Combined wet/dry cooling systems

2.6.1 Open wet/dry (hybrid) cooling towers

Technical description

The open wet/dry cooling tower or hybrid cooling tower is a special design that has been developed as an important solution to the problem of cooling water use and of plume formation.

It is a combination of a 'wet' and 'dry' cooling tower or, in other words, of an evaporative and a non-evaporative process. The hybrid cooling tower can be operated either as a pure wet cooling tower or as a combined wet/dry cooling tower, depending on the ambient temperature. The heated cooling water first passes through a dry section of the cooling tower, where part of the heat load is removed by an air current, which is often induced by a fan. After passing the dry section, water is further cooled in the wet section of the tower, which functions similarly to an open recirculating tower. The heated air from the dry section is mixed with the vapour from the wet section in the upper part of the tower, thus lowering the relative humidity before the air current leaves the cooling tower, which (almost) completely reduces plume formation above the tower.

Optimising the effect of a hybrid cooling tower means optimising the amount of dry heat transfer to meet the plume control requirements. At the same time the wet section is being used for the major part of the cooling.

Figure 2.17: Schematic representation of hybrid cooling tower principle (example applied in power industry)

[Eurelectric, 1999]

Characteristics of open hybrid cooling towers are:

- base load and partial load operation for all capacities - cooling medium being water only

- cooling tower make-up water required for most of the operating time - thermal performance being the same as in the case of wet cooling towers - reduction in make-up water quantity

- regulations for environmental protection, e.g. reduction in overall height (due to fan assistance) and plume abatement

- sound attenuation equipment required due to noise regulations.

To operate a hybrid cooling tower efficiently a number of devices are used:

- variable speed fans

- closing devices for air inlet openings (such as louvres or sliding gates) - valves for water flows to the wet and dry sections

- bypass systems

- booster pumps (for special constructions)

- system for mixing of the wet plume with dry plume.

Hybrid tower construction

Currently only mechanical draught type hybrid cooling towers are available. The hybrid cooling tower is different from the characteristic open wet tower design in that it has a dry and a wet section, each with its own air inlet and corresponding fans. Hybrid cooling towers can be found as package cooling towers, large round cooling towers with forced draught fans or as cell-type cooling towers with induced draught fans. Fill, water distribution system, drift elimination, and sound attenuation are features common to both tower designs.

Wet/dry cooling towers of the mechanical draught type are fitted with internal mixing systems to mix the wet and dry airflows. They can be automatically controlled according to the heat load, water flow, ambient air and plume conditions.

Cooling capacity

They can be built as package cooling towers, induced draught or forced draught cooling towers and – on a larger scale - as cooling towers of the cellular type or circular type with the heat rejection ranging from < 1 MWth up to 2500 MWth.

Environmental aspects

The major difference between a hybrid cooling tower and a conventional cooling tower is its comparatively lower water use (which is make-up water) amounting to 20% less than that of a wet cooling tower [tm132, Eurelectric, 1998].

The resulting annual energy consumption of a mechanical draught hybrid cooling tower can be reduced to a level of 1.1 to 1.5 times that of a comparable mechanical draught wet cooling tower since in nominal conditions, airflow is almost double (wet and dry sections). Natural draught cooling towers of the wet/dry design are under consideration.

Application

A decision to install a hybrid cooling tower is made in the light of site-specific requirements (limitation of height and plume reduction) and several can be found in the power industry, especially in Germany and in United Kingdom (in cogeneration systems). Its use is restricted to temperature ranges of 25-55ºC, because above 55ºC precipitation of calcium carbonate is observed to occur more easily on the tubes. This does not mean that no precipitation occurs below 55ºC and some care must be taken in using this as a rule of thumb.

2.6.2 Closed circuit hybrid cooling systems

Technical description

For closed circuit cooling hybrid systems, characteristics can be described in a similar way as for closed recirculating wet cooling systems concerning fans (axial and radial), airflow direction (cross or counterflow) and noise abatement systems (see § 2.4). Generally, these units have a small space requirement. Three technical modes can be applied to closed circuit hybrid cooling towers: sprayed finned coils, adiabatic cooling or combined systems.

Environmental aspects

Closed circuit hybrid cooling towers combine the advantages of closed loop cooling with significant savings of water when compared to conventional closed circuit wet cooling towers.

Compared to closed circuit dry cooling towers they offer the advantage of lower cooling temperatures. In terms of size, energy consumption and noise emission they compare with conventional closed circuit wet cooling towers. Depending on their design (sprayed finned coils) special attention may need to be paid to the quality of the water treatment. Additional costs can be more than offset by the significant saving of water, as such products require the use of water only during a very short period of the year. Closed circuit hybrid coolers also significantly suppress and, in some designs, even eliminate plume formation.

2.6.2.1 Sprayed (finned) coils

Figure 2.18: Schematic presentation of the principle of a closed circuit hybrid cooling tower

In a closed circuit cooling tower the process medium runs through cooling elements (a tube/plate bank or the finned coil) in a closed loop, the primary cooling circuit. These cooling elements are wetted via a secondary water circuit and air is simultaneously moved over the elements to create evaporative heat. The cooling water that runs off the elements is collected in a basin and can be recirculated a number of times, sometimes using another cooling tower or after blowdown (see Figure 2.19). In an indirect configuration, the medium that runs through the primary cooling circuit is not the process medium but another coolant which in turn cools the process medium in a second heat exchanger.

2.6.2.2 Adiabatic coolers, wetting and pre-cooling the air that cools the coils In the adiabatic mode the fluid to be cooled bypasses the prime surface coil. The cooling water trickles down the wet deck and the air passing the deck is wetted with as much moisture as it can take up. The wetted air passes the finned coils and will take up more heat than dry air would do. Compared to conventional evaporative cooling equipment the water consumption is much reduced. (See Figure 2.19)

Figure 2.19: Combined dry/wet operation of a hybrid cooling system [tm151, BAC, 1999]

2.6.2.3 Combined technology

In combined technology the finned coils, the sprayed prime surface coil and the wet deck are all used. In the dry mode it is then possible to close all water sprays and lead the medium to be cooled through both the finned coils and the prime surface coils, both cooled by dry air only. In the wet/dry mode the medium after passing the dry coils passes the sprayed prime surface coils before returning to the process as cooled medium. The warmed water trickling down from the prime coils will fall over the wet deck surface. Air is drawn in and passes both the prime surface coil and wet deck surface, where it is saturated and picks up heat. As it passes the finned coil more heat can be picked up (see also Figure 2.19).

2.6.2.4 Costs of hybrid systems

In the application of hybrid systems reference is always made to the investment and operating costs involved. In general hybrid systems require higher investment costs. Costs of plume suppression vary depending on the cooling system. Compared to a cooling tower with the same cooling performance, Fluor [1995] calculated that for an open wet cooling tower installation of 300 MW the cooling installation costs are about 2.5 times as high as for cooling towers without plume suppression. For closed circuit wet cooling towers, costs for plume suppression are reported to be 1.5 to 2 times as high as for towers without plume suppression (Eurovent). The costs have to be adjusted for cost savings on water intake and operational flexibility. The annual costs for water, including water treatment and electricity can represent in some cases just about 10 % of the annual costs of a cooling tower. These economic considerations depend of course on the individual application and the prices of water and energy [tm139, Eurovent, 1998]

Cost indications by the power industry show levels of EUR 40000 to 70000 per MW_th for mechanical draught type hybrid cooling towers. In this sector this means an installation cost level of 1.3-1.6 times that of towers of similar capacity without plume suppression.