Sizing a Cooling System to Control Process Temperature
When it comes controlling the temperature of process applications, it is important to make the right decisions when selecting equipment to control the rate of heating.
A chiller can be used in an industrial heating application for a number of purposes. In the most common application, a traditional cooling unit is employed to remove heat from a process using a cooled media — in most cases, water — at temperatures from 41 to 104°F (5 to 40°C). Many suppliers offer equipment — from point-of-use chillers to large industrial-scale chillers with remotely located cooling towers — for these water-cooled applications.
Two factors — plant layout, and the size and temperature requirements of the cooling application — typically dictate the selection of the chiller system. Sizing the chiller system properly is key to effective cooling process performance.
Ensuring that the cooling system has the necessary cooling power for the process is the first step. This is simple to calculate if two variables are known:
- The temperature differential (ΔT) between the inlet water temperature (Tinlet) and the outlet water temperature (Toutlet). (Note: The equation to the right is for calculating cooling power using °F temperature scale.)
- The flow rate (in gallons per minute) of the water.
If these two variables are known, the required cooling capacity can be calculated as follows:
BTU/hr = 500 x Flow Rate x ΔT
For cooling processes where the actual water inlet and outlet temperatures and flow rate are not known, calculate the cooling capacity using the following equation:
Q = [ (ρ V cp)material + (ρ V cp)bath fluid ] ΔT / t
t is time.
ρ is density.
V is volume.
cp is constant-pressure specific heat.
ΔT is the temperature differential.
Q is the required capacity of the cooling system.
This equation takes into consideration the properties of the material/product or equipment requiring cooling.
For discussion with a chiller manufacturer, have the following requirements available:
- Temperature range required.
- Cooling capacity desired at specific temperature(s).
- Integrated heater-capacity requirements (if needed).
- Desired control electronics.
- Need for external temperature control/monitoring (if needed).
- Pump requirements, including flow rate and pressure capacity.
- Inline filtration requirements.
- External communication requirements.
Daily Log Parameters
Keeping a daily log of operating temperatures, system pressure, fluid levels, filter efficiency and flow rates can assist in maintaining the most efficient operation for the cooling system. Daily log parameters to monitor include:
Typically, a safety factor of 20 to 30 percent is added to the calculated cooling capacity, and the total is specified for the chilling system. This extra cooling capacity should be calculated for the lowest temperature required in the process or application. Consult with the chiller manufacturer to review the entire process for a thorough review of the cooling system.
Careful planning will avoid the installation of a chilling system without enough cooling capacity. If it becomes necessary to remedy an under-capacity process, reduced production throughput and potentially costly retrofitting will result, so it is best to avoid the problem during the planning stages.
Monitor and Maintain the Cooling Media
Water quality management for cooling systems requires high priority. Depending on the fluid circuit, additives to guard against piping corrosion, scaling and biologic growth are added to the recirculated water to reduce fouling and help ensure trouble-free operation. Consider the use of inline filters, particularly when using plate heat exchangers in the process. Fouling from particulates can quickly degrade efficiency and lead to maintenance downtime and repairs.
For large systems employing cooling towers, similar water quality tests should be conducted on the tower water. In addition, as the recent outbreaks of Legionella in 2015 brought to the public’s attention, tower water requires extra monitoring and proper sanitation treatment to abate this organism. Conduct consistent monitoring of the water quality to maintain the most efficient operation.
Keeping a daily log of operating temperatures, system pressure, fluid levels, filter efficiency and flow rates can assist in maintaining the most efficient operation for the cooling system. Any degradation in heat transfer efficiency will affect the process. Cleaning the water circuit on an annual or demand basis will help eliminate any fouling or corrosion while helping maintain efficient operation. Periodic cooling-fluid testing should be conducted weekly or monthly. The precise schedule will depend on the process, fluid and type of chiller.
Processes operating at elevated temperatures require the use of an alternative recirculating fluid such as a silicone or a hydrocarbon-based heat transfer fluid. Obviously, the use of a non-water-based fluid increases initial operating costs. However, these fluids provide some clear advantages over water, including:
- Better heat capacities, which improve the heat removal efficiency of the system.
- Extended temperature range.
- No need for additives to prevent corrosion, scaling or biologic growth.
- No volume loss to evaporation.
Additional safety training for handling of these fluids will be required in addition to more detailed spill mitigation protocols.
Processes that require the use of elevated-temperature heat removal run the gamut and include molding, extrusion, distillation and drying. For specialized processes, a point-of-use chiller system could prove the best solution.
When Industrial Processes Get Really Hot
High temperature processes (above 212°F [100°C]) present a unique challenge. Pressurized high temperature water and hot oil circulator products are available from many manufacturers. They are designed to circulate high temperature fluid to a process. This article focuses on high temperature processes generating excess heat that must be removed from a location.
In particular, radio frequency (RF)-heated industrial processes — operating at high temperatures and requiring component cooling — can utilize a hybrid approach. In this example, a radio frequency high temperature system operating from 1472 to 1832°F (800 to 1000°C) requires an external component to be maintained at a temperature less than 572°F (300°C). Before staring the process, a hot-oil temperature control unit (TCU) supplies 482°F (250°C) fluid to the component to bring the external component to operating temperature. Once the hot-oil TCU reaches 482°F, the radio frequency heating process begins. Excess heat from the radio frequency source soon increases the temperature of the circulating hot oil to 572°F when it is returning to the TCU in the fluid return line. To control the hot-oil temperature, an internal plate heat exchanger in the TCU — plumbed to the facility cooling water system — removes the excess heat. This allows the hot-oil TCU to supply 482°F fluid back to the application. In effect, acting as a system chiller, the internal TCU plate heat exchanger removes the excess heat generated by the radio frequency source. This approach illustrates a clever engineering solution to one high temperature application.
Specialized applications require diligent communication with the chiller manufacturer to ensure proper system design and safety mechanisms. For high temperature processes, operators must be well trained in system operation and emergency shutdown procedures.
High temperature fluids should be monitored closely for signs of cracking or degradation. Regularly scheduled maintenance should include periodic cleaning of the hot-oil fluid lines and TCU with a flushing fluid compatible with the hot oil used in the process. Many hot-oil flushing fluids can be operated at elevated temperatures, which will improve residue removal in the fluid circuit. Follow the flushing procedure recommended by the fluid supplier.
In summary, take the proper steps to configure an ideal chiller system for the process:
- Outline the process parameters with as much detail as possible.
- Consult with the chiller manufacturer to review the system parameters. This will ensure proper design and performance of the complete cooling solution.
- Outline and enact a maintenance protocol for the chilling system.
- Monitor cooling fluid properties.
- Schedule regular maintenance for the chilling system, cooling fluid and fluid flow circuit.
Following these guidelines will yield a process that works right the first time and, with proper maintenance, will last a long time. PC