The principle aim of both the VOC-directive (volatile organic compounds) issued by the European Community as well as the Technical Instructions On
Air Quality (TA Luft) is to reduce the emission of noxious volatile organic compounds like solvents used in the printing industry. The Technical
Instructions On Air Quality apply for all flexo printing houses using more than 15 tons of solvents/year and for all gravure printing houses using more than 25 tons of solvents/year.
Although the main emphasis of such regulations for exhaust air purification is on ecology and health policy, also economical aspects are of particular importance due to the increasingly severe competition. In this context, solutions combining optimum energy recovery with further use of the released energy contribute significantly to cost reduction. The installation of a 3-canister RTO with heat recovery manufactured and supplied by KBA-
CleanAir (formerly LTG Mailänder) in Stuttgart (Germany) at a rotogravure printing company in Aachen (Germany) highlights the criteria for as well as the possibilities offered by such a unit.
Fundamental analysis
The middle-sized printing company prints high quality flexible packaging using 3 rotogravure printing presses with 500 to 800 mm width. In order to print on the broad spectrum of substrate including mono film and compounds based on paper, film, cellophane and aluminium, solvent-based printing inks are used. Although water-based as well as UV-inks do not require any air purification, their range of use is remarkably limited compared with solvent-based inks, which in addition are much easier to use for example due to their faster drying rate.
Due to the ongoing growth of the printing house the company decided for the installation of a new ten-colour gravure printing press. Concerning air purification, KBA-CleanAir was involved at a very early stage, as the company already installed several units at this company. At first, the issue was to upgrade the ten-year-old KBA catalytic air purification system.
However, this system obviously did not comply with modern technology any

longer and therefore had to be upgraded anyway according to the German
Technical Instructions On Air Quality. So the company decided to invest in a new technology, attaching importance to combine maximum efficiency with highest sustainable usage of energy.
At first, the manufacturing process was analysed so that one was able to design the new unit according to the individual conditions. These included:
 use of different solvents and mixtures of solvents of the three gravure printing presses running at multiple shift operation.
 very fast change of solvent concentrations due to short print jobs
 solvent concentration of 2 g/m³ at partial load operation and 10 g/m³ at full load operation.
 high air volume range for individual operation of each press without mutual impact
 high efficiency also at low solvent concentrations to facilitate heat recovery for the heating of the machines and the production buildings.
The degree of solvent concentration at the processing of a print job is mainly determined by the share of area ink covering. All printing presses are supplied with control devices to keep the air volume as low as possible at jobs with small area covering and to raise the solvent concentration. At full load of the press, the maximum allowable solvent concentration (LEL = lowest emission limit) of 25% according to he European safety standard EN
12753 is achieved.
Decision for KBA-CleanAir
Due to the manifold requirements mentioned above, the company decided for a new KBA-CleanAir Regenerative-Thermal Oxidiser RTO. This turn-key solution does not only offer a favourable price/performance ratio but also ensures optimum use of energy and ease of maintenance. The decision was also due to the perfect satisfaction with previous co-operation with KBA-
CleanAir. Catalytic air purification systems have already been installed there in 1992 and at another production site in 2000.

Regenerative-thermal oxidiser
Due to high purification performance and effectiveness, the regenerativethermal oxidiser is now dominating the market. Harmful substances are burned reliably at temperatures of 820 °C and continuously low clean gas concentrations of < 20 mg/m³ are achieved. This corresponds to a destruction efficiency of > 99,8% (!), independent of the kind or composition of the solvents. This does not only clearly fall below the required clean gas limit but also avoids odour problems. Regenerative-thermal air purification systems are characterised by a very high thermal efficiency of the primary ceramic heat exchanger and therefore require just little external energy.
Depending on the application, such systems are supplied in most different design. The system supplied consists of three separate canisters with a common combustion chamber on the top which are fully insulated inside with ceramic fibre. Therefore there are no metallic parts in the hot area and no wear-out occurs. The unit can be operated at temperatures of more than
1000 °C without any problems. Due to the 3-canister design, continuously low clean gas values also at maximum inlet concentration are ensured. The canisters are filled with special ceramic honeycombs , alternately heated by the hot clean gas and cooled by the cold raw gas. The cold raw gas flows through the first heat exchanger where it is heated up to about 780 °C. With external energy, the raw gas then is heated up to the required operating temperature of 820 °C. The ceramic honeycombs provide a thermal efficiency of 96%. This means that only 4% of the heating energy must be supplied by external sources. As in practise the heating energy of the solvents contained is used, the burner can be switched off completely at a minimum solvent concentration of only 2 g/m³ resulting in the autothermal
operation of the unit. The hot clean gas flows through the next canister and is cooled until reaching the stack. At a raw gas temperature of approx. 60 °C the outlet temperature is lower than 100 °C. Both, construction and thermal efficiency of the heat exchanger decisively affect the total operating costs.
For example, if the thermal efficiency is 92% instead of 96%, this means that the required external energy is increased from 4% to 8%, which is almost a doubling of energy requirement with according cost. In addition, the ceramic honeycombs facilitate a compact design with very low pressure loss which also results in cost savings.
Including the reserve, the unit is designed for a maximum exhaust air volume of 33.000 m³/m and a maximum solvent concentration of 10 g/m³. For cyclical distribution and control of the air, special air-tight dampers with double sealing are installed underneath the canisters. Figure 2 shows the turnkey unit. Beside the stack, the order also included the ventilation engineering, the piping for hot oil between printing presses and RTO as well as the complete wiring and assembling. The two existing gravure printing presses with catalytic air purification have been prepared while at full production and switched to the new system on the weekend to reduce production downtime to a minimum.

Pict. 1: Scheme of a Regenerative Thermal Oxidiser RTO in 3-canister design
Energy concept
The heating of the dryers of the gravure printing presses is carried out by using hot oil heat exchangers providing a leading temperature of 200 °C and a reflew temperature of 180 °C. The total installed power is about 1.400 kW.
This energy required for drying had to be generated solely with the air purification system without the need for additional hot oil burners. The maximum energy demand for drying has been determined to 600 to 700 kW, as in practise the total drying performance of all printing presses is never needed at the same time.
With an autothermal operating point of 2 g/m³ and a typical average solvent concentration of 1,5 to 6 g/m³ the unit runs at the optimum energetic state.
This means, that the unit produces surplus energy. This concept is ideal in that way that there is a close mutual relationship between the energy demand of the printing presses and the production of surplus energy at the regenerative-thermal oxidiser. At high production with high solvent emission a high degree of energy is recovered during air purification.
With an air purification system highly integrated into the energy concept, not only the operating costs are reduced due to the autothermal heating of the dryer, but also total invest costs can be reduced, as no separate hot oil boiler is needed. In this application, herewith another EUR 80,000 to 100,000 could be saved.
Recovery and re-use of the energy

In order to deal with high solvent concentrations the regenerative-thermal oxidiser is equipped with a »hot bypass«. When producing surplus energy, a part of the hot clean gas is deduced immediately after leaving the combustion chamber and bypassed around the heat exchanger. Before exhausting through the stack, it is mixed with the clean gas coming from the heat exchanger. At this »hot bypass« a hotoil waste heat boiler with a total power of 1.000 kW is integrated. This power represents the maximum surplus energy to release and therefore does not only cover the energy requirements of the printing press but also supplies energy for the heating of the production site. For this, an additional hotoil/water heat exchanger has been integrated at the reflew of the printing press’ heat exchanger, delivering at max. 500 kW for the heating system of the production site
(80/50 °C). With this configuration, primarily the printing presses are supplied with energy, using the remaining energy for the heating of process water. Both heat exchangers are equipped with an own bypass system if the demand for heat is lower than the surplus energy available. Only in this case the energy is led via the stack without being used.
Another interesting solution for the energy-saving operation of the unit is the patented burner control with direct gas injection developed by KBA-
CleanAir. This module is activated, if the autothermal peak is not reached due to insufficient load of the printing presses. The control module provides a turn-down ratio of the burner from 0% to 100%. Conventional gas burners merely have a turn-down ratio of 1:8 to 1:10. However, if the level of the actual solvent concentration allows the autothermal operation of the unit, no external energy supply is necessary anymore and the minimum power of the burner is higher than the actual heating power required. This means, that the delivery of external energy exceeds the actual demand. If just 0% to 20% of the burner power is required due to the according solvent concentration, the module switches to what is known as »lance operation«. The direct flame mode of the burner is switched off and replaced by the flameless operation with direct natural gas injection. The required amount of natural gas is injected via a lance. This does not only save energy by optimizing the burner power but also reduces the combustion air during the flameless operation.
Summary
The installation proves that ecology and economy must not be an antagonism. With a fully integrated regenerative-thermal oxidiser an optimal recovery and re-use of energy is achieved. Such a system is no longer a costly and non-effective accessory, but an essential part of the production process. The multi-stage energy recovery from the solvents in the exhaust air did not only make unnecessary the installation of an additional heating unit for the dryers but also significantly reduced energy demand to a fraction of the previous consumption. Emitted solvents are harmful to the environment and therefore have to be disposed of according to the legal regulations. However, emitted solvents are also an effective

energy supplier. Mature technology, low energy consumption and high operation reliability of the regenerative-thermal oxidiser together with the service and the experience of KBA-CleanAir ensured a good co-operation.
Exhaust Air Purification: 3-canister RTO 30.000 m³/h for treating the exhaust air from 3 rotogravure printing machines.
For more details please contact:
KBA-MetalPrint GmbH
KBA-CleanAir Division
Wernerstr. 119-129
D-70597 Stuttgart
Tel.: +49-711-69971-691
Fax: +49-711-69971-346 cleanair@kba-metalprint.de www.kba-cleanair.de
Your contact at ICE:
Dietmar Decker
Mobile: +49-172 7103565 dietmar.decker@kba-metalprint.de