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Sizing a Cooling System to Control the Temperature of Process Heating Operations

The use of a chiller in an industrial heating application can have many meanings. The most common application employs a traditional cooling unit to remove heat from a process using a cooled media, in most cases water, at temperatures from 5 °C to 40 °C. The market supporting water cooled applications has many suppliers providing solutions from point of use chillers to large industrial scale chillers with remotely located cooling towers.
 
Size the Cooling System Properly
 
The size and temperature requirements of the cooling application and plant layout typically dictate the selection of the chiller system; i.e. a point of use chiller or remote system supply chiller. Key to performance for the cooling process is sizing the chiller system properly. Ensuring that the cooling system has the necessary cooling power for the process is the first step.
 
In applications where the temperature differential between the water inlet / outlet and the flow rate of the water are known the required cooling capacity can be calculated as follows:
 
BTU/h = 500 x GPM x DT  (in °F)
Conversion to Tons:  Tons = BTU/h / 12,000
 
For cooling processes where the actual Tinlet, Toutlet and flow rate are not known calculate the cooling capacity using the following equation:
 
Q = [(rV cp)material + (rV cp)bath fluid] dT/ t
where

t
= time
r = density
V = volume
cp = constant-pressure specific heat
dT = change in temperature
Q = 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 manufacture, have the following requirements available:
·         Temperature range required
·         Cooling capacity at specific temperature(s)
·         Integrated heater – capacity requirements, if needed
·         Control electronics
·         External temperature control / monitoring
·         Pump requirements – flow rate, pressure capacity
·         In-line filtration
·         External communication requirements
 
Typically, a safety factor of 20-30% extra cooling capacity 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 chilling system without enough cooling capacity. Remedying an under-capacity process will lead to reduced production throughput and potentially costly retrofitting.
 
Monitor and Maintain the Cooling Media
 
Water quality management for cooling systems requires high priority. Depending on the fluid circuit piping anti-corrosive / anti-scaling additives and anti-biologic additives in the recirculated water reduce fouling and ensure trouble-free operation. Consider the use of in-line filters particularly when using plate heat exchangers in the process. Fouling from particulates can quickly degrade efficiency and lead to maintenance down time and repairs. For large systems employing the use of 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 public 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 of the water circuit to eliminate any fouling / corrosion on an annual or demand basis will ensure the most efficient operation.
 
An example of daily log parameters to monitor:
 
  • Compressor motor load
  • Current
  • Amps
  • Entering Chilled Water Temp
  • Leaving Chilled Water Temp
  • Entering Condenser Water Temp
  • Leaving Condenser Water Temp
  • Evaporator Refrig Temp
  • Evaporator Pressure
  • Condenser Refrig Temp
  • Condenser Pressure
Periodic cooling fluid testing should be conducted on a regular basis whether weekly or monthly dependent 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 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:
 
  • Better heat capacities which improve the heat removal efficiency of the system
  • Extended temperature range
  • No need for anti-biologic, anti-corrosive or anti-scale additives
  • 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
(>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. For the topic of this article I will focus on high temperature processes generating excess heat that needs to be removed from a location. In particular radio frequency (Rf) heated industrial processes operating at high temperatures (>800 °C) requiring component cooling can utilize a hybrid approach. For example, an Rf high temperature system operating from 800 – 1000 °C requires an external component to be maintained at a temperature <300 °C. Before staring the process a hot oil temperature control unit (TCU) supplies 250 °C fluid to the component to bring the external component to operating temperature. Once the hot oil TCU reaches 250 °C the Rf process begins. Excess heat from the Rf source increases the circulated hot oil temperature to 300 °C returning to the TCU in the fluid return line. An internal plate heat exchanger in the hot oil TCU plumbed to the facility cooling water system removes the excess heat enabling the hot oil TCU to supply 250 °C fluid back to the application. The internal TCU plate heat exchanger removes the excess heat generated by the Rf source acting as a system ‘chiller’. This unique approach illustrates a clever engineering solution to a 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 and / 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.
 
A Complete Solution that Works the First Time
 
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.