Textile dryers, what's the best choice?

When a purchasing decision has to be taken among different dryers which can perform the same operation, important comparison criteria must be taken into consideration...

When a purchasing decision has to be taken among different equipment which can perform the same operation, the most important comparison criteria to be taken into consideration are:

the quality (ie. the added value) of the products obtained from each equipment;

the overall costs deriving from the use of each equipment.

These criteria have to be evaluated when comparing equipment performing the same operation by means of similar technologies, but they are even more important when comparing equipment which apply totally different technologies.

For instance, when we consider the drying of fibres and yarns after dyeing or any other wet treatment, different technologies are available for this purpose: there are "conventional" methods, where the textile product is heated / dried by means of hot air blown onto or through it, or alternatively, the "radio frequency" technique, where the textile product is heated / dried in an endogenous (volumetric) way by means of a suitably generated electromagnetic field.

Let us analyse in more details the main features and the practical implications of various conventional drying methods.

Drying rooms - drying cabinets

These are the simplest and oldest drying systems, which play nowadays a marginal role in textile mills worldwide, due to some major drawbacks, namely:

extremely long and inconsistent drying times;

poor control of the residual moisture content within the product;

too much space / volume required to obtain a reasonable product output;

very low energetic efficiency.

As a logical consequence, drying rooms / cabinets are used very rarely, mainly for the drying of very small dye-batches or of extremely delicate products that cannot stand the thermal and/or mechanical stresses to which they would be submitted in other drying equipment.

For the above reason, we will not consider anymore this drying method in the rest of these notes.

Chamber dryers - semi-rapid dryers

These dryers became - and still are - rather popular, thanks to the reasonably low capital investment required and to the overall acceptable level of performance in terms of drying times, running costs and quality of the dried products.

On the other hand, the majority of the end-users report several drawbacks which make chamber and semi-rapid dryers far from being an ideal solution within the modern yarn dye-houses.

The most common inconveniences reported include:

long drying times and correspondingly high energy consumption are found with dense (tightly wound or highly compressed) packages, with fine count yarns and with packages having large diameter;

longer time is also required in order to dry bleached white and light shades without yellowing problems (due to the need to keep low the air temperature);

accurate loading of the packages on the carriers / trolleys is necessary to avoid preferential air paths which cause severe loss of efficiency and very long drying times;

the external ambient conditions (air temperature, humidity) strongly affect the drying efficiency, thus causing noticeable inconsistency of the drying times from batch to batch;

as the drying time is affected by too many variables which cannot be kept under proper control, it is difficult to predict and set accordingly the end of drying cycle of each batch, therefore under- or severe over-drying occur rather frequently;

the surface of packages becomes dirty (this phenomenon is very evident with white and light shades) due to the large volume of external air passing through the yarn - in spite of air filtering systems, which require frequent and accurate cleaning operations;

uneven residual moisture distribution within the same batch, from package to package, is found depending on their position in the carrier;

packages having different diameters or winding densities are dried unevenly;

the residual moisture content within each package is not evenly distributed;

overdrying of the whole batch is necessary to ensure the absence of wet packages or wet spots within the packages;

rather long conditioning time of the yarn is required after drying;

some displacement and evaporation of finishing chemicals and softeners occur;

there is no possibility to exploit the nominal drying capacity of the machine when smaller, variable size dye-lots have to be processed. The main problem is that very often small dye-lots cannot be loaded together (due to colour contamination problems and/or to the difference in the drying time needed for each lot); furthermore, partial loads solution lead to a correspondingly higher drying cost per kg of product;

loading and unloading operations of the carriers / trolleys are quite labour intensive, unless full automation is available (but it is very expensive and little flexible);

there is no possibility to use the dryer for different products other than yarn packages (this is a major inconvenience for all dye-houses where textile products in various forms such as loose fibres, hanks, tops etc. are processed).

Due to the above reasons, we can say that only in the case of rather soft, small and regularly shaped yarn packages, especially made of synthetic or blended yarns in coarse counts, dyed in medium size lots, the chamber and semi-rapid dryers are a viable and economically convenient solution within a package dyehouse, ensuring an acceptable compromise between quality and cost.

On the other hand, in case of tightly wound or compressed yarn packages, or having large diameter, or not having consistent weight or winding, of fine count or highly hydrophilic yarns, in various different batch sizes and colours, especially white and pastel shades, delicate yarns etc. this drying method is not the recommended solution.

Rapid dryers - pressurised dryers

Many yarn dyeing machinery manufacturers have included in their production range the so called rapid dryers or pressurised dryers, such that the dye-carriers bearing the soaking-wet yarn packages coming out from the dyeing vessels can be placed directly inside drying vessels - having the same shape and size - where the water extraction and drying processes are carried out as a single operation.

Actually, the main advantage of these dryers is the low labour cost deriving from the extraction of the unbound moisture followed by drying being carried out in the same equipment, this requiring only a single, fast and very simple loading operation.

Another advantage, especially if compared to chamber and semi-rapid type dryers, is the much shorter drying time required for various products, which normally does not exceed a couple of hours - at least with the most recent and efficient rapid / pressure dryers - provided the cooling water temperature fed to the moisture condensation unit of the equipment is low enough (which is not always the case, especially in countries having a hot climate).

On the other hand, rapid / pressure dryers have several technical and economical drawbacks: we can say these equipment are subjected to, and actually exasperate - due to the very strong air flow and to the high temperature gradients within the packages - all problems generally related to drying methods by hot air convection, as previously described for chamber and semi-rapid dryers (with the only exception that in this case the air circulation is carried out in a closed circuit, so that the external air humidity and dirtiness do not affect the process).

Moreover, in addition to the problems already mentioned, further serious inconveniences are specific to rapid / pressure dryers, namely:

due to the high air temperature and pressure, the yarn is thermally and mechanically stressed, so that its physical properties such as strength, elasticity, softness and hand are negatively affected;

due to the same reasons, yellowing, discoloration, colour migration are quite common phenomena;

again, the strong air flow generates a lot of hairiness in the yarn, which causes various inconveniences when the yarn is woven or knitted;

some yarn hairs accumulate in the equipment during the drying process, and this makes it necessary to carry out frequent cleaning / washing operations of the vessel, especially when light shades have to be dried after dark shades (to avoid cleaning after every drying cycle, batch scheduling / colour sequencing during the day - starting from the lighter shades - is recommended, but this is not always feasible);

the functioning of the equipment is very noisy and a lot of heat is released to the ambient.

Notwithstanding the above drawbacks, the main disadvantage of rapid / pressure dryers is the very high overall utilities’ cost.

In more details, the electricity, steam and cooling water consumption of rapid / pressure dryers is very high for two main reasons: the mechanical hydroextraction operation, which is the cheapest way to eliminate the large amount of unbound water retained by the product when coming out from the dyeing vessel, prior to the drying process, is not efficiently performed by the vacuum / pressure treatment which the product undergoes and consequently a very large amount of water has yet to be eliminated by evaporation; furthermore, when the drying process is approaching to the end (ie. when the final moisture which is strongly bound to the core of the fibres, has to be eliminated), the heat transfer efficiency from the air to the residual moisture, as well as the efficiency of the moisture condensation system, become very low, therefore large amounts of mechanical and thermal energy and of cooling water have to be wasted to eliminate just a little remaining amount of moisture.

The Radio Frequency drying technology

As an obvious consequence of what we described above, in the last three decades most textile fibres and yarns dyers adopted the radio frequency drying technology within their operations, as this method brilliantly solves all the problems related, directly or indirectly, to heat transmission phenomena, ie. to all conventional drying equipment.

We mention herebelow the main advantages of the RF technology in the drying of yarn packages and, more generally, of all textile substrates:

very short and consistent drying times (generally from 15 to 50 minutes, depending on the moisture content to be evaporated) with all products, irrespective of product size, density, yarn count, shade, etc.;

perfectly controlled and uniform drying process, leaving very even residual (regain) moisture distribution within the packages, from package to package, from batch to batch, thus eliminating the need of long yarn conditioning time after drying;

full flexibility irrespective of the product type (yarn packages, hanks, tops, loose fibres, etc.);

any product or batch size can be dried with the same high efficiency, maintaining the same unit drying cost;

high energetic efficiency (about 65-70% of the total energy consumption of the equipment is exploited for the drying process itself) with no heat dispersions to the ambient;

no influence of external ambient conditions (humidity, temperature) on the drying performance / time;

just-in-time and instantaneous operation, no pre-heating required;

no yellowing, no discoloration, no contamination between different colours;

no need for batch scheduling / colour sequencing / frequent dryer cleaning;

no hairiness generated in the yarn;

no dirtiness on the surface of the yarn due to air pollution;

negligible displacement and evaporation of finishing chemicals;

better physical and mechanical properties of the yarn, thanks to short drying time, low drying temperature and no mechanical stresses;

in many cases the rewinding operation after drying can be avoided.

From the economical and operational points of view, one may argue that in comparison with the single and very simple handling operation required by the rapid / pressurised dryer to process an entire dye-lot, RF drying implies two separate, labour intensive operations: the first one for loading and unloading the centrifugal hydroextractor, the second one to feed the packages to the RF dryer.

However, it should be noted that in spite of the costs due to additional handling and to the hydroextraction operation, the practical experience of several end-users shows that even where the hourly labour wages are quite high (like in western Europe or in the USA) total costs of RF drying are still significantly lower than those of rapid / pressure drying.

Conclusions

The main advantage of using rapid / pressure dryers is the ease of operation and, consequently, the very low labour cost. However, total utility costs are the highest among various drying methods - even if heat recovery systems are duly installed and utilised - and, even considering the high hourly wages paid in western European countries and USA, such high utility costs are not compensated by the reduced manpower requirements. Therefore, this drying method remains, eventually, the most expensive one and also the one which gives, in general, the poorest quality level in the dried products.

From the technological point of view, the main difference between rapid / pressure dryers and chamber / semi-rapid dryers is that in the latter the hot air is pushed through the product at a much lower pressure / speed, so that the drying times are longer but a lot of (expensive) mechanical / electrical energy can be saved.

On the other hand, the weak air flow through the product cannot perform the required mechanical hydroextraction operation, which has therefore to be accomplished separately in advance, exactly in the same way as when using RF dryers. It means that, even though utility costs are quite lower (not far from the ones achieved with RF dryers), the main advantage of using rapid / pressure dryers is totally lost when using chamber / semi-rapid dryers. Moreover, most of the problems related to the poor quality of the dried products and to the intrinsic lack of flexibility of all conventional drying methods by heat convection are still there, even if not as dramatically as with the rapid / pressure dryers.

On the contrary, RF drying, while ensuring the lowest possible overall running costs compared to all conventional drying methods (even including the most economical, new generation chamber / semi-rapid dryers) is the only technology which allows the attainment of the highest quality standards and the highest degree of operational flexibility, which, in their turn, result in an increase of the overall profitability within the dyeing - drying operations of all textile mills.