Travel mugs are a great way to take a cup of coffee with you on the go, while continuing to enjoy it at the temperature it was intended to be drunk at. But what is it about travel mugs that keeps your hot coffee hot? What insulation is used in travel mugs and how does it work?
Travel mugs use a thermal insulation design with an outer layer with reflective coating and a second inner layer which are fused at the top to ensure a tight seal. This vacuum seal prevents heat transference via conduction, convection, and radiation, keeping your hot coffee hot.
To understand how insulation works in travel mugs, you first need to familiarize yourself with the basic principles of thermodynamics and heat transference.
So, before we go into how travel mugs keep your coffee hot, we have to discuss why your coffee doesn’t stay hot in a regular mug. The principles of heat transfer can be broken down into three distinct phenomena: Conduction, Radiation, and Convection.
Conduction is when the heat travels through solid materials. In the case of a cup of coffee, the heat moves from when the hot liquid touches the solid object that is the cup. So when you pick up a hot cup of coffee, you may not be able to, because the mug has conducted heat from the hot liquid inside.
In short, heat moves from the hot object or the hot liquid onto a cold one when they come into contact.
For Convection to occur, objects don’t need to touch. This is the basic principle of hot air rising and pushing cold air downwards. So, with your cup of coffee, when the top layer of the liquid is exposed to the cold(er) air in the room, it generates heat (and steam) which rises and pushes the cool air down towards the liquid in a constant cycle, continuously cooling your coffee down.
Radiation occurs at an atomic level. Heat travels in the form of visible and non-visible light. When you can’t see the light, but still feel the heat, this is called infrared radiation. So, a smoking hot cup of coffee could emit heat from the cup when it isn’t insulated. Over time, this infrared radiation will transmit all of the heat from your hot cup of coffee outwards into the room.
So, if heat is conducted/transferred/emitted from your coffee to the coffee mug to the outside world, how does insulation help to prevent this process from occurring?
The first thermal travel mug was designed and patented in 1908 by Gustav Robert Paalen, and it detailed a “double walled vessel comprising inner and outer walls united with each other only at the mouth of the vessel”.
The patent is now held by Zojirushi Vacuum Bottle Co. Ltd and some minor changes have been made over the years. But it has stood the test of time and kept coffee from going cold for more than a century!
So, it’s important to understand the components of a vacuum insulated travel mug. The outer layer of the travel mug will feature reflective coating (typically stainless steel) and have the screw-on or attachable lid.
The inner layer, made of the same material, but without the reflective coating will be fused into the outer layer at the top of the travel mug, so that it is also sealed when the lid is attached/screwed on, ensuring a tight seal.
Sometimes an additional outer layer is added to protect the mug from scratches, fractures and other potential damage. But, ultimately, the key feature is the vacuum that exists between the inner and outer layer that prevents any form of heat transfer.
To put it in more detailed terms, conduction is prevented due to the vacuum, convection does not occur due to the tightly sealed lid and the reflective coating eliminates heat transference through infrared radiation.
So, vacuum insulation is based on some sound scientific principles, but how does this play out in real life?
The reason that your coffee gets cold is that there’s a major discrepancy between the temperature of your cup of coffee (which should be between 195ºF and 205ºF) and the room that you’re in (±70ºF). The major difference means that the heat will be transferred very quickly until it reaches equilibrium, which is where your cup of coffee goes cold.
However, a vacuum insulated travel mug will prevent this rapid heat transfer through conduction, convection and radiation. So, when you open the lid on your travel mug a few hours later, it will still be smoking hot!
One question you may be asking now that you’ve learned about the vacuum insulation features used in travel mugs is whether they can be as effective at keeping a cold liquid cold in rooms or outdoors when it’s very hot outside. For example, will it keep your glass of wine or beer cold? The answer is yes!
In fact, some travel mugs are designed for this very purpose! You can get insulated coffee mugs and travel mugs, as well as insulated wine cups and various custom insulated mugs that can be used to keep your coldly served drink cold. It follows the exact same principles in terms of thermodynamics and the transference of heat, but just prevents heating instead of cooling.
Travel mugs are an ingenious way to prevent a hot cup of coffee from going cold. By using vacuum insulation, with an inner layer that holds the coffee and an outer layer that’s exposed to the room temperatures, it prevents conduction. Their screw-o or attachable lids keep the hot air inside the mug, preventing convection. And the reflective coating on the layers prevents radiation. By mitigating all of the ways that heat is transferred, it keeps the liquid inside hot, no matter what the temperature is outside. However, it doesn’t last forever, so don’t leave your coffee in there for too long!
- How Does Vacuum Insulation In A Thermos Work? | Home Grounds
- How Do Travel Mugs Keep Heat | SeniorCare2Share
- How Do You Choose the Right Travel Mug? | Quality Logo Products Blog
- How Do Insulated Cups Work: Overview For Insulating Cups | Ecoway Houseware
- Principles of Heating and Cooling | US Department of Energy