Heating and cooling seems to be an afterthought for many designers. They presume they can slap any heater on their equipment and be done with it. What they don't consider is where and how the heater is attached, and how that affects equipment performance

Collaborate from the start

Reviewing design requirements at the beginning of a design cycle saves time and a lot of headaches. It's especially true for medical equipment that uses heaters, such as IV systems, fluid warming and hypothermia-prevention systems, and respiratory therapy. Good communication between application engineers and heater designers ensures the application works properly. For instance, it's necessary to understand the fluid to be heated to find the best heater. But companies sometimes will not disclose this information because it's proprietary. It's difficult to perform thermal analysis if we don't understand what we are heating. For example, water needs more heat than oil because the specific heat of water is a lot higher. If we don't know that, the best heater may not be chosen. One tactic is to have the heater manufacturers sign a nondisclosure agreement so everyone involved can be upfront about design requirements.

An example of a good intention gone wrong involves heated baby incubators. Our company provides heaters for the incubators which require stringent testing. Because specs were not clearly defined at the beginning of the design process, it took several iterations to get the best heater. Close collaboration between engineers and the customer is essential..

Another common mistake is not understanding the country or continent requirements where the equipment is to be used. These requirements are often taken for granted, but can come back to bite if neglected. For instance, European standards require electronics to be lead free and low voltage. Asian countries have their own safety requirements. Although it may seem obvious, this must be spelled out at the beginning of the design cycle.

A heater is not just a heater

A few applications can get by with a standard heater out of a catalog. But medical equipment probably needs designers to put some thought into the design. Some questions to ask include: Does the application require precise control of temperature? Does the application need uniform-heat distribution or high thermal uniformity across the heated surface? Is space an issue? What about heater response?

There are syringe heaters for medical injection applications that ensure precise liquid temperatures, reduce fluid viscosity, and have temperature-sensing controllers. Some heaters have ramping capabilities as high as 80° C (176° F)/sec. Thick-film heaters are made of film resistors and dielectric materials on quartz, stainless steel, and ceramic substrates. These provide a fast temperature response and uniformity with a low-profile.

A Guide to Good Heater Design

Most electrical heating problems are readily solved by determining the heat required for a job. To find that value, convert the heat requirement to electrical power and select the most practical heater. Whether the problem is heating solids, liquids, or gases, the approach to determining power requirements is the same.

Most heating problems involve the following steps:

Define the Problem

Your heating problem must be clearly stated, paying attention to defining operating parameters. First, gather application information and make a rough sketch of the problem. Take these into consideration:

Minimum start and finish temperatures

Maximum flow rate of materials being heated

Required time for start-up heating and process cycle times

Weights and dimensions of both heated material and containing vessel

Effects of insulation and its thermal properties

Electrical requirements such as voltage

Temperature sensing methods and location

Temperature controller type

Power controller type

Electrical limitations

And because the thermal system you're creating may not take into account all the possible or unforeseen heating requirements, don't forget a safety factor. A safety factor increases heater capacity beyond calculated requirements.

Calculate power requirements and determine:

• System start-up power
• System maintenance power
• Operating heat losses

The total heat energy (kWh or Btu) will be the larger of the two values shown below.

• Heat required for start-up
• Heat required to maintain the desired temperature

The power required (kW) will be the heat energy value (kWh) divided by the required start-up or working cycle time. The kW rating of the heater will be the greater of these values plus a safety factor.

Review System Application Factors

Before choosing a heater make sure you've determined:

• Operating temperature
• Operating efficiency
• Safe/permissible watt densities
• Mechanical considerations
• Operating environment factors
• Heater life requirements
• Electrical lead considerations
• Safety factor

Always include a safety factor of varying size to allow for unknown or unexpected conditions. Generally speaking, the smaller the system with fewer variables and outside influences, the smaller the safety factor. Conversely, the larger the system and the greater the variables and outside influences, the greater the safety factor.

The size of the safety factor depends on the accuracy of the wattage calculation. Heaters should always be sized for a higher value than the calculated figure. A factor of 10% is adequate for small systems; 20% additional wattage is more common. Safety factors of 20% and 35% are not uncommon, and should be considered for large systems, such as those containing doors that open or are large radiant heat applications. Also determine how long the system must operate without failure. To do so, examine the needed heater life.

The safety factor should be higher in production operations with equipment cycles subjecting them to excessive heat dissipation, such as large radiant applications, opening doors on furnaces, and introducing new batches of material that can be of varying temperatures.