Help for CBE Thermal Comfort Tool for ASHRAE-55: Ranges Section
This web-based tool for thermal comfort calculations according to ASHRAE Standard 55-2013 is developed at The University of California at Berkeley. Its aim is to provide a free, cross-platform tool that allows designers and other practitioners to perform thermal comfort calculations.
See the bottom of the web-page for acknowledgments, contact information, and citation.
This guide aims to explain the main features of the Compare Section, and demonstrate how best to use it. In most cases, the interface is intuitive and does not require explanation. To get information quickly, click on the input headers to be directed to a Wikipedia article relevant to that input.
This section of the tool allows the user to explore the concept of thermal comfort flexibility, by representing on both the psychrometric and the temperature-relative humidity charts a range of comfort zones and the respective range of air temperatures in which occupants will find comfort.
Firstly you change the inputs, then draw the range on either one of the charts, finally you can see the air temperature ranges and interact with the visualization with the mouse, hovering on the chart.
For the ranges section, there are three main parts:
• Left-hand side: this is the user interface. It contains the input fields with the values that drive the comfort calculations and visualizations. To change these input values, you can type directly in the boxes or manipulate the up and down arrows. At the right hand side of each input box, except for air temperature and humidity, there are mote options that you can select to set the representation of the comfort ranges. First the extremes of the input range, which can be specified both with the input box and the sliders below it, second the step or increment that you want to use to display the gradient of comfort zones. The bigger the step, the fewer zones are represented, but the results don't change. Finally, click on the "R" button to draw the range on the chart.
• Top-right: this section contains a visualization of the thermal comfort conditions in the input. There are currently two types of charts that can be used in this section: the psychrometric chart and the temperature-relative humidity chart. You can switch between them by clicking on the button next to the name of the chart. Apart from the standard functionality of the tool, where you can type and change the inputs to see the comfort zone dynamically move on the chart, in this section you can draw ranges of comfort. By clicking on the button in the inputs section, several comfort zones are drawn for each step of the selected variable, given the other three parameters plus humidity. A line is also displayed, showing the range of air temperatures according to that particular value of relative humidity in the space. Interactivity is added to the visualization, to enhance the legibility of the representation.
• Bottom-right: Below the chart, a summary and brief explanation of the ranges of air temperature is presented. This section is displayed only when you click on the "R" buttons, and it is automatically updated for any change you make on the inputs, after re-clicking on the range button.
For the ranges section, only the PMV/PPD method is used.
For air speeds greater than 0.2 m/s (39 fpm) the PMV calculations employ the elevated airspeed method, which calculates and reports the cooling effect of the air movement.
For more information about the comfort models, you can follow the link to Wikipedia by clicking on "select method".
There are six primary factors that affect thermal comfort. These include environmental conditions such as air temperature, and personal factors such as metabolic rate.
By modifying this value, you will notice the red dot on the psychrometric or temperature-humidity chart moving. This value corresponds to the x-axis of both charts.
Depending on which specification of humidity is being used, the red dot may follow the lines of constant relative humidity, or move horizontally. This value does not affect the comfort zone itself, but only one condition. The air temperature input box is hidden when you draw the ranges, because it is not meaningful anymore if you consider ranges of conditions.
MRT represents the mean of the radiant temperatures of the enclosing surfaces of a space, which is determined by the emissivity and the temperature of the surfaces. This value affects the location of the comfort zone, since it may affect the range of acceptable air temperatures. For example, higher radiant temperatures allow the occupant to feel comfortable at lower air temperatures, or vice versa. Thus, an increase in MRT shifts the comfort zone to the left side of the charts.
Giving a range to the MRT means for example allowing the temperature of one or more surfaces in the space to change in time between two extreme values, and see how this can affect thermal comfort, given the other parameters of the environment or the occupant.
This is the rate of spatial change of air in a space, which is used to calculate convective heat transfer and thus changes the comfort zone. Higher air speeds allow higher temperatures and humidity, due to the cooling effect that air movement has on an occupant.
A range of air speeds means that the air movement intensity can be modified in time to satisfy a larger number of occupants under different conditions.
Relative Humidity is the ratio of the partial pressure of the water vapor in the air to the saturation pressure of water vapor at the same temperature. You can also input dew-point temperature, humidity ratio, wet bulb temperature, or vapor pressure, by selecting it through the expandable box.
Humidity will change the position of the dot in the standard visualization. It doesn't affect the comfort zone boundary since the boundary represents a range of temperature and humidity, but it does affect the PMV/PPD calculations.
For this reason, in the ranges visualization, the level of humidity is important, and this can be seen thanks to the red curve, which can be changed – once the range is displayed – by clicking on the humidity input box. The RH curve will be updated according to the input, changing its position on the chart.
Metabolic rate is the rate of energy production of the body, which varies for different activities. A list of common activities and correspondent metabolic rate in met units is available next to the input box.
Increasing the metabolic activity means moving the comfort zone significantly towards lower temperatures and vice versa, since higher activities make the body produce more heat and thus be more comfortable in colder environments. Elevated metabolic rate can also result in decreased effective clothing value and increased relative air velocity (as air is pumped through clothing).
The metabolic activity can change in time due to personal adjustments to the thermal environment. If this aspect is considered, a wider range of comfort can be represented.
Clothing is probably the most important variable in terms of adaptation to a thermal environment, and this means that acting on the clothing level may be very effective to reduce energy consumption. You can use the clothing creator and dynamic calculator in the main section of the tool, and then use such value to compare conditions in this section.
For the concept of thermal flexibility, adaptation in clothing represents a great opportunity. If you consider that the occupants can adapt to their environment by wearing different layers of clothing, or simply you want to see what happens in a space where people dress in different ways, you can use the ranges visualization giving a minimum and maximum value at the clothing level. This typically greatly affects the width of the comfort zone.
At the bottom of the input section of the tool, you can find more clickable buttons to set more parameters and open some dialogs.
Clicking on this button, a new window pops up, letting you type the following inputs: air temperature, air speed, globe temperature, globe diameter, globe emissivity, to calculate the correspondent Mean Radiant Temperature.
This feature allows you to have a more precise evaluation of the MRT by taking measurements with a globe thermometer.
The button is disabled when the operative temperature is used.
You can change the barometric pressure to account for changes in altitude. The standard atmospheric pressure is 1 atm = 101.325 kPa = 101325 Pa.
Remember to input the value in Pascals!
Click on this button to switch between the International System of Units (SI) and the Inch-Pound system.
This button sets the default values for all the input variables and removes the comfort ranges, to restart the visualizations.