Thermal wheel

A rotary heat exchanger ( also rotary heat exchangers, heat wheel, air preheater or LUVO called ) is a heat exchanger which allows two airstreams a heat recovery. Heat is transferred from one air flow to another, by a rotating storage mass is alternately heated by a stream of air and cooled by the other.

Functional Description

The rotor consists of a number of parallel channels to the axis, the heat storage capacity is utilized. Through one half of the rotor, the warm exhaust air is blown, the walls are warmed by it. As the rotor turns on, the warmed channels reach the area where it flows through cold outside air; this is heated by the warm walls of the channels, the walls are cooled. When the two air streams are arranged in the opposite direction, the rotor has a cold and a hot side and corresponds to a counter-current heat exchanger.

In the simplest arrangement shown in the figure, the passages are still filled with hot exhaust air, when the rotor turns further and reaches the area of the opposing air flow. The co-rotation generated independently of the pressure gradient always an undesirable amount of recirculated air, because the air is mixed with the incoming air. In addition, resulting from the pressure gradient between the air streams leaks. Because of these unwanted air currents, the fans must provide for supply and exhaust air more power than is needed for the actual air exchange.

The proportion of recirculated air can be prevented by the supply air is not passed from the outside into the building in a small area (sector), but is deflected in a so-called flushing chamber and is blown in the opposite direction through the rotor into the air duct. A pressure gradient between the outside and exhaust air is additionally imperative to ensure. This device leads to a further increase of the fan power. Both measures reduce the efficiency, which is why the fans must now provide at least 3-10 % more power.

Buildup in the ventilation and air-conditioning technology

For fresh air circulation in buildings, the rotor is usually made from thin films. As in a corrugated cardboard sheet is wrapped in a change to the desired outer diameter is reached depending on a smooth and a corrugated layer. Between the "waves" or triangles of the film are the required air channels. Typical film thicknesses are in the range of 0.05 to 0.12 mm, the channels are typically between 1.4 mm and 2.5 mm, and as long as the rotor is thick, usually 200 mm. Since it can come in the heat exchanger for condensing water, the material for the rotor must be corrosion resistant. Most used is aluminum foil or stainless steel foil. Asbestos, plastic film or glasfasergestützes fabric was also used, but these materials are not recommended because they bring due to fouling health risks.

Rotary heat exchangers are manufactured with a rotor diameter of about 40 cm to about 6 m and run depending on the size of rotational speeds from about 3 to about 25 revolutions per minute. The drive is usually a three-phase motor, which rotates the rotor via a belt transmission. The motor drive power is about 30 W per m³ / s of air.

An air preheater is a heat exchanger for utilizing waste heat from a variety of industrial processes.

Air preheater ( APH ) in power plant technology

The air preheater is heated by the flue gas of the boiler and is used to preheat the combustion air. The air preheater is usually the last heating surface in the flue gas stream upstream of the flue gas vent or chimney.

He cut off the flue gas heat and warms the incoming fresh air via fans. This transferred heat term should no longer be provided by the fuel, which is why the heating surfaces can be made smaller. For this reason, air preheaters are designed to the maximum possible limit of the process technology, which is determined by the dew point of the flue gas. In addition, the heated combustion air ensures a better ignition of the fuel - air mixture. Since the air preheater flue gas extracts heat, this is cooled and exits at a lower temperature the chimney. The exhaust loss of the boiler is reduced, and the boiler efficiency increases to the same extent.

Are air preheater before all other components of the boiler particularly susceptible to corrosion because they are located at the cold end of the flue gas path and the lower temperature favors the fresh air close to the dew point temperature of the flue gas. Therefore, the laminations forming the storage mass of the air preheater, executed or enamel made of corrosion resistant materials. Often a vapor recuperator leading the air preheater is connected upstream to exclude below the dew point with certainty.

In most of the great achievements Ljungstrom regenerator is used which regards their type is a cross - countercurrent. Meanwhile, slowly rotating storage mass consists of densely packed and folded sheet steel packages, which alternately absorb or emit flue gas heat according to the rotation.

For small performance air preheater can be used in prefab or steel tube bundle.

Conveying energy

The operation of a Rotationswärmeübertragers requires additional energy. This is even the conveying energy of the air flows and on the other hand, the drive energy of the motor rotor. The demand of energy delivery is highly diverse and can be broken down as follows:

Overcome the pressure losses in the rotor wheel
More air flow rates and increased back pressures in supply and return air duct by co-rotation on the warm side.
More air flow rates and increased back pressures in the outdoor and exhaust air duct by co-rotation on the cold side.
More air flow rates and increased back pressures in the rotor wheel in the mud.
More air flow rates and increased back pressures in the outdoor and exhaust air duct in flushing.
More air flow rates and increased back pressures due to leaks.
Overcome the pressure drop of the throttle valve in the pressure warranty
Overcome the pressure losses in bringing together the exhaust and outside air ducts.

The complexity of the situation is exacerbated by the varying pressure difference between the outside air and exhaust air duct, and the slip-ring seals require very complex flow conditions.

Skip to first estimate the following surcharges may be made to the pressure loss of the rotor wheel for conveying energy and the drive:

Rotary heat exchanger with Carryover: 120-180 %
Rotary heat exchanger with flushing: 200-280 %

Example: pressure drop of a rotating heat exchanger with co-rotation = 140 Pa, charge = ~ 210 Pa. Total pressure loss = 140 Pa Pa 140 210 Pa = 490 Pa.

Leaks

Generally, a distinction is made between external and internal heat exchangers leak. Under external leakage refers to an unwanted air exchange across the outer housing wall. In contrast, internal leaks occur only within the housing between the two air streams and can be broken down as follows:

Carryover occurs in two places - one on the warm side and one on the cold side - regardless of the prevailing pressure gradient on and inevitably entails an unwanted amount of recirculated air. Carryover requires increased air volume flows on the hot and cold side.

Purge air is an additional measure to prevent carryover on the warm side ( the co-rotation on the cold side is not thereby prevented ). Purge required increased air flows through both sides of the rotor wheel.

Leaks occur depending on the local pressure gradient. This pressure gradient are different on the hot and cold sides and differ by twice the Rotorraddruckverlustes (~ 300 Pa). The manufacturer's specification for leakage ( air and supply air ) is given only at 0-20 Pa pressure drop ( inlet air to the exhaust air) on the warm side and is in accordance with DIN maximum of 3 % of the supply air flow. The fact that the leakage from the air and supply air despite opposite pressure gradient occurs, is due to the co-rotation mitgemessenen. This is different for the leak on the cold side: It can be estimated because of the local pressure drop increased with the ~ 5 times the leakage on the warm side. Decisive for the amount of leakage is the actual pressure gradient. This differs usually considerably from the above Test 0-20 Pa. Leaks require increased air volume flows on the hot and cold side.

Applications

Rotary heat exchangers are used in ventilation and air conditioning systems in ventilated buildings and in the offshore area (eg cruise ships ). Also in the process air technology, both in the hot-air range up to 650 ° C as well as in drying of paint shops they find increasingly application.

In temperate latitudes, especially the heating of the supply air in the cold season is in the foreground; at high ambient temperatures, the heat exchanger can be used as well, to cool the cooling air, the sucked external air. However, may be undesirable in winter the desired moisture transfer in the summer.

Advantages:

Condensation unilateral efficiencies up to 80 % by countercurrent process, high energy- savings and reduction of CO2 emissions through lower power requirement at the air heater or air cooler.
Low payback periods ( between 1-5 years )
Depending on size suitability for very high air volumes up to 180,000 m³ / h air.
Self-cleaning effect compared to coarse ( eg insects, leaves ) or dry dirt ( eg dust) by the constant change of air direction between supply air and exhaust air. That Dirt of the outside air is transferred to the exhaust air and is blown out or dirt of the exhaust air is transferred to the supply air and is blown out again.
Low depth ( 100-250 mm)
Supply air can be regulated ( if only small heat recovery is required, the speed can be reduced).
Side panels can be used also to regain humidity ( with water-absorbing coating of the heat exchanger ) and thus the energy requirements for air humidification in winter to reduce.
Usually no condensation draining is necessary, as there may be any condensation is taken from the supply air without water-absorbing coating of the heat exchanger. Only in extremely humid air, as occurs in a swimming pool, or already largely saturated supply air can accumulate condensation.

Cons:

Mechanically moving parts, therefore susceptible to interference than eg Plate heat exchanger.
Merging of outdoor and exhaust air ducts leads to increased investment and energy consumption of the fans.
No preventive protection against smoke and fire spread.
Recirculation units by co-rotation
Leakage through sliding seals
Recirculation units and the possibility of leakage beautiful the recovery efficiency or efficiency
No complete separation of supply air and exhaust air. Remains of the pollution in the air can pass through the heat exchanger in diluted form back into the supply air. This is especially problematic for noxious fumes or strong odors. A flushing chamber can be reduced greatly with proper fan arrangement, the mixing of the air streams.
Seal wear in the supply air
Icing of the ( cross ) seals at condensation in winter
Pressure gradient between the outside and exhaust air with the use of rinsing chambers required. When too much pressure gradients could be very large rinsing and air leakage amounts to deform the cabinet with impairment of the operation.
Electricity consumption due to increased amount of air for co-rotation, irrigation and leakage.
Size ( wheel diameter ) is substantially larger than the cross -section.
Life of only 10-15 years.

Countercurrent exchange Membrane fouling Boiler (power generation)

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