9.2.2 Thermal performance properties of tubing
Section 4 of this chapter covers condenser tube material selection with regard to improving its resistance against the effects of corrosion/erosion. In-service titanium tubing trials were carried out at a number of power stations under varying operating conditions, see Table 4.3. As a result of the success of the trials and increasing economic competitiveness, titanium tubing is now used in all new power stations where impingement attack of the condenser is considered possible.
Advanced geometry tubing
Another development in condenser tubing which is currently being investigated is the use of advanced geometry (roped) tubing, to satisfy items (a) and (b) in the list of aims and objectives given in Section 9.1 of this chapter.
Figure 4.37 is an example of roped tubing. It has a rolled helical ridge on both steam and water sides of the tube. One effect of this advance design is to augment the waterside heat transfer coefficient by perturbing the water flowing in the tube, so that boundary layers do not build up along the tube wall and inhibit the transfer of heat. It also augments the steamside heat transfer coefficient by assisting condensate drainage.
The total increase in overall heat transfer coefficient is estimated to be approximately 30% above plain tubing; however, waterside frictional losses are increased.
By maximising tube performance, the design allows the overall tube length and tubeplate size to be reduced, without incurring operational penalties.
Optimisation studies have been carried out on a transverse underslung condenser, using 25 mm OD roped titanium tubing, and 25 mm OD plain titanium tubing. Water velocities varied from 1.2 m/s to 2.2 m/s, and tube lengths varied from 12 m to 18 m.
Table 4.5 shows the minimum cost, optimum CW quantity and condenser parameters possible within the constraints imposed on the system being studied, i.e., a CW flowrate of not less than 1.68 m/s and a CW temperature rise of less than 12°C.
This suggests, for the two optima in Table 4.5, savings of £200 000 by using roped tubes, however, when compared to the existing condenser using conventional tubing materials, savings in the order of £2.6 million are calculated.
An improvement in the overall heat transfer coefficient of roped tubes is clearly illustrated by the defined reduction in tube length and CW quantity, and hence the size of tubeplate required to achieve these savings.
A comparison of the mechanical behaviour between roped and plain tubing is given in reference .
As far as reliability and availability are concerned, research continues in the following areas:
- Tube maintenance.
- Tube cleanliness.
- Modular construction.
In particular, the maintenance of condenser tubes without access to the waterbox is being investigated.