Material solutions

Keep the air flowing for productive drying

1 December 2011



Ian Harris sets out some simple guidelines that will improve tumble drying efficiency


The efficiency of a tumble dryer is dependent on the available airflow. Dryers can be designed to use the air in the environment around the machine, draw fresh air from outside the building or recycle the hot air already in the machine.

Many dryer installations draw their air supply from the environment where they are situated and then pass this air through the heat source for the tumbler before it reaches the fabric in the machine.

On steam-heated dryers, the air is drawn through a steam battery that acts like a radiator and warms the air. To prevent contaminating the battery and the work in the tumbler, the incoming air passes through filters to remove the dust, lint and debris.

The benefit of using air from around the machine is that it will be quite warm, especially if the dryers are mounted at a high level – as could be the case with dryers serving a tunnel – thus taking less heating. But this air inevitably contains a lot of lint that needs filtering out and will be laden with humidity.

Air drawn from outside the building should be a lot less contaminated but will also be a lot colder and therefore more energy will be required to heat the incoming air to the set temperature.

One option that seems to make economic sense is to recirculate the exhaust air from the tumble dryer and filter this into a fresh air supply.

This air will be quite heavily contaminated with water vapour and lint that will need to be removed before it can be re-used.

The advantage is that the increased inlet air temperature reduces the energy required to operate a dryer by up to 26%.

Because tumble drying can use as much as one-third of the total steam consumption of a laundry, heat recovery systems can save a lot of money. It is also far more cost-effective to remove water from fabric by extraction at the washing stage than by tumble drying.

Never be tempted to compensate for poor extraction by extending drying times. It is essential, when tuning the press from a tunnel washer, that the maximum press pressure is maintained for the maximum period of time.

This will involve adjusting the speed at which the maximum pressure is achieved.

However, care must be exercised to avoid sheeting being damaged because the press is too inflexible in its programming.

Filters must be cleaned regularly to maintain good airflows through a tumble dryer. The operator will usually be responsible for the regular cleaning of the air outlet filter screen and maintenance.

Ideally this should be done after every cycle to maintain airflow.

In practice, the frequency of cleaning on the type of textile being dried, the length of time it is in the dryer and its propensity to lint.

The operator must be vigilant to ensure that the filter is not damaged in any way.

A damaged filter may let more air through at first but it will eventually result in lint entering the steam dryer’s battery and blocking the ducting. Specialist cleaning personnel may then be required at considerable cost.

Therefore, it is important that all exhaust ducting is regularly checked, on a quarterly basis for example, and cleaned if a build-up of dust, lint and debris is detected.

Correct duct size

There are a factors about the duct that should be considered. It must be sized correctly with help from the manufacturer. This will ensure that all exhaust gas is efficiently removed from the dryer without restriction. If the duct is too small in diameter, this will reduce the airflow and increase drying times.

Channelling the exhaust from two or more dryers into a common duct will also have a negative effect on drying performance, as this could result in a backflow of the exhaust air.

If the physical restrictions within a building mean using a common duct, then it is important to minimise the potential effect of exhaust air backflow.

This could include sweeping the individual ducts at an angle of 45 degrees where they meet the common duct or introducing baffles.

Flexible ducting should not be used, as its concertina-like characteristics will encourage dust, lint or debris to lodge in this area.

A well-designed duct will be as short as possible with the minimum number of bends, as bends will restrict airflow.

When exiting a building through a roof, ducts should always terminate with a “swept bend”. The so-called “chinaman’s hat” should never be used as the duct cover will restrict the exit and create a back pressure.

Fans are used to draw the heated air through the load in the tumble dryer. To provide the necessary airflow, the fans must be operated at the correct speed and rotate in the right direction.

Airflow is reduced when the perforations in the dryer cage become blocked. Lint and debris will block the perforations but the most common blockages are caused by plastic bags melting in the dryer. To maintain airflow and reduce the additional possibility of a washer blockage, it is important to make rigorous checks to prevent all debris – plastic bags in particular – from entering the washing machine.

If the tumble dryer is not sealed properly – due to badly fitting filters, door seals or even damage to the machine’s outer casing – then air will leak in. Because this air is unfiltered, the effect will be the same as that attributed earlier to damaged lint filters. The cool air will also restrict the dryer’s efficiency.

The dryer should be regularly checked for evidence of air ingress.

Correct loading of the tumble dryer is critical to airflows and to the effective evaporation of moisture. The load’s residual moisture after washing, the fabric type and the size of the items to be dried should all be taken into account when maximum loading weights are determined for different classifications.

Cotton items are generally loaded to the machine’s full capacity, but polyester cotton should be loaded to approximately 75% of the tumbler’s capacity and will require much shorter drying cycle times to prevent pressure creases forming during the drying cycle. Very bulky items like duvets may have to be dried at only 35% of the maximum capacity.

For stand-alone batch dryers, it is critical that the operator chooses the correct cycle. Taking into account fibre content and drying instructions, the operator should have enough information to choose the most suitable cycle. Once the cycle has been started, the dryer should not be stopped for any reason – for example, to check if the load is dry – as this will extend drying times.

A cool-down must always be selected with any fully dried cycle to minimise the possibility of the load spontaneously combusting after it has been taken out of the dryer.

Fire is the biggest risk in any laundry and spontaneous combustion contributes to the majority of fires. So a well-managed shut down procedure is essential to ensure that all items are removed from a dryer at the end of the day and that all fully dried items, especially kitchen linen and mops, have been completely cooled.




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