Chilling Process Control
One of the most significant problems facing processors is determining the correct type of fluid-heating system that is required to control a given process. While temperature is certainly a primary factor, many other aspects of the process also must be taken into consideration.
Use the following list of important questions to determine the best system for your application:
What are the minimum and maximum required operating temperatures of the process?
The temperature will help determine what type of fluid-heating system is required. For temperatures up to 250oF (121oC), water is the likely choice. For temperature to 750oF (399oC), thermal heat transfer fluid systems would be recommended. Liquid salt systems will provide temperatures from 800oF (427oC) to beyond 1,200oF (649oC).
Is there a possible future process requirement that could force the operating temperature to extend beyond the limit indicated above?
It is not uncommon for processors to believe that a maximum operating temperature will never be exceeded, only to have this limit increased due to new process materials or technology.
Does the process require heating, cooling or both?
The processor needs to identify whether the process to be controlled is an exothermic or endothermic reaction requiring cool or heat inputs, or one that requires heat as a catalyst to the reaction and then is cooled as it exceeds the process temperature limit.
Some processes initially require a significant load followed by a considerably lesser load to maintain the process temperature within the specified range. When identified, the fluid-heating system can be designed to meet these process load requirements.
How much time is available to preheat the process equipment before the process can be started?
Less time available for preheat can translate into a much higher load requirement load at startup. A very short available preheat time can make it difficult to reach the required startup temperature without risking product damage.
Is the fluid circulating piping and equipment new, or are you retrofitting existing equipment?
For new processes, proper fluid circulating port and line sizes can be incorporated into the design. Problems can occur in systems that are being retrofitted or where the process sizing is being based on previous designs. This often is the case when converting a process from steam heating to heat transfer fluid (hot oil) heating. Typical steam processes have relatively small piping inlets -- often only one inlet per heated zone -- and a similarly sized (or slightly larger) outlet for the condensate return. When converting these processes to hot oil to attain higher processing temperatures at relatively low pressure, the restricted line sizes and process loop limit the amount of oil flow -- and heating or cooling ability -- that can be transferred to the process.
Does the process have any thermal limitations whereby the product being heated could be jeopardized by excessive temperature?
When scaling up a process, a typical theory is that a higher process temperature will permit shorter dwell times and thereby increase the output to increase productivity. These can be referred to as "high change in temperature processes." This may work for some applications, but it is important to take into consideration the effect the increased (for heating) or decreased (for cooling) temperature will have on the processed material in the event of a process interruption. If the process material's maximum tolerance temperature is not lower than the fluid medium temperature, and if a process interruption occurs, the end result often is damaged or destroyed product -- and a potential safety hazard. This problem is typical in web printing and laminating applications and in chemical processing applications where heat transfer is a function of maintained processing speeds. Ideally, the fluid temperature should be as close as possible to the required process temperature with consideration for the temperature lag that exists between the fluid and contact surface of the process equipment.
What is the process delivery and supply piping size as well as the overall pressure drop of the entire circulating loop, including all processing equipment?
It may be desirable to heat (or cool) a process extremely fast with a high heating (cooling) input. However, if the fluid-circulating loop has a limited flow capability, it will not be possible to attain the theoretical desired result.
Is the process heating or cooling load constant or cyclical?
For continuous processes, the heating or cooling load is constant, while for batch processes the load will be cyclical.
What is the heat loss from the circulating piping loop and the process equipment?
While heat loss can be somewhat limited with the application of insulation, this is another factor that needs to be addressed when sizing the fluid-heating system.
Does the process require any control interface with the fluid-heating system for monitoring or control purposes?
If the fluid control system is acting independently of the process control system, there must be adequate process monitoring and safety devices to determine if the fluid system is performing properly.
What services are available for operating the system?
The processor needs to evaluate all available utilities, including electrical, gas, steam and water.
What temperature accuracy is required by the process?
The process equipment and fluid-heating system must be capable of attaining this requirement.
Could the process or fluid system be affected by seasonal temperature changes?
These seasonal changes result not only in variances in ambient air temperature, but more importantly, in fluctuating supply water temperatures where cooling water is used to cool the fluid-circulating system. If seasonal weather changes significantly affect process efficiency, the fluid system should be designed to limit this effect accordingly (for example, an additional chiller system may be required).
When changing from one heating/cooling medium to another, what is the heat transfer efficiency difference between the two mediums?
It is important to know whether the process design allows for the use of a less efficient heat transfer medium. In some cases, changing the heating or cooling medium will require some improvement such as increased flow, pressure or surface area.
Ultimately, the reason for using a fluid-heating system is to optimize process control. Once the process requirements have been established, choosing the correct fluid circulating control system is a key step in achieving a successful process.