by Jim Kreisel, Director of Industry Marketing, Watlow
Laboratory managers, scientists and technicians want near perfect conditions to assure accurate and repeatable test results, so they pay very close attention to the temperature of their test samples.
Icubating and protecting cell cultures is only one reason for optimizing the heat source in equipment. Temperature is also a common variable in evaluation and testing most organic and inorganic materials in pharmaceutical, clinical diagnostics, chemistry, forensic, petrochemical, biology and other laboratories.
In its most basic form, an electric heater is a resistive wire, foil or film element that become hot when applying electricity. Since an open wire is not practical for most laboratory equipment, applications need a more complete heater or thermal subassembly including a heater element, electrical insulation, lead wires and a mounting method.
Safety Considerations
A typical wiring diagram for an integrated temperature controller and safety limit. Click to enlarge.
Safety is paramount in equipment and heater design. Design considerations for safety include electrical (low-leakage current), fire, gas or particulate outgassing and temperature stability.
The primary global standards for equipment safety are found in a combination of several international standards, such as Underwriters Laboratories (UL®) 61010-1, UL® 991 and others. Depending on the equipment type, the standards may be used individually or in conjunction to cover all the equipment requirements. The entire laboratory equipment is evaluated under UL® 61010A.
There are a variety of temperature safety devices. Bimetal snap and bulb and capillary devices are the lowest cost. These devices have limitations in accuracy and limitations in the locations where they can be used. Electronic safety limit controls are more common in laboratory equipment because of the need for precision, over or under temperature monitoring, and shut down. Laboratories that use Factory Mutual (FM) as an underwriter may also need safety limit devices that meet the requirements found in FM Class 3545 temperature limit switches.
Electronic temperature limits can be either standalone devices or integrated in the process temperature control. New electronics technologies give customers the ability to integrate a high amperage power controller output with a high-performance PID controller and an FM approved over/under limit in one panel mount package.
Heater Performance
The goal of all heaters in laboratories is to supply the best performance for the equipment’s intended use. Basic technical questions a thermal engineer will ask a device designer include:
1. What is the application?
2. Is the device heating air, gas, fluid or a solid?
3. What is the process temperature?
4. What is the temperature ramp rate?
5. What are the dimensional requirements?
6. Are there material compatibility issues?
7. Are there voltage limitations?
8. Are there special agency approval requirements?
9. What is the heater life expectancy?
Watlow’s ULTRAMIC heater.
A thermal engineer will need to decide whether the best thermal solution uses convection, conduction, radiation or a combination. Convection transfers heat through gasses and liquids from a region of higher temperature to a lower one. Conduction transfers heat within a body or between bodies in contact, and radiation emits energy in the form of waves.
Portability and space requirements also drive design decisions. This usually means less space is available for the thermal solution, which drives heater engineers toward smaller designs. For example, in a mass spectrometer, a ceramic heater with an integrated temperature sensor can be used to reduce the part count and size in a mass spectrometer while increasing the response time. In a hematology analyzer, an assembly with a tubular heater is cast into an aluminum housing. The housing is treated with a nylon overcoat to allow easy cleaning. The cast aluminum heater assembly assures a uniform temperature profile for accurate and repeatable test results.
Other factors to consider include usage and reliability. Many laboratories and tests require around-the-clock operation while others are used for short test cycles.
There are many applications for testing in extreme environments. For example, petrochemical analysis equipment may be located in a desert or in frigid conditions, and gas chromatographs need to be portable for military and home security usage. For these applications, the robustness of the heater is a requirement for precision temperature.
Sizing a heater for optimum wattage requires knowing the flow rate, material to heat and a change in temperature. The design engineer needs to calculate the heater wattage for a unit where there is a rise in temperature. A thermally optimized application is well within reach by properly defining scope up front with a thermal expert.
For more information, contact Jim Kreisel, director of industry marketing, Watlow, at jkreisel@watlow.com or by phone at 507-494-5369.
