Medical practitioners want near perfect conditions for patient recovery and so pay close attention to applying ideal temperatures in new medical devices. For example, injecting fluids into a patient at room temperature and in post-operative recovery can reduce body temperature, possibly leading to hypothermia. Adding an electric heater, however, helps warm the fluid and avoid the condition.

But preventing mild hypothermia is only one reason for adding a heat source to equipment. Heat may also be used to incubate cultures, add warm humidity to respiratory equipment, stabilize equipment performance, enhance surgical procedures, and sterilize instruments.

Before sizing an electric heater, let's define its scope. In its most basic form an electric heater is simply a resistive wire, foil, or film element that gets hot when applying electricity. For most medical applications, an open wire is not practical. These need a more complete heater or thermal subassembly, such as a heater element, electrical insulation, lead wires, and a mounting method. For safety in medical applications, a redundant over-temperature device may be incorporated. A thermal subassembly may include a heater and other electro-mechanical components, the combination of which improves equipment design and performance.

Safety considerations

For obvious reasons, patient 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. Medical and diagnostic equipment is covered by several regulatory organizations including FDA, UL, and CE.

In the United States, specs for electrical leakage current are based on the values and requirements in standards NFPA 99, “Health Care Facilities” and ANSI/AAMI “Safe Current Limits for Electromedical Apparatus.” The U.S. standards differ from the International Electric Code IEC 60601-1. The U.S. standards modify the acceptable passing-current limits for the earth and enclosure leakage tests, but maintain the same values for the patient leakage tests.

The base IEC 60601-1 standard does not directly differentiate between inside or outside the patient. IEC 60601-1-1, “Medical Electrical Systems,” which addresses a combination of several pieces of equipment, does make the distinction with respect to leakage current testing. UL 60601-1 differentiates between patient-care equipment (6 ft around and 7.5 ft above the patient) and nonpatient-care equipment for these leakage current tests. In UL 60601-1, leakage current values are specified in Tables 19.5DV.1 and 19.5DV.2. These values are given as:

Class I product (typical value) = 300 µA patient-care area

Class I product (typical value) = 500 µA nonpatient-care area

UL 60601-1 allows opening the ground conductor and one supply connection simultaneously for nonpatient-care equipment. In most cases, the earth-leakage-current test per UL 60601-1 provides a worst-case condition within the patient area. The enclosure leakage current test per IEC 60601-1 is the worst-case test in the normal condition.

To meet these low current leakage safety requirements, many device manufacturers have resorted to step-down isolation transformers and low-voltage supplies. The step-down isolation transformer adds significant cost, space, and weight to the equipment. An alternative design approach would be to use a heater with low-current leakage that could eliminate the isolation transformer and save cost and space. Advanced ceramic heaters with aluminum nitride (AlN) as a base material and a dielectric strength of >15KV/mm can meet the leakage current requirements. Medical equipment such as renal dialysis, insufflators, lithotripters, and thermal abrasion could benefit from the performance and cost benefits of AlN heaters.

Heater performance

A goal of all heaters is to supply the best performance for the medical device'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?

Decide whether the best thermal solution uses convection, conduction, radiation, or a combination of these. 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. For the heater designer, this usually means less space is available for the thermal solution, and that drives decisions toward a smaller design rather than just a traditional heater component. For example, a combined syringe-heater assembly might include a foil heater element embedded in polycarbonate housing, temperature sensors, temperature controller, and a high-limit controller in a compact reusable assembly that can be snapped on to a fluid-delivery system.

Another example: a hematology analyzer assembly with a tubular heater cast into an aluminum housing. The housing is treated with a nylon overcoat to allow for 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. Some applications require 24/7 operation while others are disposable so a one-time use works fine. For example, in respiratory therapy equipment, the heater may warm air, humidity, and medication. The heaters are part of the base unit and expected to have years of reliable operation. Conversely, heaters used in colonoscopy procedures may be part of a point-of-use device, and are therefore disposable. While patient well being is critical in both applications, the long-term reliability and cost of the respiratory therapy heater will be different than the one-time use colonoscopy heater.

Calculating power

Sizing a heater for optimum wattage requires knowing the flow rate, material to heat, and a change in temperature. Calculate a heater wattage for a unit where there is a rise in temperature with:

W_h = (0.076 V³T) / ³t

where W_h = heater wattage, kW; V = volume, liters; ³T = temperature rise, °C; and ³t = time allowed for ³T, minutes.

W_h = (0.16 V³T) / ³t

where V = volume, gallons; and ³T = temperature rise, °F.

If the application heats air to, for example, warm a blanket or infant incubator, calculate the heater wattage with:

W_h = (V³T) / 3,000³t

where V = volume, cubic feet; and ³T = temperature rise, °F.

W_h = (V³T) / 47³t

where V = volume, cubic meters; and

³T = temperature rise, °C.