Topic last reviewed: 1 February 2014
Sectors: Downstream, Midstream, Upstream
A cooling system is used to reject heat from a process or plant. There are many types of cooling systems available that are used in the oil and gas industry. 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 requirements.
In order to determine whether efficiency gains in a cooling system may be achieved, one should understand the types of systems and their strengths and shortfalls. Cooling systems are available in many types of design and construction, each with its advantages and limitations. In general, all cooling systems will utilize a combination of several of these design features. The main cooling system principles are:
Open or closed system, designates whether the coolant is allowed to contact the environment or not.
Open systems – process medium is in contact with the environment. 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 avoided2.
- 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. Or a fin fan cooler could reduce a plant’s water consumption especially in dry locations. 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.
When selecting a cooling system, a Best Available Technologies (BAT) evaluation should be performed, (this is also referred to as Best Available Techniques) 3. BAT evaluation includes an integrated examination of the heat flows within the plant, as improving plant efficiency and reducing heat rejection demands directly reduces the demands on the cooling system.
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. A simple treatment using affinity laws suggests that halving the speed of a pump or fan will reduce its energy demand by 7/8ths. Once through systems - These systems may be subject to cost penalties due to heat rejection limit violations. Alternatively they may experience reduced cooling capability due to low water levels or by avoiding discharge temperature penalties, which leads to lower plant efficiency and capacity.
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 temperature4. 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). Water cooled systems tend to have smaller approach temperatures than air cooled systems, because it is easier to reject heat into water than air. Hence water-cooled systems may be preferable in situations where small approach temperatures are needed, both in terms of cost and energy efficiency.
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.
|Years experience in the industry:||21+|
|Range of application:||Upstream and downstream, LNG compression, gas reinjection, gas lift, cooling hydrocarbon gas and lube oil, production, refineries, power stations, and transportation|
|Efficiency:||Efficiency can be measured by power consumption for pumps and fans as well as make up water, and chemical treatment usage rates. Return process fluid temperature dictates effectiveness|
|Guideline capital costs:||Heat exchangers, cooling tower, controls, connections, controls, inlet and outlet piping, inlet filters, instruments, valves, fans, pumps, tanks, chemicals, redundancy, as well as installation, start-up and commissioning expenses. Wide range of costs from $50,000 to $1M+.|
|Guideline operational costs:||Includes routine maintenance like cleaning of tubes and plates, repairing leaks, rebuilding pumps, cooling towers fill replacement. Additional costs or lost revenue are related to plant outage time while equipment is off line. Operational costs include power for pumps, fans, and controls, and water treatment chemicals|
|GHG reduction potential:||Increasing the efficiency of cooling systems reduces the amount of energy consumed, resulting in a reduction of GHG emissions|
|Time to perform engineering and installation:||1-24 months|
|Typical scope of work description:||
Cooling systems are used in a large variety of applications and locations. A typical project will consider the use of cooling systems during initial project planning, determine operating conditions, and evaluate site conditions, environment, layout, available water, power consumption, operation, applicable regulations, in addition to energy efficiency before selecting a type of cooling system. Existing systems with worn or outdated equipment may be improved by looking at new technology that operates more efficiently after performing a complete system assessment.
|Technical:||Optimization of primary process first, heat reuse
Range of system operation, flows and temperatures
Availability of water for cooling
Water temperature, air dry and wet bulb temperatures
Permitting related to water, land, emissions
Land space available, site location, orientation
Power, water, noise and chemical consumption
Level of automation
|Commercial:||Parasitic power demands
|Environmental:||Water resources and availability
Protection of aquatic organisms at water intake
Chemical substances to water
Leakage and biological risks
Fish impact study may be needed
Dredging associated with the installation of the withdrawal pipelines
Optimization of the primary process design and control modifications will save energy on the front end and may avoid or reduce the need for cooling systems. By decreasing the amount of non-recoverable heat to be rejected to the environment, a plant can reduce its need for cooling systems and increase overall plant efficiency. Adding variable flow fans and pumps will allow scalable operation and improve efficiency of a cooling system.
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 require 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.
If cooling demand increases or was underestimate at installation, a cooling system must be derated or extra capacity should be added by increasing heat transfer surface area and pumping capacity.
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.
Some cooling systems such as cooling towers have a narrow range of operating BEP and may run less efficiently at higher and lower flows versus nominal flow rates.
Many cooling system designs are available. Some can be customized for specific applications as well as standard designs that are available with minimal lead time at lower costs. Several reasons to upgrade or add cooling systems are listed below:
- Upgrade existing equipment to newer more efficient designs
- Right sizing of equipment due to initial over or under design
- New systems needed due to regulatory changes, water use and effluent temperatures
- Reduction of energy usage, make up water, GHGs, and emissions
- Replace existing equipment due to wear and tear, and lower efficiency
- Additional cooling capacity due to increase in plant output
Industry case studies
Automatic Disc Filter Keeps University’s Cooling Water Clean
This study looks at an application of an automatic filtration system to a cross-flow cooling tower to removed particulate and control contamination levels. Most cooling towers should have some type of water treatment system to add corrosion inhibitors, adjust Ph, and anti-fouling treatments to the cooling water as well as a water blow down system. But despite those measures, cooling towers will capture particles from the air which end up in the cooling tower basin and leads to corrosion problems, reduced cooling efficiency, and downtime. Particle buildup creates an opportunity for algae and other biological growth to occur.
An array of disc filters were added to the system to pull water out of the cooling tower basins, filter it, and return it to the system. The system features an automatic back flushing feature to keep filters clean and reduce maintenance. The filter system lowers the plants water consumption due to basin blowdown and reduces the use of water treatment chemicals. This type of cooling tower is very common to power plant cooling systems and many other applications. And the filtration of basin water is an often overlooked feature in cooling system design. Although the additional equipment adds some new O&M costs, the filter system lowers the risk of reduced heat transfer capacity, increases plant efficiency for increased and reduces overall cooling system O&M costs.
- HydroThrift Corp., Closed-Loop Cooling Systems 101, 2002. (Retrieved 3 February 2014)
- Thermopedia, Heat Exchangers. (Retrieved 3 February 2014)
- Reference Document on the application of Best Available Techniques to Industrial Cooling Systems, Integrated Pollution Prevention and Control (IPPC), European Commission.
- Thulukkanam, K, Heat Exchanger Design Handbook, 2013. (Retrieved 3 February 2014)