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Tin Bath Atmosphere Recycling
Nitrogen / Hydrogen Atmosphere Recycling System For Tin Baths In Float Glass Plants

What is it?
The recycling system is a further development toward tin bath atmosphere quality improvement. Instead of venting the atmosphere outside the tin bath, the mixture of Nitrogen/Hydrogen is pumped out continuously, purified to fresh N2/H2 specifications and re-injected back into the tin bath.

The tin bath atmosphere is extracted at several points on both sides of the tin bath casing. As the tin bath atmosphere is hot, the gas is cooled down for further processing via heat exchangers.

Then the cooled gas passes through a first filtering assembly where the main particles are trapped before being pumped and discharged to H2S removal columns.

After having passed through the H2S removal columns, the gas is filtered again and then fed to the Oxygen purifier device where oxygen content is removed from the gas.

The water particles and vapor are removed via a chemical dryer. The purified gas is then cooled down, filtered and re-injected into the tin bath atmosphere.

Facts about tin bath atmosphere recycling
We have conducted an evaluation of more than 20 float lines in operation with and without recycling system to determine the influence of the recycling system.

Below are summarized the main conclusions:

• A recycling system allows to recycle up to 50% of the total fresh atmosphere injected in the tin bath (N2 + H2). Up to 70% of the atmosphere can be recycled in some cases. This means that if a total 2000 Nm3/h N2 + H2 is injected in the atmosphere, with a recycling system only 1000 Nm3/h are needed as fresh supply and the other 1000Nm3/h are recycled for a constant turnover ratio.
• The H2 is recycled as well. If the N2 contains for example 7% H2 at the recycling inlet, the output recycled N2 will have minimum 6.5%H2 with 0,5% being losses in the deoxo to reduce the O2 ppm.
• All tin defects included as a percentage of total defects in glass were from 9,5%  to 15% in float lines without recycling as compared with less than 0,5% in lines with a recycling system.
• Tin baths without a recycling system add each year 4 to 5% of the total amount of tin to compensate for tin losses, dross and evaporation as compared to less than 1,5% with tin baths equipped with a recycling system. The dross is reduced significantly with a recycling system.
• Drips, micro drips, top specks are reduced to virtually zero in tin baths with recycling systems. This compared to 40 to 90drips in average for 10m2 of glass in tin baths without recycling system.
• Tin baths with a recycling system work with pressures less than 20 Pa as compared to higher pressures in other tin baths.
• The N2 turnover in a tin bath with recycling can be increased 2 to 3 times more than in other baths (N2 turn over means how many times per hour is the N2 inside the tin bath completely replaced with fresh N2. Good practice recommends minimum 3 to 5 times per hour total N2 turnover).
• The bigger N2 turnover has a great effect on glass quality and can only be achieved with a recycling or extraction of N2 without significantly increasing the amount of fresh supply.
• Tin baths with a recycling system have decreased the concentration of H2 in nitrogen from 8 to 9% down to 4 to 5%. As a result they use two times less H2 and recycle up to 50% of the injected H2.
• Tin bath roof and cooling equipment is cleaned up to 30 times a year in tin baths without recycling. This compares to less than 6 times a year in tin baths with recycling.
• Cooling capacity in the float bath is reduced by approximately 20% ( less flat coolers are used) 
• The recycling can be installed or retrofitted any time during production without generating production losses or deteriorating glass quality. It is typically done during a change of thickness or color.
• The recycling is a must for tin baths with intra float beams used for CVD (Chemical Vapor Deposition) inside the tin bath.

Space requirements
See the sketch below with the dimensions and space requirements. All dimensions are in mm. Minimum height required under the tin bath is 3000mm. Other heights can be accommodated, to be discussed.

How much will you save ?
Savings in N2 fresh supply
In normal production depending on the float line capacity, an average of 1500 to 2000 Nm3/h of pure N2 is supplied to the tin bath roof.  With a recycling system, at least 50% of this flow is recycled back in to the tin bath reducing the fresh supply in half.  (this does not take into account production savings due to minimum  tin bath roof blow off, five to ten times less frequent cleaning of the equipment around the tin bath etc.)

Savings in H2 fresh supply
In normal production depending on the float line 150 to 200 Nm3/h of H2 is supplied to the tin bath roof.  With a recycling system this will be reduced by at least 30 to 50%.

Production savings due to improved quality
Production loss due to tin bath defaults is specific to each plant and it is not possible to quantify without plant specific information.

The chart below shows the impact of a recycling installation on tin bath drips in an actual plant in France:

How much it costs?
The recycling system is tailored to each float glass plant and the cost will be dependent on many parameters that are inherent to each plant. Our specialist will be glad to make a detailed quotation for each specific case.

Depending on the budgetary constraints, plant size, lay out and configuration of each particular plant, there are several alternatives, however the most common are:

• Option 1: Turn key installation.  The budgetary cost will be 2 to 2.5million USD
• Option 2: Supply of equipment only. The plant makes all utilities (civil works, electrical, piping, etc.) interconnections on site itself under the supervision of our engineers. The budgetary cost will be 1.5 to 2 million USD.
• Option 3: We install the recycling system at our own cost.  The plant pays for the amount of recycled gas supplied into the float bath by the recycling system and for utilities cost.

The price of recycled gas will be the same as the price paid for the fresh supply.

This is the most optimum solution as the recycled gas is equal to the amount of reduced fresh supply which the plant won’t have to pay to the supplier of N2/H2.

Basically it costs nothing to the plant, but it brings tremendous advantages in quality improvement and tin bath operation. In order to qualify for this option, the amount of recycled gas must be greater than 700Nm3/h. (fresh supply of 1200Nm3/h or greater)

Utilities and consumptions
Following would be the required utilities consumptions for a standard recycling equipment

Total electrical power :

	1. Supply	:	400 volts,  50 / 60 cycles
	2. Installed Electrical Load	:	300 kW
	3. Electrical Consumption	:	150 kW/h
	4. 3 - phase power cable from your mains to our electrical cabinets

Chilled Cooling water for Deoxo:

	1. Supply Temperature	:	12°C
	2. Supply Pressure	:	2 to 5 bars
	3. Flow Rate	:	20 m3/hr
	4. Pipe Size	:	ND80, Flanged
	5. Cooling Water Outlet T°C	:	10°C temperature rise

Chilled Cooling water for blower:

	1. Supply Temperature	:	12°C
	2. Supply Pressure	:	2 to 5 bars
	3. Flow Rate	:	5 m3/hr
	4. Pipe Size	:	ND50, Flanged
	5. Cooling Water Outlet T°C	:	10°C  temperature rise

Cooling water for condensers

	1. Supply Temperature	:	30°C
	2. Supply Pressure	:	2 to 5 bars
	3. Flow Rate	:	200 M3/hr
	4. Pipe Size	:	2 x ND150,  Flanged
	5. Cooling Water Outlet T°C	:	Max 3°C Temperature rise
	6. Cooling water return pressure	:	0.8 bar lower than  in inlet supply pressure.

Total instrument air:

	1. Flow	:	3 Nm3/hr
	2. Quality	:	Dry & Oil free, from plant instrument air network
	3. Pressure	:	Approx 5 to 6 bars
	4. Pipe connection	:	ND 20

Total instrument N2:

	1. Flow	:	1 Nm3/hr
	2. Quality	:	From float bath instrument N2
	3. Pressure	:	Approx 3 to 6 bar
	4. Pipe connection	:	ND 15

Some Pictures of a Typical Recycling Installation
Below are some pictures from most recent installation of a complete recycling for a capacity of 1300Nm3/h.