubes with aluminum fins arranged in a staggered order.

The freon cooler is distinguished by the design of the distribution unit ("spider") and the specifics of the refrigerant supply.

The collectors of the freon heat exchanger are made of copper tubes.

The drip trap (Fig. 12) is a set of special plastic plates that effectively trap condensate and collect it into a tray located in the lower part of the cooler body.

Fig. 12. The shape of the droplet trap plates.

The pallet is additionally thermally insulated and equipped with a discharge pipe for draining condensate

When installing the air cooler, it is necessary to ensure its horizontal position.

Filters are divided into three classes according to their effectiveness. Class I filters trap dust particles of all sizes (the cleaning coefficient is not less than 0.99), Class II filters – particles of more than 1 micron (cleaning coefficient is more than 0.85), class III filters – particles of more than 10–50 microns (cleaning coefficient is not less than 0.60).

The noise level generated by ventilation systems is an essential criterion of ventilation quality. The sources of noise in ventilation installations are fans and electric motors, as well as the movement of air in the ducts and its exit from the holes. Two types of noise are considered: aerodynamic and mechanical.

The reasons for the appearance of aerodynamic noise in fans are: the formation of vortices and their periodic disruption from the blades of the impeller, local air flows at the entrance to the wheel and at the exit from it, leading to unsteady flow around the blades of the wheel, perturbation of the medium by rotating blades. Mechanical noise occurs in bearings, in the drive, in the installation sites (fasteners) of the ventilation unit on building structures, etc. The degree of noise increases with insufficient balancing of the fan impeller.

Of all the sources of noise generation, the dominant ones are fans that create aerodynamic noise. The noise generated by the ventilation system can be reduced by the following measures: installation of fans with the most advanced acoustic characteristics, in particular, fans with blades bent backwards; the choice of fans with the highest efficiency (not less than 0.9 of the maximum), with a minimum angular velocity of the impeller (not higher than 30 m/s), i.e. fans with a small diameter of the impeller and a small number of revolutions (at the same time, you should not overestimate the pressure against the calculated one, since this causes an increase in noise level); careful balancing of the impeller.