The Keys to Effective Kiosk Design
From retail environments to airports to hotels, interactive kiosks are practically everywhere you turn in many public venues.
From retail environments to airports to hotels, interactive kiosks are practically everywhere you turn in many public venues. Enabling users to conveniently perform a range of tasks, including retrieving information and buying tickets, these devices are only useful, however, when they’re working properly. When a kiosk malfunctions, which most of us have experienced, the failure is often due to overheating of electronic components inside the enclosure. That’s why anyone designing a kiosk containing such electronic devices must know the keys to keeping it functioning properly. Here’s a look at how to ensure reliability in your kiosk design, starting with an understanding of the basics of heat and airflow.
Inside the enclosure
Any system that requires power will generate heat. Electronic equipment is typically designed so that when it’s located in a relatively open space indoors at room temperature, heat dissipation through the air is sufficient to keep the equipment from overheating. However, when you place a piece of electronic equipment in an enclosure, you’re creating a local ecosystem for that equipment. The heat produced by the equipment must somehow be removed from this local environment to prevent failure.
Ambient and operating temperature
Ambient temperature is the temperature in the area immediately surrounding a piece of electronic equipment. When the device is sitting on a table in an open room, the ambient temperature is the room temperature. Inside an enclosure, the ambient temperature is likely to be significantly higher than the room temperature outside the enclosure.
If a device is operating in an ambient temperature above the operating temperature range specified for the device, it may overheat, reducing the life of the unit or causing it to fail. A typical operating temperature specification is 10 degrees C to 40 degrees C or 50 degrees F to 104 degrees F.
Types of heat transfer
Heat transfer takes place in three ways:
Conduction takes place when heat travels through material.
Convection takes place as air circulates and carries heat with it. Natural convection is the result of hot air rising. Forced convection occurs when air is blown across the surface of a device, moving heat away.
Radiation occurs when heat is transferred electromagnetically away from a device such as a stovetop burner, for example. In an enclosure, radiation is typically not a significant heat transfer mechanism.
In most enclosures, the rate that heat is dissipated depends primarily on heat transfer by convection. Convection can be enhanced by increasing air circulation through appropriate placement of vents, and in some cases, by installing fans.
After getting a better grasp on the basics of heat and airflow, it’s necessary to consider how these factors should influence your initial design.
Estimating heat generation
Due to conservation of energy, all power used in a system is eventually converted to heat. Some energy is immediately converted to heat due to inefficiencies in the system, while the remaining power makes the transformation as it’s consumed by equipment in the system.
Therefore, an estimate of the power used in a system can be used to represent the amount of heat generated. Although an estimate based on the assumption that equipment is running at its maximum power rating will typically overestimate the amount of power that’s consumed, this allows for other variables that are more difficult to take into account.
The example in Table 1 shows an estimate of heat generated in an enclosure that contains a computer, display, printer, and fan. The estimate was developed using specifications such as voltage, current, and power ratings for the equipment — often found on the device label or in the owner’s manual.
Table 1. Estimating heat generated in a sample kiosk enclosure.
For equipment other than external power supplies, first look for a power specification to use as the estimate (see the estimate for the computer and fan). If a power specification isn’t available, calculate the power by multiplying the DC voltage and current ratings (see estimates for the display and printer).
A typical external power supply is about 80 percent efficient. This means 80 percent of the power goes to the device being powered, and 20 percent is immediately dissipated in heat. To estimate the contribution of a power supply to the overall heat generated in the enclosure, calculate 20 percent of the power rating (see estimates for the external power supplies for the display and printer). The maximum amount of heat that could be generated by the equipment in the example shown in Table 1 is 277 watts.
Airflow can be passive, enhanced by strategically placed vents, or it can be supplemented by adding fans.
Adding vents in an enclosure is a great way to provide openings for air to enter and exit. When doing so, keep the following considerations in mind:
Placement. Vents should be placed so that air flows around the devices that need to be cooled. Placing a vent at the bottom on one side of the enclosure and a vent at the top on the other side may help direct air around devices. Vents should also be placed so that rising warm air doesn’t become trapped in a “bubble” at the top of the enclosure.
Size. Larger vents present less resistance to air flow. For the best airflow, make vent openings as large as is practical for the environment in which the enclosure will be placed.
Types of vents. A vent may be as simple as a hole or collection of holes cut or drilled through the side of the enclosure. A vent opening may be uncovered or may be protected by a cover with slots, holes, perforations, a screen, or louvers. Remember that any covering increases resistance to air flow.
Vandal-proofing. Vandalism is often a concern for kiosks in public places. Vents must be sized and placed so that people can’t reach in or insert objects or spill liquids into the enclosure. Sometimes vents can be placed out of reach on the back side or top of the kiosk. In other cases, vents with perforated coverings or louvers will need to be considered.
Dust. Electronic equipment must also be protected from dust. Accumulated dust on equipment can reduce the effectiveness of convection to carry heat away. Dust can also affect the visibility of displays. Flammable dust, such as sawdust, can present a fire hazard. In some cases, dust entering the enclosure may be minimized simply by placing the lowest vents high enough above the floor of the enclosure to reduce the entry of dust from the floor outside. In more extreme cases, you may need to install vents with perforated openings, or even air filters, to exclude dust. (Air filters, however, reduce air flow and require regular servicing, so consider other alternatives before incorporating air filters into your design.)
If your enclosure design doesn’t provide adequate ventilation without a fan, and you can’t use fans in your enclosure, you may want to consider hiring a consultant to help optimize your design.
Figure 1. Temperature change in an enclosure.
Adding fans is another effective counteractive measure to offset the effects of energy. This involves estimating the airflow necessary to dissipate heat. The equation below shows a simplified method for determining the temperature rise based on air flow through an enclosure.
?T = 1.76Q/G
?T = T2 – T1
Q is the power dissipated by the equipment in watts
G is the volumetric airflow in cubic feet per minute (CFM)
T2 is the exit temperature in degrees Celsius
T1 is the inlet temperature in degrees Celsius
1.76 is a constant.
Figure 1 shows how these parameters relate to an enclosure. This equation can be rearranged to determine the airflow required to adequately cool the enclosure.
G = 1.76Q/?T
?T = T2 – T1
G is the airflow out of the enclosure in cubic feet per minute (CFM)
Q is the estimated temperature dissipated by equipment in the enclosure (277 W for the example in Table 1)
?T is the calculated temperature rise the enclosure can sustain
T2 is the exit temperature. The exit temperature can’t exceed the lowest maximum specified operating temperature for any device in the enclosure. For the example in Table 1, T2 is 40 degrees C.
T1 is the inlet temperature. The inlet temperature is the highest ambient temperature anticipated at the inlet vent. In a room where the temperature is controlled at 68 degrees F, T1 = 20 degrees C.
This equation can be solved for the example in Table 1.
G = 1.76 (2.77)/40-20 = 24.4
Thus, a flow rate of about 25 CFM is required to keep the temperature inside the sample enclosure within the required operating temperature range.
Selecting a fan
Figure 2 shows how the efficiency of a fan is affected by the environment in which it’s used. Fan manufacturers describe the capabilities of a particular fan using a fan curve such as the one shown in the illustration. The fan curve shows how the static pressure, or back pressure, of the system affects the amount of air the fan moves. Back pressure affects how freely air flows through the fan. As the back pressure increases, the amount of air flow from the fan decreases.
The manufacturer’s fan rating is the highest airflow the fan can achieve in the most ideal conditions. As the graphic shows, a fan in an enclosure will move significantly less air than represented by its rating.
An enclosure resistance curve shows the amount of pressure required within a particular enclosure to achieve a certain air flow. As more air flow is needed, more fan pressure is required to achieve that airflow. The enclosure resistance curve can be moved lower and to the right by reducing the back pressure of the enclosure for example, by adding more vents. The point where the enclosure resistance curve crosses the fan curve indicates the amount of airflow the fan will produce inside the enclosure. Any given fan can only deliver one flow rate at one pressure in a given enclosure.
Figure 2. Determining fan efficiency in a system.
The sample graph in Figure 2 indicates that, as a rough rule of thumb, to achieve the desired air flow in your enclosure, you should select a fan rated for at least twice the flow rate you calculated. For example, because the flow rate needed is about 25 CFM, at least one 50 CFM fan or two 25 CFM fans should be installed to help ensure delivery of a 50 CFM flow rate. (Note that adding fans to the enclosure will increase the amount of heat generated in the enclosure.)
The fan rating is typically provided on the manufacturer’s label on the fan, and a fan curve is usually included in the specification sheet. Most fan manufacturers also provide specifications and design guides on their websites.
As Figure 2 shows, lowering the back pressure of the system allows the fan to operate closer to its rated value. Increasing the number and size of vents decreases the back pressure of the system, allowing the efficiency of the fan to increase. Placement of equipment within the enclosure can also affect back pressure. The more open the pathways where air can flow, the less back pressure.
Another critical aspect of successful kiosk design is planning for maintenance. Following certain guidelines will help you streamline your efforts and maximize the end result.
All systems require maintenance. Consider the following as you develop your design:
Allow for easy serviceability of fans, which often have a high failure rate over time.
Provide access to filters for cleaning or filter changes.
Provide access to printers that may require regular replenishment of ink or paper.
Provide access to equipment in case of failure.
Also consider developing a maintenance plan for the kiosk with a checklist and schedule for maintenance tasks.
After considering all of the potential maintenance issues, it’s important to implement a prototype of your design before putting it into production. Install all of the equipment to be used, and test the kiosk for a period of time in an environment typical of the worst-case conditions in which it will be deployed. This will give you an opportunity to optimize the thermal aspects of the design before deploying multiple units. Once you’ve built this unit, it’s time to test it.
Checking temperatures at various points in the enclosure will ensure that all electronic equipment is operating within specified operating temperature ranges. To do so, complete the steps below.
1. Use a thermocouple to measure the air temperature at all vents in the system.
Measure the temperature at all inlet vents and exit vents (see T1 and T2 in Figure 1). The temperature difference between any two vents should be no more than ?T, the calculated maximum temperature rise the enclosure can sustain. The temperature at any vent shouldn’t be higher than the calculated value for T2.
2. Use the thermocouple to check that all devices in the enclosure are operating within their specified operating temperature range.
Even though overall airflow through the enclosure may appear to be adequate, areas within the enclosure may not be receiving adequate airflow. A critical component located in a hotter area inside the enclosure may be operating outside its specified operating range, putting it at risk of failure.
Optimizing your design
If measurements show that the airflow through the enclosure isn’t adequate or that hot spots exist within the enclosure, there are a number of options available to resolve these problems:
Redirect air to optimize air flow to hot areas using deflectors.
Estimate the amount of heat that will be generated by electronic equipment in the kiosk.
Consider how vents can be used to help dissipate the heat.
Plan for maintenance. Consider how access will be provided for maintenance as you develop your design.
Build a prototype of the design with all equipment installed.
Test the prototype. Conduct measurements to ensure all equipment is adequately cooled.
Adjust the design. If tests show some devices are overheating, change the design to address these issues.
Reposition components to provide more open space to improve air flow.
Place components more directly into an existing air flow.
Add more vents or make existing vent openings larger.
Position vents to improve airflow, such as placing intake vents below equipment and output vents near the top of the enclosure, so that air is directed over hot components.
Replace vent covers with covers that have more or larger openings, if requirements for excluding dust or minimizing the risk of vandalism don’t preclude doing so.
If airflow from an existing fan isn’t adequate, replace the fan with a higher rated fan or add more fans.
Check that hot air isn’t being trapped at the top of the kiosk or elsewhere within the kiosk.
Replace electronic components with equivalent models that generate less heat, and don’t oversize components.
Once you complete these steps, you’re well on your way to a more successful interactive kiosk — regardless of the setting.
John Jedrzejewski, Neil Brashnyk, and Scott Vahlsing are product engineers with Planar Systems, a Beaverton, OR-based display solutions provider.