Optimum Moisture Control In The Vertical Drying Range

A Vertical Drying Range (VDR) is a series of dryers used to dry woven fabric in open width condition in the textile manufacture process. It is the simplest and cheapest mode of drying fabric, mainly used for intermediate drying rather than final drying, and for pre-drying prior to stentering.

The VDR is made up of several cylinders. In the drying process, fabric is passed around a series of the steam heated cylinders.

The total number of cylinders can be anywhere from 6 to 40, grouped in stacks. In order to generate heat to dry the fabric, steam is injected into these cylinders, at pressures varying from 2 bar g to 3.5 bar g. The fabric to be dried is passed between the mangle press. Moisture from the fabric is squeezed out due to pressure from the mangle, so moisture percentage is reduced before passing through the VDR.

Moisture is further evaporated by convectional heat transfer between the cylinder surface and the fabric, resulting in complete drying.

The VDR can be used to dry down a wide range of fabrics.

Typical Variations Observed in the VDR

Moisture percentage

Inlet moisture varies depending on the preceding process, mangle pressure and type of cloth. Higher inlet moisture leads to increased evaporation load, resulting in increased steam demand in order to maintain the moisture percentage. This scenario is reversed in case of lower moisture percentage.

Steam pressure

Operating steam pressure varies from plant to plant. However, most plants operate at a high pressure of up to 5 Bar g. Optimising the VDR to operate at lower pressures of 1.5 to 2 bar g helps reduce steam consumption. Radiation losses and loss through non-working traps also reduce.

Parameters generally maintained at textile plants

	Minimum	Average	Maximum
Pressure	0.88	2.73	4.5
Speed	15	47	106
Inlet Moisture %	20%	62%	113%

Typical Problems Identified

Higher surface area

The VDR is designed to cater to varying GLM of cloth (eg: 200 to 400 GLM). Since surface area is constant, at lower GLM there is more surface area available to evaporate less water, reducing the drying rate of VDR. This leads to inefficient capacity utilisation and lower productivity on the VDR.

Over drying of cloth

Often, the machine is oversized for the GLM of cloth being processed, causing over drying of the fabric much before it reaches the last cylinder of the dryer, resulting in excess energy consumption.

High pressure of steam

Amount of latent heat reduces when the VDR is operated at pressures higher than 1.5-2 Bar g, leading to excess steam consumption.

Inefficient Condensate Evacuation and Recovery

Most plants adopt a group trapping system so as to reduce capital costs. Since the steam load at each of the drying cylinders varies, in a reducing pattern from input to output, the condensation rate and condensate pressure also varies. A group trapping system, with single trap at a common outlet, cannot therefore handle condensate effectively, leading to bypass valve opening, leaking rotary joints and syphon issues.

Capacity utilization

Steam to water ratio is better when drying rates are high. When the VDR is not utilised to full capacity, the steam consumption of the machine is impacted.

Measuring VDR efficiency

Specific steam consumption (SSC) is the amount of steam consumed to evaporate water per kg of fabric. The water content in unit weight of fabric depends on the inlet and outlet moisture and GLM of the fabric. Hence, the SSC varies with the variation in moisture per cent of fabric. SSC is therefore not an accurate parameter to measure the performance of the VDR.

To nullify the impact of varying moisture and hence varying SSC, Steam to Water Ratio, i.e. quantity of steam used to evaporate a kilogram of water, is a better parameter to measure the efficiency of the VDR.

Conventional VDR control systems

In conventional steam systems for the VDR, steam is controlled manually. A single trap is installed at a common outlet point, resulting in inefficient condensate evacuation and increased backpressure. They do not offer any provision for moisture control, as a result of which the fabric may retain excess moisture and the finishing solution does not adhere properly to the fabric.

The Forbes Marshall Solution

The vdrMAC from Forbes Marshall is a complete automation package for optimim moisture control. Equipped with the vdrMAC, VDRs operate on higher drying rates. Outlet moisture is maintained at the desired limit and at maximum permissible (mechanical limits) speed for superior quality of fabric. The system helps enhance productivity with better capacity utilisation and offers maximum steam savings.