Textile industry

Technical advantages

• Reduced space. No general installation of tubing is required, or space to install a boiler or auxiliary equipment.
• Quick start-up, as there is no intermediate exchange fluid to be heated beforehand.

Technical disadvantages

1. There is a general conviction that a direct heating system provides high energy yields.

   Although the system filters the combustion gases, the presence of unburned products or ash must be avoided in all these gases, as it would affect the quality of the finished product and the energy efficiency - fuel used but not burnt. This necessarily leads to combustion with a high excess of air and therefore moderate, if not low, yields.

   This is because, although the smoke outlet temperature is not high, its flow rate is important and the amount of energy expelled without being used by the product is important.

   Although it is difficult to evaluate the overall difference in yield, indicative figures are shown in Fig. 2. 2.

   Thus, for a normal and proper excess of air in combustion with natural gas of 10-20%, the yield is of the order of 89-90%. For higher air excesses to avoid unburned products, of at least 50-70% excess air, the yield is around 85-86%. For values of 80-100% excess air, the yield is 80-83%.

   Thus, yield reductions of 5-8%, favourable to indirect heating, is believed to be about right.

   

   Fig 2. Yield vs excess air for a given exchange surface.

   Although it is not the usual option, there is the possibility of not working with high excess air. The consequences of this working method, however, are even worse from an energy efficiency point of view.

   In effect, low excesses of air involve a high flame temperature, on the one hand, (see Table 1 attached) and on the other hand, a low coefficient of heat transmission, as the velocity of the combustion gases is low, due to low flow rates.

   As a result, the temperature of the gases in the smoke outlet is very high and therefore energy efficiency is not satisfactory.

   Excess air (%)	0	10	20	30	40	50	60	70	80	90	100
   Flame temp (ºC)	1545	1465	1400	1335	1270	1205	1140	1075	1010	945	880

   Table 1. Flame temperature vs excess air for natural gas combustion

   Obviously, a high flame temperature for low excess air and low combustion gas flow rate are also found in indirect heating. However, one of the great advantages of direct heating - reduced space - works against it in this case.

   The exchange surface cannot be very large, as it would mean an increase in the size of the machine, which would be a significant, if not decisive, disadvantage. Therefore, for high flame temperatures, this exchange surface is unable to absorb the energy properly and the smoke temperature increases, decreasing energy efficiency.

   In a boiler, with a proper exchange surface, with no space problems in consumer appliances, combustion gases can have moderate temperatures and give satisfactory yields.

   In addition, as mentioned above, there is a greater risk of unburned fuel in this working method and therefore a significant decrease in energy efficiency.

2. Maintenance is a critical point.

   Gas burners do not require large maintenance operations. However, there are usually a large number of burners installed in each stenter: one for each field.

   Therefore, there is an increased risk of a breakdown, and combustion and its adequate adjustment are important, requiring special attention to this section, as it can directly affect the reliability and quality of production.

   Also, a change of fabric or process conditions requires a change in the burner combustion parameters. This point is generally overlooked and undoubtedly affects the final quality of the product.

   Finally, the maintenance is performed on equipment inside the stenter, which involves stopping production to perform most of the important operations.

3. Reliable and stable gas supply.

   This is a problem only in certain circumstances, as a stable gas supply is currently present in most places.

   However, this may be a problem in certain countries or rural locations. The use of propane or butane gas in individual facilities is not a good alternative, as the cost of these fuels is higher.

4. Possible yellowing of the product.

   The combustion of natural gas produces NOx which can react with the fibre and cause yellowing. To prevent this, precisely adjusted, low NOx burners are required, which makes implementation of the equipment more expensive and affects maintenance.

Financial advantages

Low implementation costs. As no significant installation of heat transport pipes is required.

Financial disadvatages

Depending on the distribution of the stenter in the production area, the installation of gas may be complex and expensive.

Technical advantages

1. Operating flexibility. A change in the burner combustion parameters, following a change in fabric type or process, is immediate to get the required results.

2. Less maintenance and fully centralized in few components external to the machine. For continuous work, these components can have replacements on stand-by, so production is not stopped.

3. Auxiliary fuel. The need for an auxiliary fuel supply to ensure production is maintained may not be due only to an unstable natural gas supply.

Technical disadvantages

Time required to put into service. This disadvantage is exclusively for equipment that does not work continuously.

It can be minimized by maintaining a moderate temperature in the facilities, even if the machines are not in production.

It must be remembered that energy losses with this option are minimal if the facilities are properly insulated.

Economic advantages

Low operating costs due to lower maintenance, stoppages not affecting production, fewer critical components, for example.

Economic disadvantages

High implementation costs compared to the direct heating option.

Care must be taken in this regard, as some of the direct heating options – e.g. burners with low NOx index and heat recovery from combustion gases - may involve higher investment.