Feasibility Studies

Stewart Engineers can help you quantify risks and ensure financial feasibility before investing in glass manufacturing.

Investing in float glass is challenging for new entrants. How can you verify the viability of an idea when there are many small glass importers making gathering market data tricky? There is a broad range of quality, and therefore cost, available in glass industry equipment making a cost benefit analysis difficult.

About Stewart Engineers

Stewart Engineers is a privately held and family-owned company led by glass industry experts. Our company headquarters is in Wake Forest, North Carolina with branch offices in Kyzylorda, Kazakhstan and Altendorf, Switzerland. Les Stewart founded Stewart Engineers in 1986. We have served the glass industry since our inception with engineering and glassmaking equipment.
We have a deep understanding of the glass industry due to our expertise and experience in the glass industry. Our knowledge enables us to complete projects, with thoroughness and accuracy, which companies outside the glass industry cannot.

Our Process

We conduct a market and competition analysis for the local market to determine the market size and competition. Our staff travels to the location to verify the site and market feasibility.

We develop the technical strategies and solutions for civil and process engineering that best fit the project. After we conduct a feasibility analysis, we can shorten the project timeline because we determine the technical requirements during the feasibility study.

We model the plant performance for five years to understand the initial capital payback period, OPEX, profit and other financial metrics. We also consider risk and consolidate the local data, technical feasibility, and business analysis into a business plan that is used to attract investors and secure financing.

Why Choose Stewart for a Feasibility Study
Accounting firms conduct many feasibility studies, but the results are questionable because they have no process knowledge. Stewart Engineers has a global network of glass partners to determine market data; we design and build high-quality glassmaking equipment, like tin baths and CVD systems, as well as float facilities.
Everyone builds feasibility studies on reasonable assumptions. However, without process knowledge, there is no way to evaluate the reasonableness of the premises. For example, assessing raw material availability is contingent on knowing the glass recipe as well as acceptable quality levels, neither of which is common knowledge.

Stewart Engineers can empower you to make high-quality glass profitably.

Glass Manufacturing

Stewart Engineers builds high-quality float glass manufacturing facilities around the world. We use our considerable experience in float glass to design and build float facilities that maximize profit. Technologies like the StewartFloat® tin bath and AcuraCoat® CVD system combined with Stewart’s international project management and procurement experience result in float facilities that are built on schedule, on budget, and on quality. Beyond the initial project metrics, Stewart Engineers delivers a total solution by providing highly automated and robust solutions along with glass manufacturing process and business operations training.

Stewart Engineers specializes in helping new entrants to float glass manufacturing understand what they need to succeed. Determining ideal tonnage, staffing, product mix, glass recipes, and other critical design decisions can be tough for groups entering the float glass market. A feasibility study is the best tool for determining the market strength and market positioning. Stewart has been in glass for many decades and can assist customers in making the decisions that will determine their future success. Whether or not a customer purchases a feasibility study, Stewart Engineers advises our customers on the best paths to success.

Unlike other engineering companies, the facilities we build are designed first, and foremost, to be profitable.

Plant capacity, location, product mix, product quality, rebuild period, and equipment selection all affect profitability. We consider how every aspect of the plant affects profitability before finalizing it. One example is plant capacity. The higher the plant capacity, the lower the cost per ton of the glass produced. On the surface, it seems like plants with very high capacities would be best but other factors come into play like risk and market size. A float facility has a limited ability to produce less than its design capacity and as glass production is reduced the profit per ton drops precipitously. Even when only considering operating expense (OPEX) this means it is better is better operate a 600 tons per day (TPD) plant at 600 TPD rather than operate an 800 TPD plant at 600 TPD. The plant design must be carefully tailored to the local market and have the appropriate technology for the endeavor to be profitable. We have many decades of glass industry experience to guide the decisions that will affect profitability.

Properly designed workflow can be a significant advantage to a manufacturer. We develop our facilities to promote logical and efficient workflows. An example of a good workflow design is the placement of the quality lab. We place the quality lab directly beside the production line; this enables the Quality Control (QC) operators to pull samples and return to the lab to conduct their measurements speedily. Ultimately the efficient workflow empowers the production supervisors and operators to respond to process conditions more quickly—leading to higher quality and yields.

We use our experience and process knowledge to prequalify our sub-suppliers to guarantee the quality of our facilities.

Stewart Engineers guarantees and warranties our facilities. Most engineering companies will only guarantee the equipment; leaving the customer at risk of having functioning equipment but poor product yields or quality. Stewart can ensure not only correctly functioning equipment but also successful glass production because we have extensive experience in glass.

We have decades of experience with most of our suppliers, so we know how to deploy capital efficiently. Glass making suppliers can present customers with a wide array of options. Batching configurations can range from one to more than five times cost for exotic arrangements. Stewart has the glass manufacturing process expertise to select the best recipes for our clients.

Our team travels globally installing glass technologies. We ensure our systems whether built internally or purchased from sub-suppliers meet our rigorous quality and maintenance standards. We use the most straightforward technology that meets our production goals, ensuring replacement parts and knowledgeable technicians are available in any part of the world.


Continuous Improvement
  • We can transform a greenfield into a float glass facility in 24 months.
  • Our layouts are the most effective in the industry.
  • Our equipment uses the latest technology.
We use automated process controls when it makes sense.

We automate jobs where operators make errors. For example, as the molten glass floats atop the liquid tin in the float bath, the speed the glass is pulled from the bath is automatically maintained. By automatically controlling many of the variables involved in the forming process, product quality is improved.

We automate jobs that would be dangerous for an operator. Stacking glass by hand is a dangerous job, years ago it was common to have 100 or more people handling glass in our factories. As glass handling technology has gotten better, faster, and cheaper, we have reduced the number of people handling glass—leading to a considerable reduction in injuries.

We automate jobs where operators cannot keep pace. The feeding of all raw materials to the glass furnace is automated. Over 700 TPD of raw materials must be weighed to produce 600 gross TPD of glass, and it is common to have over 1,000 weigh-ups (weighing of raw materials) every day.

Let us be your trusted technical partner for float glass projects.

Make CVD Part of your Future

Glass manufacturers began to use CVD for online coatings in the 1960’s. Pilkington was the first to develop a marketable product, Reflectafloat. Over the lifetime of the product, it was made in both the tin bath and the lehr. Reflectafloat is a reflective product that doesn’t have Low-E properties; it is still used in developing glass markets. Pilkington licensed the Reflectafloat technology to some of their float bath licensees but never made it widely available.

Stewart Engineers was founded and committed to developing CVD so independent glass manufacturers could have access to the technology. With a concerted effort from the entire engineering staff, Stewart Engineers commissioned its first CVD system in 2005. Pilkington and AGC continued development of new CVD coatings, and then in 2006, Pilkington was acquired by Nippon Sheet Glass (later known as NSG). Stewart Engineers, NSG, and AGC all continued with CVD development in the 2000’s with Stewart having the most significant gains by percentage and the others by scale.

Stewart Engineers continues to develop cutting edge technology and offers a comparable product to that offered by NSG and AGC in every category. Stewart Engineers is developing a breakthrough in CVD, and we expect to launch it within 18 months. About half of the ten largest float glass manufacturers use CVD. The reason is clear; it is profitable. Since Stewart Engineers has made CVD available to independent float glass producers, it makes sense for the independent float glass producers to embrace CVD technology which has been profitable for half a century.

History of Glass

How to turn Sand into Glass

The raw material from which glass is made is silica, the most abundant of all the earth's minerals. Milky white in color, it is found in many forms of rock, including granite. And as every beach in the world has been formed by water pounding rocks into tiny particles, sand is the major source of silica.

When you examine a handful of sand, any grain that is semitransparent - rather than black, red, yellow or some other definite color - is a grain of silica. Sand also contains other minerals, but silica is the main component because it is hard, insoluble and does not decompose, so it outlasts the others.

Pure silica has such a high melting point that no ordinary fire would convert it into glass. Today, lime and soda are combined with silica to produce soda-lime glass, used for making bottles, window panes, flat glass products, and cheap drinking glasses. When glass cools, its structure does not return to the crystalline structure of silica, which is opaque. Instead, it forms a disordered structure rather like a frozen liquid, which is transparent.

Making window panes, the old way

The technique for making thin, flat window glass was perfected in Normandy, France, in the 14th century. Known as crown glass, each piece was blown by a craftsman. An accomplished glass-blower could make only about a dozen windows in a day, making medieval window glass an expensive luxury.

For each pane, the molten glass is blown into a large bubble using a blowpipe. The bubble is then flattened and attached to the end of an iron rod, called a "punty" which is rotated as fast as possible by the craftsman.

The flattened bubble of glass fans out to form a circle 1m to 2m wide, depending on the size of the original bubble and the skill and strength of the craftsman.

The round, flat glass sheets were then cut for use as small window panes, particularly in churches. The "bullseye" at the center of the disc was the least transparent section, but because glass was so expensive, it would have been used anyway.

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Float Glass Cutting and Packing

The final online manufacturing process for float glass is the Cutting and Packing line. The cutting and packing conveyor is immediately downstream of the annealing lehr. It is comprised of special roller sections that are capable of transporting glass product without damage.

All cutting equipment and conveyor sections along the length of the conveyor are computer-operated and capable of handling "Jumbo" size (full ribbon in width, longer than ribbon width in length), "LES" size (full ribbon in width, ribbon width or less in length), and "SS" sizes (less than full ribbon in width, less than full ribbon width in length).

The various pieces of equipment on the cutting and packing line are:

Float Glass Annealing Lehr

What is a lehr?
A glass annealing lehr oven- often just referred to as a 'lehr', is a long, temperature controlled, kiln. Lehrs are typically 6m wide and 120m long, with an adjustable temperature gradient through which the glass passes. In the case of containers and tableware, the product is transported on a conveyor belt and for flat glass on a special roller conveyor. Adjustable electric heating elements and air heat exchangers are located in the lehr to maintain a consistent temperature profile across the width of the glass ribbon.

Why is a lehr used?
If the glass ribbon were to cool in ambient temperature air, the surfaces would cool much more rapidly than the internal body of the glass. This temperature gradient would cause the surface to be in severe compression, which will cause the glass to break spontaneously as the stresses exceed the strength of the glass.

What is annealing of glass?
After the formed glass ribbon leaves the tin bath, the glass is at a temperature of nearly 600°C, well above its annealing point. The rollers convey the glass into the A0 section of the lehr. This first section of the lehr only maintains a uniform temperature across the ribbon, usually with edge burners.
As the glass enters the B sections of the lehr, the glass temperature is cooler; around 540°C which corresponds to the upper end of the glass annealing temperature range. The annealing temperature for float glass is from 540°C to 470°C. The B zones use heating and cooling to cool the glass—in a controlled process— below its strain point, the lowest temperature at which permanent stresses within the glass can form.
The glass enters the mass air section of the lehr next. The goal in the mass air section is to cool the glass, to near room temperature, quickly and uniformly without causing the glass to break. As the name of this section implies large fans blow air directly onto the top and bottom of the glass ribbon. Dampers are used to bias the air so that temporary internal stresses are minimized.
At the end of the lehr, the glass is cool and ready to be cut and stored.

Tempering vs. Annealing
The glass annealing process differs from the glass tempering process in that the glass after annealing is only slightly in compression, but the glass after tempering is in extreme compression. Tempered glass production begins by loading precut annealed glass into a tempering oven. The oven reheats the glass to its annealing temperature and then cools it quickly compared to the annealing process of slowly cooling the glass below the strain point.
The faster the glass is cooled, the more compression the final product is in. Glass can’t be tempered on-line during its original production because it is still a constant ribbon of glass and cannot be cut into sizes after the tempering process.

Glass Forming

Forming Technology

Flat glass is manufactured using one of three processes: the sheet process, the plate process, or the float process. The float glass process has, almost entirely, replaced the sheet glass and plate glass processes.

Sheet Glass

The sheet glass process began with the mechanically drawing cylinder process, where large glass cylinders were made to be split and flattened into plates, and has developed into continuous vertical drawing processes. These processes depend on the stretching and cooling of viscous glass, and their produced glass typically has a desirable bright and transparent "fire finish" due to the surfaces of the glass ribbon cooling down without contact with solid forming elements until they are relatively hard. However, in stretching the molten glass, small differences in viscosity due to chemical and thermal in-homogeneity give rise to thickness variations in the glass ribbon which cause undesirable optical distortions.

Plate Glass

The plate glass process was an early process of casting molten glass onto a metal table and rolling it into a plate. Manufacturers improved the process by continuously rolling a glass ribbon between water-cooled rollers. This forming process yields surfaces which copy any undesired imperfections of the forming machines; consequently, the glass must be mechanically ground and polished to produce a transparent, smooth sheet of glass. Plate glass has a plain polished surface with a high degree of parallelism and is generally free from the optical distortion of the less expensive sheet glass.

Float Glass

Sir Alistair Pilkington conceived the float process as a means of combining the brilliant fire-finish of sheet glass with the freedom from optical distortion of plate glass. It has substantially replaced the plate and sheet processes worldwide since its introduction in 1959.

A StewartFloat® tin bath

StewartFloat® Tin Bath Technology

From the working end of the melting furnace, glass flows through the canal area into the StewartFloat® tin bath.

This tin bath consists of support steel, upper and lower casings, refractory liner, molten tin, heating elements, a reducing atmosphere, temperature sensors, temperature and flow control systems and is typically 8m wide and 60m in length. It is designed to control heat flow balance, desired ribbon width and thickness, and glass optical quality. The float bath contains nearly two-hundred tons of molten tin at an average temperature of 800°C (1472°F).

A continuous stream of molten glass, at a temperature of around 1050°C, is poured onto the bath of molten tin. Movement along the bath is created by the tractive force that is applied by a roller conveyor. This force transports the formed ribbon from the bath and through the annealing lehr.

The tin bath maintains the molten glass ribbon at a high enough temperature and for a sufficient amount of time to spread laterally and "float" out any surface irregularities. As the glass thickness falls towards the equilibrium value of 6mm thickness the rate of spreading decreases, and thickness and width then decrease under the stretching influence of the conveyor rollers. As the ribbon moves further along the bath, it is progressively cooled, until it is rigid enough to be bent and lifted from the bath by the conveyor rollers without breaking or damaging the continuous glass ribbon.

For thin or thick glass ribbons (other than the equilibrium thickness of 6mm) pairs of driven toothed wheels—called top rollersare applied to the top surface of the ribbon edges. The top rollers, which can be angled with respect to the longitudinal tractive force, grip the ribbon and apply a force which can control ribbon width and thickness.

Float Material Selection

Molten tin is the premier material to support glass in the float process. Pure tin provides a combination of properties uniquely suited to the process. Despite an extensive search, no better pure metal or alloy has been identified that meets the following material requirements:

Molten tin, like other materials, does introduce complications. It oxidizes rapidly when exposed to oxygen; therefore, a carefully controlled reducing atmosphere composed of a blend of nitrogen and hydrogen is introduced to the tin bath, keeping the entire bath at a positive pressure.

Float glass products are manufactured daily in a thickness range of 0.4mm to 12mm using StewartFloat® Tin Baths.

Glass Materials and Batching

Glass contains three major categories of constituents - formers, fluxes, and network modifiers. Silicon dioxide (SiO2), or sand, is used as the former and basic constituent with soda ash (Na2CO3) as the flux. Lime (CaCO3) and dolomite (CaCO3MgCO3) are network modifiers that stabilize the chemical properties of the glass.

Batch Plant Storage

The batch plant receives the delivery of raw ingredients for glass making. Delivery of these ingredients varies from site-to-site, particularly when the plant is installed in a country where some of the ingredients are imported rather than domestic. The types of materials can be divided into two principal groups: bulk ingredients including sand, cullet, soda ash, limestone, etc., and "small ingredients" - various combinations of minor ingredients that are used to change physical properties of glass including color, transparency, and refraction.

Batch plant storage systems are almost entirely dependent on the quantity of materials to be stored and the period of time for delivery of replacement materials. Production demand and delivery time of materials are critical factors in deciding how to design your batch plant.

Bath Plant Mixing

Poor materials, inefficient mixing, or excessive segregation of the batch following mixing can adversely affect the operation of the melting furnace and quality of melt. Therefore, it is necessary to ensure that the materials, as fed into the furnace, represent a mixture suitable for the melting furnace. Handling of the required ingredients, their weighing and mixing, requires special considerations. The number of ingredients, plus quantities to be handled, are such that normal bulk handling techniques cannot normally be used.

Batch Plant Control

The degree and measure of plant control systems are a matter of economics and desired mode of plant control. Ideally, each section of the plant should, where possible, be autonomous with respect to its ability to function separately from other sections of the plant.

The batch house system should have a control system that will permit automatic start-up and control of the batch plant for filling the furnace hopper as required, with automatic cut-off as soon as the furnace hopper is full. This is usually about an 8-hour shift worth of production needs.

Glass Melting Process


The typical melting furnace is a Six Port Cross Fired Regenerative furnace with a capacity of 500 tons per day. Cross fired regenerative furnaces have been built for very small and very large melting areas. The smallest units may be uneconomical, although in certain cases they may provide the only technologically possible route. The major sections of the furnace are melter refiner, working end, regenerators, and ports, and are constructed of specialized refractory material with an outside steel framework. The largest units are found in flat glass manufacture, where furnaces with total areas of 500m2 and above are found. The typical 500 ton melter has an area measuring 23m x 9.5m (218.5m2) with a glass depth of 1150mm.

The batch mixture is delivered to the melting furnace where the batch is heated to approximately 1580oC (2875oF). Insulation, special airflow features, and combustion air heating enable the furnace to operate at maximum fuel efficiency with negligible pollutant emissions. The batch is melted by fossil fuels, natural gas, or fuel oil.

The melting furnace consists of refractory bricks, in both standard and special shapes, support, and binding steel, insulation, the fuel firing system, temperature sensors, and the necessary controls. The design of the furnace is adapted to meet the plant's specific tonnage goals.

From the melter the glass flows through the waist area, where stirrers homogenize the glass into the working end. The waist is a refractory lined canal that connects the melter to the working end.

Melter Combustion

A cross fired regenerative furnace uses regenerators to store waste heat that is contained in the exhaust gases that are developed during a firing cycle of the furnace. The waste heat is then used to preheat the combustion air during the next firing cycle, resulting in a considerable improvement in fuel economy.
As an example, we will describe the firing cycles of a cross fired regenerative melting furnace. The first cycle will begin with firing from the North side of the furnace. The fuel, either natural gas or fuel oil is introduced to the furnace through several burners that are located under the ports. As the fuel enters the melter it is mixed with combustion air that has traveled through and has been pre heated by the regenerator. The fuel to air ratio and the total amount of fuel to each burner and port are very closely controlled. As the fuel air mixture enters the melter, the fuel is ignited by the intense heat of the melter and a continuous flame develops that will extend nearly the entire width of the melter. The combustion exhaust gasses leave the melter through the ports on the South side of the melter, make a 90° turn and flow downward through the South regenerator. Regenerators are refractory structures with an area of 11m x 3m and a height of 10m. Inside the regenerators, a matrix of refractory bricks is stacked from the bottom up to the port level. The bricks take on heat as the waste gasses pass through the openings in the brick matrix. At the bottom of the regenerator is a refractory lined exhaust flue that channels the exhaust gasses through a reversing valve to the chimney for discharge. The waste gas passing through the regenerator heats the brick (checkers) to 650oC at the bottom of the checker matrix and to 1320oC at the top of the matrix.

When the checkers of the South regenerator have reached their desired maximum temperature, the furnace reverses. This means that the fuel feed is stopped on the North side and started on the South side along with the reversal valve changing position so that the North exhaust flue is open to the chimney and the South exhaust flue is closed to the chimney and open to the supply of combustion air that can flow upward through the South regenerator, taking on heat, and combine with fuel from the South port burners to provide combustion to the melter.

A regenerative furnace changes it's direction of firing on a periodic basis, usually every 15 to 20 minutes, to enable it to recover some of the heat that is lost in the waste exhaust gasses and therefore to make it operate in a more efficient manner.


The refiner is an extension of the melter with an area of 13m x 9.5m (123.5m2). When all raw materials have been fully melted, large amounts of gases that remain in the glass can form bubbles, seeds, or blisters. The purpose of the refiner is to remove these gaseous inclusions.

Working End

The working end is a separate refractory chamber with an area of 14m x 8.5m (119m2) and a glass depth of 1150mm. Glass flows through the canal, where it is stirred, into the working end for additional conditioning and to establish a laminar glass flow to the next furnace, the float bath furnace.

The temperature conditioning, of the working end, cools the glass from a temperature of 1315oC (2400oF) at the entrance to 1065oC (1950oF) at the exit.

Glass Coating Technology Comparison

A variety of techniques are available to deposit thin films onto flat glass. The most widely used of these for producing high-quality functional coatings can be subdivided into two classes: Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD). PVD processes include many approaches of which sputtering is one, and is also the one most widely used for glass. Sputter coatings are generally referred to as "soft coat glass" and are applied using PVD processes. Pyrolytic coatings are applied using CVD methods and are referred to as "hard coating".

Both coating methods have advantages and disadvantages. It is essential to consider the performance and handling factors that best meet product and manufacturing needs when evaluating which system is right for a glass manufacturing facility.

A Stewart Engineers AcuraCoat® CVD Under-Coater for on-line pyrolytic CVD glass coating.

Disadvantages and Advantages of Magnetron Sputtering Technology

Sputtered coatings are applied off-line, independently of the float glass manufacturing process. Thin films are formed by accelerating high-energy ions from targets toward the glass surface at low temperatures. The ions bombard the glass surface, forming uniform, thin layers. The bond is weak, which is why the process is called "soft coating".

Commercial sputtered coatings are produced by depositing between six and twelve layers of thin metallic and oxide coatings onto the surface of the glass in a vacuum chamber. Silver is the active layer for low-emissivity sputtered coatings. Additional layers include barriers, color modification, oxide layers, and sacrificial metal layers.

Glass distributors have little choice but to install sputter systems to grow their businesses, which accounts for the popularity of these types of systems; however, glass manufacturers have other options and must consider the disadvantages of sputter coating glass:

Advantages and Disadvantages of CVD process

Chemical Vapor Deposition (CVD) is used to produce aesthetic and functional coatings as an alternative to PVD. Specialized coaters produce CVD hard coatings by passing metal oxides over semi-molten glass (600 - 700C) in the tin bath or annealing lehr. A chemical reaction occurs, joining the vapor with the glass surface permanently, through a strong covalent bond. The result is a hard and robust coating that enhances the strength and stain resistance of the glass.

These hard coatings, which cannot be accidentally wiped off during normal handling, are more durable than soft and fragile sputtered coatings. Manufacturers and fabricators handle CVD hard coatings with the same procedures and equipment as standard float glass, resulting in higher yields, higher profit, excellent lead times, and improved customer service.

Pyrolytic CVD hard coatings offer other benefits as well:
Which is best?
The type of coated glass that is best depends on several factors for a glass manufacturer, including where customers are located, the size and type of operation, inventory turns, and required durability.

For most glass manufacturers, pyrolytic CVD technology is superior due to lower capital and operating costs and higher throughput.

Downstream customers view both sputter coated glass and pyrolytic glass as high-performance glass products. Architects and building owners are generally open to using both products and are primarily concerned with aesthetics that favor CVD.

If your company is interested in determining the feasibility of a pyrolytic CVD hard coater, check out the Stewart Engineers AcuraCoat® CVD coater today.