Interactions are at two levels: between the components and with the environment. Some interactions can be complex. For example, engineers often request heaters with very specific temperature uniformity; power dissipation is an attribute of a heater not uniformity. Temperature uniformity is a characteristic of the interaction between the heater and the environment. Similarly, when it is discovered that a new thermal system design doesnt control temperature within desired tolerances its often concluded that the accuracy or precision of the temperature control unit must be the problem. What is more likely is that the sensor or heater response is unstable due to mechanical fit or related assembly tolerance problems.
To make the best thermal component choice for any given application, its essential to remember that a systems performance isnt simply a sum of its parts. Considering interactions will avoid start-up problems during the checkout of new equipment and improve process reliability during its lifecycle.
Even for very simple thermal systems, taking the time to develop a complete system description before starting detailed design can save time. Many potential headaches such as mismatched components and escalating development costs can be avoided by preventing the quick fixes that are typical for a sketchy system description.
In its most basic form, a thermal system consists of the following primary components, as illustrated in Figure 1:
Figure 1. A system scheme for temperature control.
- Work load the fluid, to be heated or cooled (e.g., a liquid or gas);
- Heat transfer medium the materials and environment that must transfer heat to and from the work load (e.g., a vessel or pipeline);
- Heat/cool source a device that changes the input power source into heating/cooling energy (e.g., a band heater or chiller unit);
- Process temperature sensor an instrument that indicates the temperature of the work load (e.g., a thermocouple or infrared pyrometer);
- Process temperature control manages the temperature of the work load (e.g., a thermostat or electronic control); and
- Power control connects/disconnects input power to the heat/cool source as determined by the process temperature control (e.g., a thermostat or solid-state relay).
Some thermal systems require additional safeguards. If the system or process is not inherently safe (meaning it is capable of posing a hazard to people, equipment or the environment in the event of a malfunction), good thermal system design must include the following secondary components, also shown in Figure 1:
- Limit temperature sensor provides a redundant or back-up indication of the temperature of the work load, the temperature of the heat/cool source, or both;
- Limit temperature control prevents the temperature of the work load from reaching a hazardous level; and
- Safety contactor an independent means of removing or disconnecting input power from the heat/cool source in the event a hazardous condition occurs.
When schedules are short, design engineers frequently dive right into the details. A specification of a particular component is generated with little effort spent understanding the relationship between other parts. Just as it is the interplay of ingredients that gives richness to the flavor of a great bowl of chili, power dissipation across a heater surface is just one ingredient that adds up to temperature uniformity in a thermal system. A heat/cool source can be designed such that the system attribute of temperature uniformity emerges. To do so requires that the heat/cool source be combined with the other system components in a very specific and controlled manner. Defining the relationship of system components such that the desired system performance emerges is the primary challenge when describing a thermal system.
Describing the system
Many different methods have been developed for describing a system or defining system relationships. The important aspects of the most effective approaches are summarized in Figure 2.
Figure 2. Describing the system.
System development should start with a clear statement of need from the users perspective. In the case of a bowl of chili, the corresponding statement of need might be I want to win the chili cook-off championship. More relevant to thermal systems, this statement of need might be I want to maintain the temperature of the reactant flowing in my stainless steel gas delivery line between 170°C and 180°C to prevent it from condensing or degrading.
Once the need has been clearly established, begin the system description. This document defines how the system will fulfill the need, its behavior: what it does. For our cook-off need, one characteristic of behavior is taste in this case award-winning. For our gas line heating need, one characteristic of behavior is temperature control range in this case 175 + 5°C. Often behavior is referred to as function and is defined in a functional specification describing the following:
- Mission objectives to be fulfilled, information to be collected or received, plan or process to be followed and degree of cooperation with others in the environment;
- Viability the extent the system is able to maintain a separate existence in the environment; and
- Resources what is required for pursuit of mission and maintenance of viability.
Next, its helpful to describe the systems structure specifically, how its built and how each component interconnects with others. That chili needs to be competition chili, not eatin chili. Its going to need good fresh spices, and lots of them. Some good lean beef cut into cubes will be a good host for the spices. Maybe adding some fresh peppers instead of dried peppers will improve the flavor lets skip the beans to avoid distractions. Yes, Texas Style is potentially a good structural solution for our chili. But lets not forget about that gas delivery line. The 175 + 5°C temperature control range is a very tight tolerance and is going to require knowledge of the flow rates involved and consideration of the specific physical geometry of the line.