/ Design guidelines
Version 4 - Internal
NF-EQ / tuohir / OU500141608 Rev 1 / 1 (6)

,

Design guideline for demister maintenance solution

  1. Introduction

Outotec has developed a new forced draft solution cooling tower design. Tower development began with need of streamlining the maintenance work of a solution cooling tower while achieving improved cooling efficiency. The improved maintenance efficiency of the new design is largerly achieved with the use of horizontal flow demisters.

Conventional solution cooling towers have vertical outflow demisters, i.e. the air out of the tower is flowing upwards from the top of the cooling tower. In Outotec Cooling Tower the air exits the tower flowing horizontally.

Figure 1. Geometrical presentation of solution cooling towers, on the left vertical outflow cooling tower, on the right horizontal outflow tower. Arrows show the flow direction of the air flow.

Figure 2. 3D render of the cooling tower

The tower gas outlets are equipped with demisters to remove the droplets, most commonly used are the vane type demisters. Vane demister is based on the momentum of the droplets in the gas flow which causes them to collide with the vanes and thus be removed from the gas flow.Mesh type demisters that are used in water cooling tower could be of higher efficiency, but very difficult to clean and easily blocked.

Figure 3. Presentation of the functioning of vane demister, vertical and horizontal air flow through the demister

Usually there are two layers of demisters; first one is coarse layer for removal of larger droplets and second one fine layer, for removal of the smaller droplets, with smaller spaces between the vanes. The larger caps in the first layer also allow for easier cleaning of the demister.

When air flows vertically through the demister out of the tower, the drainage of droplets out of the demisters is counter-current to the gas flow as seen in Figure 2 In the horizontal demisters the drainage is perpendicular to the air flow.

  1. Problem

For the maintenance it is sometimes required to lift the demisters out of the cooling tower. Demisters are large elements that are housed in the demister boxes as seen in Figure 4. Each demister box has two elements, in the Figure 5 the coarse demister box hatch is open and the fine demister hatch is closed. The coarse demister needs the most maintenance work of the two.

Figure 4. Demister element and workman

Figure 5. Coarse Demister maintenance cover removed

The problem we’re presenting is this document is how to remove the demister. What kind of crane or other lifting solution to be used. In following chapter we have presented requirements for the lifting and also rough outline of different solutions. However, we encourage the group to find innovative solutions to pursue.

2.1.Lifting device

The purpose of thislifting device is the ability to raise the demister up for maintenance (mainly washing) and then lower back down at its place thus not requiring anyone to go inside the demister box to wash it. If possible it would be suitable if the demister element can be lowered to the ground with no need for extra equipment to enable the change of element.

The lifting capacity required to lift one demister element is described in below table.

Table 1. Crane capacity requirement.

Weight kg
Demister element / 135 kg
Maximum gypsum load / 100 kg
Total load / 235 kg

Five main types of lifting devices that can be used has been identified so far.

2.1.1.Crane located at the center of the tower

If possible, have crane in center. This probable has highest effect on the tower cost, but could be the most efficient solution for maintenance with shorter boom than the side boom crane.

2.1.2.Side boom crane

Crane could be located at the side of the tower and supported either to the ground with steel beam or alternatively to the access platform. If supported to ground with the beam, it can be tied up to the tower shell. This was checked earlier with fabricator and required no extra strengthening of the structure.

This design has additional benefit that in principle one crane could maintain two towers when located optimally.

2.1.3.Overhead crane

Overhead crane is probably best for maintenance, but it also is the most expensive, however cost would be separate and for use of this crane no change is needed on the cooling tower design compared to no crane situation.

2.1.4.Mobile crane

Client can also use mobile crane that is driven in to the place when required.

2.1.5.Demister local crane

In some instances client doesn’t purchase overhead crane or boom crane for the tower. In this case would local movable crane that is easily installed to the roof of the tower be suitable.

  1. The task for the student group

Student task is divided several phases and continues to next one when issues are complete in each one.

Development phasesare estimated to be as follows. However if the team sees need for additional step, it is encouraged to use initiative and modify the plan:

  1. Students are basically introduced to machine directive and what is required for lifting devices ( Machine directive EU)
  2. What lifting tools are available from different manufacturers
  3. Basic level idea for lifting tool
  4. New Ideas are compared by benefits and problem points
  5. Compare ideas to previous ideas and choose one to development
  6. After basic solution is founded, basic dimension drawings for structure are made by Solidworks or Autodesk AutoCAD.
  7. Strength calculation (FEM) for critical parts is made, and extra stiffeners are added ifneeded.
  8. Safety mapping for the equipment is done and the design is changed if required
  9. Manufacturing drawings are updated based to calculation.
  10. Testing plane for lifting and stability test is constructed
  11. Prototype is manufactured on workshop
  12. Testing is made and result are write up
  13. Report is written base on project and results, this can include development ideas for device in the future.

Design guideline for demister maintenance solution / Outotec (Finland) Oyj
PDM, OU500141608 version 4, Rev 1, State: Complete / Riihitontuntie 7 C, PO Box 86
FI-02201 Espoo, Finland