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Industria Textil Pirobloc

PROCESSES

Pirobloc provides efficient, safe and custom solutions to heat different types of equipment used on the textile industry, among which there are:

Stamping tunnels

Calenders

Rollers

APPLICATIONS

In the textile and dyeing industry, Stenters are machines to set, dry and finish fabrics, to widen them and correct distortions in the weft. Stenters are associated with a thermal effect, the mechanical action of cross-stretching or heat setting of the fabric; thus, a heat source is required for the equipment.

Many years ago, this heat was provided indirectly by steam.  Later, the option to provide this heat with thermal fluid became very common and, later still, the option of direct heating via combustion gases from gas burners: Direct gas heating.

In recent years, the option of mixed direct/indirect heating with gas burners and thermal fluid has also emerged; although this system is exclusively applicable for situations where the gas supply is unreliable or not constant.

The advantages and disadvantages of heating systems from both a technical and economic point of view are evaluated below.

Direct heating by gas burners

Its operating scheme is shown in Figure 1.

A burner (1) is inside a chamber (3) within the machine. The combustion gases are sucked by a fan (2) through a filter and distributed through pipes with distribution holes (5) to make the temperature on the fabric as uniform as possible and pulled through these pipes by chains (6).  Some of the gas is recirculated (7), but most is evacuated at the smoke outlet (4).

There are different by-passes to recirculate at different levels according to the machine production needs.

Industria Textil - RAME

Figure 1. Stenter with direct heating by gas burner

1. Gas burner
2. Gas feed/recirculation fan
3. Combustion chamber
4. Smoke outlet
5. Gas distribution holes
6. Drag chain
7. Gas recirculation

Advantages and disadvantages of direct heating

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.

    Industria textil - Maquina RAME

    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 (%)0102030405060708090100
    Flame temp (ºC)154514651400133512701205114010751010945880

    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.

Indirect heating by thermal fluid

Its operating scheme is shown in Figure 3.

Thermal fluid from a boiler (6a) passes through a heat exchanger (2) and heats air sucked by a fan (1) through a filter. As in the direct heating design, this is distributed through pipes with distribution holes (5) to make the temperature on the fabric as uniform as possible while being pulled through these pipes by chains (4). Some of the gas is recirculated (7), but most is evacuated at the smoke outlet (3).

After the heat from the thermal fluid has been transferred to the air, it is returned (6b) to the general network to the boiler.

There are different by-passes to recirculate at different levels according to the machine production needs.

Industria Textil - Maquina RAME

Figure 3. Stenter with indirect heating by thermal fluid

1. Air feed/recirculation fan
2. Air/thermal fluid heat exchanger
3. Air outlet
4. Drag chain
5. Gas distribution holes
6a. Exit from the general thermal fluid supply. Fluid inlet from boiler.
6b. Return to the general thermal fluid supply. Return to boiler
7. Gas recirculation

Advantages and disadvantages of indirect heating

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.

Conclusions

For all the above reasons, we consider that direct heating machines are suitable if the number of stenters is not too high, operation is non-continuous, with few variations in fabric type or process parameters, and there is a reliable gas supply.

This minimises the disadvantages, as the maintenance of the numerous burners can be performed when the machines are stopped. Also, gas installation will not be significant due to the low number of machines, and implementation costs will be moderate, which is a great advantage.

However, indirect thermal fluid heating facilities are more suitable for high capacity production processes, with numerous stenters in constant 16/24 hour operating times. This is because this system lets production continue without stopping for maintenance or adjustments. Also, equipment parameter changes due to different fabric types are practically immediate with no delays in start-up. Also, having the facilities in constant operation maximizes its advantages: e.g. energy efficiency and low and external maintenance.

Table 2 attached summarizes this study.

Direct heating
Gas burners
Indirect heating
by thermal fluid
Advantages (+)FinancialImplementation costsOperation costs
TechnicalReduced space
Quick start-up
Maintenance
Continuous work
Energy efficiency
Flexibility
Disadvantages (-)FinancialOptions
  • NOx burners
  • Heat recovery
Possible installation of complex gas
Implementation costs
TechnicalMaintenance
Energy efficiency
Yellowing
Gas supply dependency
Start-up in non-continuous facilities
ApplicationFew stenters
Non-continuous operation
No changes in fabric conditions
High production
Continuous operation

Table 2. Summary: advantages and disadvantages of each system



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