Mean radiant temperature

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Mean Radiant Temperature (MRT) - Part IV
Copyright (c) 2011, Robert Bean, R.E.T., P.L.(Eng.) and

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Mean Radiant Temperature (MRT) - Part I: Introduction/theory
Mean Radiant Temperature (MRT) - Part II: Application of theory
Mean Radiant Temperature (MRT) - Part III: Spreadsheet tool for thermal bridging
Mean Radiant Temperature (MRT) - Part IV: Project examples & ASHRAE research
Mean Radiant Temperature (MRT) - Part V: Calculation example (REHVA GB #7)

Mean radiant (MRT) and operative temperature (Top) calculation

Below you will find screen shots from an Excel based MRT/Top calculation example using, Figures 2.1, 2.2 and 2.4 from ''Babiak, J., Olesen, B.W., Petras, D., REHVA Guidebook #7, Low temperature heating and high temperature cooling, 2007"; and its comparison against the beta test version of ASHRAE RP - 1383. This Excel tool is not freely available as yet to our visitors but we do give away a thermal bridging tool for anyone wanting to evaluate net insulation values in wood framed walls. Click the images below for an enlarged view.

REHVA example and formulas

Figure 1. Using the REHVA example and formulas, we have set up the office room geometry with dimensions for a seated person located centrally in the room ignoring the facing direction for simplification.

tabular inputs and results

Figure 2. Shown is the tabular inputs and results for each of the surfaces where the results are unity (equal to one) Note the error is due to rounding.

inputs into a surface temperature calculation

Figure 3. Once the angle factors are established they can be used as inputs into a surface temperature calculation which then drives the mean radiant temperature (Tmr) and operative temperature (Top) .

Software for modeling radiant effects on thermal comfort

ASHRAE commissioned through research project 1383 to, "Develop a Radiant System Module for the Simulation and Analysis of Spaces and Systems". This software tool can be used for evaluating the effects of radiant energy on thermal comfort vis a vis the PMV/PPD methodology developed by Fanger et al.

As of July 12th, 2012, the tool developed under contract by Wrightsoft for ASHRAE, has been released to the T.C. 6.5 RP-1383 evaluating committee as a beta version. Using this version we've taken the angle factor methodology defined by REHVA Guidebook #7 and developed the Excel tool illustrated above, and compared it to the results from the RP-1383 software with results being close or identical in some instances. We're currently working to resolve any differences and will keep our readers posted.

Ultimately the final version of the RP-1383 tool will be available through ASHRAE and will incorporate the ASHRAE Thermal Comfort Tool. If you join our Linked-In group you get first hand notice of the release or you could become a member of ASHRAE and get access to the tool at member price! In the meantime, below you find screen shots of the input/output screens from the beta version of RP 1383, using the REHVA Guidebook #7 angle factor example.

Figure 4. The input/output screen shot (Beta test version) for the REHVA room example showing angle factors, emissivity and surface temperatures which were defined by our Excel tool.

Figure 5. The 3D fully rotatable output screen shot (Beta test version) from ASHRAE RP-1383 illustrating the MRT color contours at the seated level (0.6m). The 3D tool can also illustrate PMV/PPD as per ANSI/ASHRAE Standard 55 - Thermal Environmental Conditions for Human occupancy.

Figure 6. The PMV/PPD results (Beta test version) for the Excel model and indication of compliance. As noted this is a screen shot from the pre-release version. ASHRAE members will be notified via press release when the tool is available through the bookstore.

For more on this topic read operative temperature.

Suggested reading:
  • Karlsson, H., Modelling of Long Wave Radiation Exchange in Enclosures with Building Integrated Heating, Chalmers University of Technology, Dep. of Civil and Environmental Engineering, Sweden, 2005

  • Fanger, P.O., Thermal Comfort: analysis and applications in environmental engineering, Danish Technical Press, Copenhagen, Denmark, 1970

  • ASHRAE Research Project 657. Simplified Method to factor Mean Radiant Temperature (MRT) into Building and HVAC System Design

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