Temperature Control - Expert Article

Temperature controls can factor into litigation in a variety of cases including the loss of perishable goods, lost product in manufacturing or processing facilities, fire damage, or personal injury matters involving scalds and burns. In this article, Electrical Engineer, Kenneth J. Kutchek, P.E. and Mechanical Engineer, René Basulto, P.E. provide an introduction to temperature control as it relates to expert witness investigations.

Their discussion includes an overview of temperature control systems, the major components, and potential failures.

Temperature Control - Expert Article

Temperature control is a process by which the temperature of a material or space is adjusted and controlled to achieve a desired temperature, or to keep the temperature at setpoint and within safe or desired limits.

Engineers have always been charged with the design of control systems. It was in the 1980’s that controls began a shift towards digital/electronic devices for some types of systems. Design of a control system requires a thorough knowledge and understanding of what is being controlled and how it needs to function.

Today, direct digital controls are widely used in building management systems, HVAC controls, and consumer products. However, pneumatic controls are still commonly used in many large industrial processes and systems.

Temperature control is common in consumer products such as home heating/cooling, space-heaters, refrigerators, water-heaters, dishwashers, coffee pots, ovens/ranges, heating pads, ice makers, and clothes washers/dryers. Vehicles also have multiple temperature control systems including cabin air heating/cooling, seat heaters, and engine and transmission cooling systems.

In industrial applications, temperature control is critical in manufacturing and process systems involving food products, chemicals, electronics, plastics, metals, coatings, and more. The failure to properly control temperature within an industrial process can compromise sanitation, ruin processed material, or in extreme cases cause fires and explosions.

TEMPERATURE CONTROLS & ENGINEERED SAFETY FACTORS

A home heating system is a relatively simple example of temperature control and safety factors that are engineered into the system. The major components are the furnace, which generates heat, and the thermostat, which acts as the brain of the system and performs three primary functions:

  • Measuring the current air temperature,
  • Allowing the user to enter or set a desired temperature setpoint,
  • Turning the furnace heating ON and OFF, as needed, to maintain the desired temperature.

The consequences of failure for a home heating system can be severe. The failure to activate the system can cause water damage by way of frozen pipes, or personal injury due to exposure to extreme cold temperatures. On the other end of the spectrum, the failure to deactivate the system can create dangerous fire hazards.

To improve safety, thermostats are engineered with upper and lower operating ranges that reduce the likelihood of extreme temperatures. The furnace also contains a critical temperature control safety device called a high temperature safety cutoff or a combination fan limit control that turns the furnace off if the temperature exceeds a specific upper limit.

Commercial and Industrial Ovens

Temperature control systems in ovens are designed to maintain a desired temperature for cooking/baking or a broad range of industrial purposes. The major components are the gas burner (for a gas oven) or electric element (for an electric oven), the thermostat, and temperature sensors. Using an electronic or analog input, the user can enter or set a desired temperature setpoint, which the controller references to monitor the current temperature and turn the heating device ON and OFF, or modulate the output as needed. The oven also contains a high temperature safety cutoff, which turns the oven off if it gets excessively hot.

Refrigerators & Freezers

Refrigeration systems can be as small as mini fridges used to store prescription medications, to large scale commercial freezers or refrigerated warehouses. The cost associated with a refrigeration failure in a specialized facility can easily reach millions of dollars and affect global supply chains.

Refrigerators and freezers use a vapor compression cycle to cool by removing heat from the inside air. A temperature sensor measures the temperature of the air in the cooled space. An input allows the user to enter or set a desired temperature setpoint. The controller monitors the current inside temperature and turns the cooling ON and OFF, as needed. Heat is generated and also absorbed through the refrigeration process and a failure of the temperature control system can result in loss of operation and product. A failure of the compressor high temperature safety cutoff along with secondary safeties could cause damage to the compressor resulting in refrigerant contamination, system failure, and loss of product.

Saunas

Similar to a home furnace, saunas utilize a temperature sensor, controller, and heater. However, a sauna maintains a much higher temperature of 175° to 195°F and typically utilizes a safety shutoff timer. The failure of a sauna to limit temperature and/or regulate exposure has been linked to fatal incidents in individuals. Like other systems mentioned, overheating incidents also have the potential to cause fires.

Consumer Products and Appliances

Many consumer products utilize safety devices to prevent an excessive over-temperature condition where overheating presents a danger.

  • A thermal fuse (also known as thermal cut-off or TCO or high temperature cut-off) uses a non-resettable one-time fusible link to cut off the electrical power when an over-temperature condition is experienced. The thermal fuse is like an electrical fuse: a single-use device that cannot be reset and must be replaced when it is triggered.
  • Thermal protectors and temperature limit switches (thermal switch) detect an over-temperature condition and disconnect (open contacts) the electrical power in a circuit. A thermal protector can automatically reset itself when the temperature drops within an acceptable range or may require a manual reset.
  • Overload protectors (or thermal protector) are used in electric motors to prevent overheating caused by an electrical overcurrent condition, which can cause motor failure.
  • Freezestats are used in HVAC systems to detect a low temperature condition and disconnect (open contacts) the electrical power in a circuit. A freezestat is used to shutdown fans and blowers and prevent a freeze condition of HVAC equipment or internal building systems.

The type of thermal safety device needed in a product is determined during the engineering and design process. Read the article “Is Your Product Liability Case Really an Electrical Controls Case?” to learn more about where a control systems engineer may benefit your case.

INVESTIGATING TEMPERATURE CONTROL INCIDENTS AND INJURIES

Temperature controls are a critical part of products and processes, because when temperature goes uncontrolled it can lead to burns, property loss, or damaged equipment. Each product, process, or piece of equipment is unique including its design, function, layout, operational requirements, materials processed, and the hazards involved. Therefore, temperature control decisions must be made on a case-by-case basis after a detailed process and risk assessment. Proper temperature control design, manufacturing, programming, installation, calibration, and commissioning are important and must be considered as part of the hazard analysis during product design, system design, and temperature control selection and programming.

The engineers at Robson Forensic regularly investigate equipment, consumer products, industrial machinery incidents, and fires to determine if the cause is related to a temperature control system or component malfunction. The knowledge of how the system is to be controlled is essential to determine these failures. Our engineers have the education, training, and experience to analyze equipment and system failure modes, and determine how temperature control malfunctions contributed to the failure, loss or injury. These investigations typically include an evaluation of the system and electrical design, components, manufacturing, installations, industry standards, maintenance and repair issues, presence of defects, and the adequacy of warnings and instructions.

For more information, submit an inquiry or call us at 800.813.6736.

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