Tuning a garment tunnel finisher

A garment finishing tunnel is the workhorse of a modern garment rental plant. The process: the wet garment first passes through a steam spray where it is rapidly warmed up, enabling the polyester cotton fibres to relax. The weight of the garment causes some smoothing and this is accentuated by a strong downward flow of recirculated air which contributes towards the drying process.

For a tunnel finisher to give optimum performance there are some basic rules for the engineer to follow.

Most complaints surrounding tunnel finishers normally concern their failure to remove all of the creases. Stubborn creases are, however, usually created by incorrect washing and not by incorrect settings in the tunnel.

An open pocket washer-extractor with good lifters will ensure the work lifts and drops without rolling and twisting, which tends to cause long roping creases. Correct loading and good dip control will avoid pressure creasing and correct temperature control will avoid creases caused by thermal shock. A well-washed garment can be well-finished by most tunnel finishers. However, the creases in a poorly washed garment will not be removed, even by the very best tunnel finisher in the very best condition.

Good tunnel finishing relies on a good hanger on which to mount the garment. It is also important to dress the garment correctly on the hanger, so that it sits properly with the top button fastened. Slipshod garment mounting will produce a poor result.

It is important to load garments on every peg on the conveyor, if this is possible without compromising quality – the garment spacing must be sufficient to allow a blast of air down between each garment to smooth the fabric.

It is the frequency of peg loading together with the conveyor speed which determines the residence time in the tunnel and hence the drying capacity. There will be a loss of output if garments are hung, for example, every third peg if every other peg would suffice. However, loading on every peg is preferable The steam spray at the tunnel entrance is critical to good crease removal performance and to operating economy. Too low a steam flow will give inadequate relaxation and crease removal. Too high a flow will waste steam and, importantly, slow down the production rate – excess steam will raise the humidity in the rest of the tunnel.

Given the importance of this, it is surprising to see how few tunnels are equipped with easily adjustable steam spray rates.

Even more important is the quality of the steam supplied via the sprays. A few droplets of boiler water or condensation from the steam pipework will convert a dry steam spray into a source of contamination that results in yellow-brown marking to the shoulders and upper arms.

The temperature of the recirculating air flow to the drying zone in the tunnel is usually finely controlled and displayed on a digital monitor. The temperature setting needs to be as high as the fabric will withstand. Cotton settings usually run up to 165°C whilst polyester cotton settings are set much lower.

The higher the temperature the faster the garments dry, hence the higher the throughput speed. The limit to this is set by the degree of garment damage, apparent in discolouration of the cotton fibres or thermal degradation of the polyester fibres.

The air recycle rate in the tunnel finisher is usually set up at commissioning and is rarely adjusted. It is important to use as high a recycle rate as is necessary to maintain the humidity in the first section of the tunnel whilst the fabric is being smoothed down – too rapid drying will produce a high output but poor crease removal. Furthermore, the higher the recycle rate, the lower the net energy input into the tunnel and therefore the cheaper the operation.

Air extraction from the tunnel is usually by forced ventilation through an extract fan and vent. The extraction fan setting should be sufficient to avoid hot moisture laden air spilling over into the workroom. A higher extract rate will simply push hot air out into the atmosphere. The design of the hoods over the inlet and the outlet to the tunnel will assist in maintaining a comfortable working atmosphere for staff.

Filters are also essential in the circulating air flow to remove lint and the contamination it carries. It is lint which builds up on the upstream face of the filter and must be removed every two hours.

Far more insidious is the build up on the reverse of the filter screen of other trace contaminants, particularly softener and other residues from the wash process. If contaminants are not removed the drying performance and crease removal performance of the unit will still suffer because of deposits on the downstream side.

The fundamental rule to remember is that high crease removal performance and good colour retention is as dependent on the finishing tunnel as it is on the wash process and washing machine loading.