Open cooling

Open cooling of turbine blade rows—detailed fluid mechanics and thermodynamics... 

Fig. 4.8 shows the open cooling process in a blade row diagrammatically. The heat transfer Q, between the hot mainstream (g) and the cooling air (c) inside the blades, takes place from control surface A to control surface B, i.e. from the mainstream (between combustion outlet state 3g and state Xg), to the coolant (between compressor outlet state 2c and state Xc). The injection and mixing processes occur within control surface C (between states Xg and Xc and a common fully mixed state 5m, the rotor inlet state). The flows through A plus B and C are adiabatic in the sense that no heat is lost to the environment outside these control surfaces thus the entire process (A + B + C) is adiabatic. We wish to determine the mixed out conditions downstream at station 5m. 

Change in stagnation enthalpy (or temperature) through an open cooled blade row... 

Nitrite formulations are employed for both hot and cold water closed loops (and also occasionally for open cooling systems). Unfortunately, nitrite is easily oxidized to nitrate and is very susceptible to microbiological attack (by Nitrobacter agilis and other microorganisms). Nevertheless, it is a good low-cost passivating inhibitor. 

NOTE Strongly alkaline, closed loop LPHW system formulations, such as nitrite/borates, typically include an azole product at about 4 to 5% (50% solution). Lower alkalinity products (often the molybdates) have significantly less, perhaps only 0.5%. For comparison, open cooling water and chilled water system formulations typically contain about 2 to 3 % (50% solution) azole. 

Interpretation of Corrosion Rates in Open Cooling Systems ... 

Molybdate is never used on its own in open cooling systems, as the dose rate for protection of steel is on the order of 75 to 150 ppm Mo. It tends to be formulated primarily with various organic corrosion inhibitors in alkaline solution, and certain synergistic ratios permit the molybdenum content to be reduced so that the reserve of Mo in the cooling water is only 3 to 10 ppm. 

Polysilicates, incorporated in various formulations that may include P-P04 or molybdate and phosphonates/TTA/polymer, have found application for smaller, light duty, open cooling systems and closed-loop circuits, especially where naturally soft or ion-exchange-softened makeup waters have been used. The reported effectiveness of such programs has been mixed. 

As with cleaning closed-loop systems, it is always necessary to first identify the basic types of foulants and problems likely to be found in any new open cooling system and to then select both an appropriate chemical treatment and a cleaning process. 

Nitrite is suitable as an initial passivator in open cooling systems, but it requires dosing at 1000 to 1500 ppm NaN(>2 to compensate for the loss of nitrite in the system, due to conversion to nitrate. 

Fig. 9.1 Sales aid to demonstrate that daily manual addition of chemical inhibitor to an open cooling system consistently fails to maintain reserves within prescribed levels, resulting in either overdosing (higher costs), or underdosing (inadequate protection).

Dosing and Control Equipment for Open Cooling Systems to Meet Current Basic Needs... 

For most open cooling systems, the samples that will need to be taken most regularly will include the recirculating cooling water and the immediate makeup water source(s). If the makeup is a blend of waters or has been subjected to pretreatment processes that can alter its natural characteristics, such as softening, additional samples will need to be taken, albeit probably less frequently. 

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