Cooling System and Petrochemical Industry
Pranjal Kumar Phukan
Chief Manager
Dibrugarh, Assam

Petrochemical industries generate large quantities of waste heat. Therefore, cooling system considerations constitute an important aspect of petrochemical development schemes. Technically feasible and economically favorable industrial cooling solutions are essential. To best optimize the efficiency of a cooling system, a "systems approach" should be used to identify potential savings and performance enhancement. This approach looks at the entire cooling system, including the pumps, motors, fans, nozzles, fill, drift losses, evaporative losses, blow down, makeup rate, chemicals, flow rates, temperatures, pressure drop, as well as operating and maintenance practices. By focusing on the whole system as opposed to just individual components, the system can be configured to avoid inefficiencies and energy losses. Cooling systems do not operate under one condition all the time and system loads vary according to cyclical demands, environmental conditions, and changes in process requirement.

Cooling systems are either "Oncethrough" or "Recirculating". A Oncethrough system uses the cooling water only once before it is discharged. A Recirculating system recycles the cooling water after it has been cooled at a heat sink. Cooling systems are also classified as "Closed" or "Open".

Open Systems: Process medium is in contact with the environmen. Only applied to a wet system but may be a once through or recirculated design.
  • Forced and natural draft cooling towers, cross flow towers (water/air)
  • Cooling ponds use evaporation to discharge heat, prior to reuse in the process.
  • Some systems such as a wet surface air cooler combine open and closed design
Closed Systems: Process media is inside tubes or heat exchanger and is not in contact with the environment. May be a wet or dry system and may be a once through or recirculated design1.
  • Heat exchangers of the shell & tube or plate & frame type.
  • Tubed fan cooler - fluid in tubes, air blowing over the tubes for cooling.
Once through or recirculated design. Designates whether the primary coolant is returned to its original source or returned to the process for reuse. A direct cooling system may contain one of these design features whereas an indirect system may contain both.

Once Through: Coolant passes through heat exchanger once before returning to its source.
  • River/lake/ocean/sea to process and return back to source.
  • This is the easiest and most efficient system to use although high discharge temperatures must fall within permissible limits.
  • Sensitive to fouling, scale, corrosion, and fish intake. Uses large amount of water and risks putting additives into water source.
Recirculated: Primary coolant is reused whereby heat is absorbed in one exchanger and then transferred to a second coolant in secondary exchanger.
  • Eliminates environmental impacts to water supply.
Direct or indirect systems, also known as primary and secondary systems. This term indicates where the primary process media is discharging heat directly to the environment or to a secondary media.

Direct: System with one heat exchanger or cooling tower, and only the process media and a coolant.
Indirect: There are at least two heat exchangers, and a closed secondary coolant between the process media and the primary coolant. Indirect cooling systems are applied where leakage of process substances into the environment must be strictly avoided.
  • Efficiency is not as high as direct due to extra heat exchanger stage.
  • Common in nuclear plants or with hazardous chemicals.
Wet or dry cooling system, refers to whether or not cooling water or ambient air is used as primary cooling media.

Dry: Uses forced air over tubing with fluid process media.
  • Only applied to closed systems.
  • Typical in areas without cooling water source available
  • Tubed fan coils Fin/fan coolers - fluid in tubes, air blowing over the tubes for cooling.
Wet: Involves either the use of the process fluid being cooled with air in an open cooling tower, or being cooled by water in a closed heat exchanger
  • Cooling towers - Evaporative heat transfer. Include cross-flow cooling towers, hyperbolic towers. The fluid to be cooled is in contact with cooling air flow and there are some evaporative losses.
  • Shell and tube or plate and frame heat exchangers.
The type of cooling system chosen may also reduce or eliminate environmental impacts. An air/water cooling tower may be used instead of a once through cooling system to minimize water usage or thermal water pollution. The air and water permits will generally specify certain design features such as the type of cooling system, maximum permissible withdrawal volume & discharge temperature for once through systems, cooling tower drift rate, and other permits may specify water consumptive use, cooling water discharge temperature, noise levels, etc.

Application of Technology

Efficiency gains are available with each cooling system design. New systems have the most potential for optimization using the latest technology, although existing systems have potential as well but will generally be limited by layout and construction issues. The type of cooling system selected requires extensive evaluation at the design stage of a project using many design inputs including costs, layout and size, water availability, energy consumption, energy efficiency, ambient conditions, seasons and weather, and many others depending on the project. Annual variations in local water and air temperatures have the largest influence on the efficiency of the cooling system. System efficiency is a function of the costs of the energy and resource input needed to operate the system vs the amount of cooling achieved. Electricity is used to operate fans and pumps, and other costs incurred include make up water costs as well as regulatory costs and penalties.

Cooling Towers - Wet evaporative systems are limited by the wet bulb air temperature and dry systems are limited by the dry bulb air temperature both which fluctuate throughout the year. These limitations may cause a plant to run at reduced capacity or run at lower cooling efficiency. Cooling capacity may be increased by adding additional cooling cells or by correcting design sizing errors.

Fans and Pumps - Fans, blowers, and pumps may be idled or slowed during times of favorable weather conditions or low plant load to reduce energy consumption Variable Speed Drives (VSDs; also called Adjustable Speed Drives, or ASDs) are commonly used on fan, blower and pump motors because they greatly improve cooling system energy efficiency at partial loads, relative to continual operation.

Automation - Modern controls offer ways to improve efficiency by continuous monitoring of key system parameters with automated adjustments to pumps and fans.

Cooling Medium Temperature - The efficiency of cooling systems depends on the temperature of the medium to which the heat is being expelled. Cooler mediums are easier to transfer heat to, so less, cooling medium flow is necessary, reducing pumping/blowing energy demands. In many cases, the temperature of water sources is lower than the surrounding air temperature, so using water-based cooling systems can be more energy efficient.

Exchanger Approach Temperature - The temperature difference between the cooled working fluid (as it leaves the cooler) and the incoming cooling medium is called the approach temperature. It is important for designers to not specify approach temperatures any smaller than required, as smaller approach temperatures require greater cooling capacity (e.g. larger cooling equipment, higher flow rates).

Offshore Cooling Systems - Cooling systems on offshore facilities often use seawater as the cooling medium, given its plentiful availability and low, steady temperature. Such systems, however, must resist corrosion from this salt water.

Operational Issues/ Risks

Cooling systems require regular cleaning, maintenance, and scheduled overhauls to operate at high efficiency. This can range from simple preventative maintenance activities (i.e. flushing) to repairs that requirem the tube bundle to be removed from the heat exchanger shell for cleaning or even replacing entire cooling towers. This down time should also be taken into consideration when sizing the heat exchangers.

Different cooling systems may operate at high pressures and temperatures or with hazardous fluids and adequate operating procedures must be followed to avoid personnel risks and system outages.

Challenges for Oil and Gas

In oil-and-gas refineries, industrial equipment is often jacketed or sleeved with flowing water to cool fluids and absorb process heat. This equipment generates a large heat load that can disrupt operations if not properly handled. Cooling towers are used to cool this process water for reuse. The towers must be able to handle the heat load as well as any problems unique to the industry. Leaks in the process can cause hydrocarbons to contaminate the cooling water and vice versa. Lower quality, no potable water often is used to cool plant processes. Meeting environmental regulations limiting hydrocarbons, water usage and drift rates present challenges to plant operators. New industry regulations often require cooling towers to lower the drift rate to 0.0005 percent of circulating water flow, which can be difficult to achieve in older evaporative cooling towers. Many cooling towers currently in use are not equipped to handle these demands.

A few decades ago, field-erected towers commonly used wood structural members. These cooling towers are coming to the end of their service lives, experiencing mechanical failures, requiring unplanned maintenance that interferes with production and impacting facility productivity. The heat transfer fill in the towers also degrades over time, getting plugged with waste from dirty cooling water and breaking down from the weight of debris.