Cooling Towers reduce the temperature of cooling water by the evaporation of water into the air and are designed to promote the maximum possible contact between air and water. This feature results in the cooling tower having the capability to produce performance degrading ice formations during winter operations.
Ice formations in cooling towers are categorized as either acceptable or unacceptable. "Acceptable" ice is a fairly thin cross section of ice which forms on the louvers or air intake structure of the tower and poses no structural concern. This retards the air inlet flow through the louvers, which has a similar effect as the air-side control of louvers that can be manipulated by the operator. Thus, it is self-regulating to some extent. As performance decreases and the circulating water warms up, the ice is melted. "Unacceptable" ice is a significant amount of ice that has formed on the fill, jeopardizing the operation and existence of heat transfer surface, and threaten the integrity of the tower structure (Figure 8-3).
For mechanical and natural draft towers the formation of ice is influenced by the following conditions, which are usually under the control of an operator.
Ice formation is promoted by the air flow rate (more air, more ice)
Ice formation is promoted by the lack of heat load (less heat load, more ice) Ice formation is promoted by a slower water rate (less water rate, more ice)
The design features for winterization of cooling towers include: Air-Side Control
Multiple fans with two speed motors Multiple fans with reversible motors Water-Side Control
Send high water flow nearest the tower air intakes Bypass circulation
System draining Immersion heaters
8.7 FIRED HEATERS, HRSGs AND GAS/STEAM TURBINES
The fired heater itself needs minimal winterization. Firebox, smoke stack and air preheater ducting are fully insulated, and external surfaces are jacketed and properly sealed which make them freeze proof. The related items that need protection from adverse effect of low ambient temperature are piping, valves, instruments, air intake and drain lines around the heater.
Fuel gas lines and low point drains are heat traced and insulated from the source to the burner to prevent hydrate formation which can plug the line or condensed hydrocarbon liquids that can cause burner sputtering. Low point drains in close proximity to the heater should be a double block and bleed arrangement for safety reasons. If the heater is fired with fuel oil the entire fuel oil loop and return circuit must be heat traced and fluidity must be maintained at all times (generally above 30 centistokes (30 mm2/sec) viscosity).
Atomizing steam lines should be insulated to blow out condensate and drain lines must be heat traced completely .
Snuffing steam, soot blower, and superheater coil low point drains are also heat traced and insulated. Valve bodies, flow meters, and pressure gauges should be winterized. The air intake for an air preheater should be protected from snow by installing a weather hood. As an added protection for the air intake, upstream of the air filter and silencer, baffles and steam coils are often used.
Equipment associated with steam and gas turbines (e.g., lube/seal oil tanks and coolers) which are not within the heated enclosure must be heat traced and insulated. Air intakes of gas turbines have a built-in anti-icing and icing protection system. Winterization techniques discussed in section 8 for exchangers should be followed.
Heat recovery steam generators, similar to fired heaters, are fully insulated to conserve heat and in general the same methods for winterizing can be applied as previously mentioned. Fuel oil tanks, fuel gas knock out drums, fuel gas lines and air intake and related accessories are the ones considered for winterization.
8.8 TANKAGE
It is the responsibility of the process engineer to determine the freeze or pour point of the product, and specify a safe storage temperature to be maintained. A heat loss calculation should be performed based on the storage temperature, minimum design ambient temperature, insulation thickness, and wind. Heat losses on tank surfaces on dry-side wall, wet-side wall, tank roof, and tank bottom should all be accounted for. The total heat load requirement should be noted on the equipment data sheet and passed on to Mechanical for sizing an internal coil.
Aside from tank bulk heating, the economic viability of an alternative method; i.e., lowering the storage temperature and using a suction heater or a combination of both should be evaluated by the process engineer. The economic comparison should be based on the ease of maintenance, installed cost and heating cost in conjunction with insulation. In areas of mild winter where the ambient temperature is often above the freezing point, tank heating without insulation may be considered.
In general, winterization is required for storage tank whose fluids freeze point, or pour point temperature is above the minimum design ambient temperature.
Light ends products (e.g., butane, propane, MTBE, methanol) usually have lower freezing point than the minimum design ambient temperature and do not require winterization when stored. However, they do need some insulation for heat conservation purposes .
Because of a high pour point, storage tanks containing crude oil and heavier end products are normally winterized by heating using internal coils heated with steam or heating fluid and insulated. Asphalt tanks are commonly heated with direct fired internal tube heaters. Agitation is applied near the tank bottom to ensure good thermal mixing. Other storage tanks (e.g., raw water, demineralized water, waste water, caustic and acid tanks) which are outside are also winterized by heating using internal coils and insulation. Since they are less dense, they do not need agitation and heat distribution in the tank is accomplished by natural convection. Materials of construction for steam coils needs to be carefully evaluated in some services. For example, it may be less expensive to provide an external heat source for caustic heating, than to provide an alloy coil.
Firewater tank may be winterized by steam sparging without insulation, although this is only practical in areas where winter is milder. Although inexpensive, it is not a very effective design in cold climate region, since not only is steam wasted, but after operating for some time steam condensate can build up, causing the tank to overflow.
For large tanks the temperature changes very slowly with changes in ambient temperature. Insulation may therefore be based on an average winter temperature instead of the minimum design ambient temperature.
The roofs of cone and dome roofed tanks are not normally insulated. In locations where extremely low temperatures may prevail (below - 13 o
F or - 25 o
C) or where the tank liquid has a high freezing point, roof insulation may be required. The roofs of floating roof
Tank water draw valves should be freeze proof type (Fluor Engineering Standard ST-2-4129).
Conservation vents or pressure relief valves should be traced to prevent freezing.
8.9 REFERENCES
1. Shipes, K.V., Air-Cooled Exchangers in Cold Climates, Chem. Eng. Progress, July 1974.
9.0 APPENDICES
9.1 APPENDIX I - CALCULATIONS FOR HEAT LOSS IN PIPING