Help for CBE Thermal Comfort Tool for ASHRAE-55: Compare 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.
The Compare section of the tool allows you to work with different conditions of comfort at the same time, to be able to see directly and interactively how different design decisions can affect thermal comfort.
At first, you will see only one input column with the six variables explained below. You can add up to three columns of inputs to dynamically modify the conditions and compare them both in the compliance section and in the graphic representation. The number of conditions that you can compare at once has been limited both for computational reasons and to maintain a good legibility. More than three comfort zones make the understanding of the chart and the comparison of the results too complicated.
This section of the tool maintains its three main parts:
• Left-hand side: this is the user interface. It contains the input fields containing 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. You can do this for one up to three conditions. Click on the upper buttons to display or hide the respective inputs.
• Top-right: this section contains the results of the calculations for all the conditions. The raw output of the comfort model calculations (such as PMV, PPD, etc. for the PMV method) as well as compliance information.
• Bottom-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-humidity chart. You can switch between them by clicking on the button next to the name of the chart.
For the compare section, only the PMV/PPD method is used, since it is meaningful to represent more than one condition at a time and compare such conditions.
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 output in the upper-right region changing, as well as 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, since the zone represents a range of air temperature and humidity values.
Next to the air temperature box, you can click on the use operative temperature button. When this option is selected, it will be assumed that the air temperature and mean radiant temperature are equal to the value in the operative temperature input field. Note that the x-axis of the psychrometric and temperature-humidity charts still represents dry-bulb temperature.
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.
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.
Local air speed control is the ability for the occupants to modify the local air flow, and if this is not available in their space, limits apply to the range of temperatures that can be covered. Therefore, availability of local control allows wider ranges of air speed that can be used to offset higher temperatures.
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. 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.
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).
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.
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 some default values for all the input variables of the different conditions, to restart the calculations and visualizations.
Operative temperature can be selected as an input. This will hide the air temperature and mean radiant temperature input boxes.
You can select this button when the occupants have control of the air movement, e.g. if they can operate a fan and set its intensity.