{"id":19380,"date":"2024-02-02T16:18:45","date_gmt":"2024-02-02T08:18:45","guid":{"rendered":"https:\/\/aircondlounge.com\/?p=19380"},"modified":"2024-03-26T12:03:35","modified_gmt":"2024-03-26T04:03:35","slug":"sensible-heat-ratio-formula-calculation-psychrometric","status":"publish","type":"post","link":"https:\/\/aircondlounge.com\/sensible-heat-ratio-formula-calculation-psychrometric\/","title":{"rendered":"Sensible Heat Ratio: Formula, Calculation & Psychrometric"},"content":{"rendered":"\n
Engineers who are dealing with air conditioning and psychrometrics often come across the term sensible heat ratio. It is said to be one of the most important parameters in HVAC. So, what is sensible heat ratio?<\/p>\n\n\n\n
Sensible heat ratio (SHR) is the ratio between the sensible heat and the total heat. SHR is expressed in percentage and it is calculated by dividing the sensible heat by the total heat. SHR 80% means the sensible heat is 80% of the total heat while the other 20% is latent heat.<\/strong><\/p>\n\n\n\n The sensible heat ratio formula is expressed as:<\/p>\n\n\n\n SHR = QS<\/sub> \/ (QS<\/sub> + QL<\/sub>) In practice, sensible heat ratio lets us find the required supply air temperature based on the desired room temperature and the sensible heat load present in the room.<\/p>\n\n\n\n Sensible heat is the heat that can cause an object’s temperature to rise.<\/p>\n\n\n\n Sunlight, lamps, people, printers, laptops, microwaves, ovens and other office equipment and cooking appliances emit sensible heat. The heat emitted by these equipment and appliances will raise the temperature of the room.<\/p>\n\n\n\n On the other hand, latent heat is the heat involved during phase change (e.g.: from vapor to liquid). If there is a lot of moisture in the air, we say the latent heat is high.<\/p>\n\n\n\n Apart from emitting sensible heat, people also emit latent heat. Meaning people release moisture into the air, causing more moisture need to be removed by the HVAC equipment.<\/p>\n\n\n\n The purpose of calculating sensible heat is for HVAC equipment sizing. Sensible heat is contributed by multiple heat sources including windows, walls, lightings, people and office appliances.<\/p>\n\n\n\n For each heat source, the sensible heat is calculated differently.<\/p>\n\n\n\n Nonetheless, one of the most popular sensible heat formulas is:<\/p>\n\n\n\n Qs<\/sub> = 1.08 x CFM x \u0394T<\/em><\/p>\n\n\n\n where, The 1.08 in the sensible heat formula is the product of the density of air (0.075 lb\/ft3<\/sup>)<\/em>, specific heat of air (0.24 btu\/lb\/\u00b0F) <\/em>and the conversion from cubic feet per hour to cubic feet per minute (60)<\/em>.<\/p>\n\n\n\n The original form of the sensible heat formula is:<\/p>\n\n\n\n Qs<\/sub> = mc\u0394T where, In practice, the 1.08 x CFM x \u0394T is not a feasible formula to calculate sensible heat because the CFM is unknown. In fact, it should be the opposite where the sensible heat is first determined and then use the said formula to calculate the CFM.<\/p>\n\n\n\n The sensible heat of an interior wall (not exposed to the sunlight) can be calculated using the following formula:<\/p>\n\n\n\n Qs<\/sub> = UA( to<\/sub>– ti<\/sub> )<\/em><\/p>\n\n\n\n where, For example, a curtain wall that has a layer of R-5 insulation sandwiched between two layers of bricks has a combined U-value of 0.124 btu\/hr\/ft2<\/sup>\/\u00b0F. Given that the surface area of the wall is 150 ft2<\/sup>, the adjacent space temperature is 90\u00b0F and the design indoor temperature is 75\u00b0F, the sensible heat emitted by the wall is:<\/p>\n\n\n\n Qs<\/sub> = UA( to<\/sub>– ti<\/sub> )
SHR = Sensible Heat \/ (Sensible Heat + Latent heat)
SHR = Sensible Heat \/ Total Heat<\/em><\/p>\n<\/div><\/div>\n\n\n\nWhat is Sensible Heat?<\/h2>\n\n\n\n
How to Calculate Sensible Heat?<\/h2>\n\n\n\n
Qs<\/sub> = Sensible heat, btu\/hr
CFM = Airflow in cubic feet per minute
\u0394T = Room temperature – supply air temperature, \u00b0F<\/em><\/p>\n<\/div><\/div>\n\n\n\n
Qs<\/sub> = (\u03c1v)c\u0394T<\/em>
Qs<\/sub> = (0.075 x 60 x 0.24) x CFM x \u0394T<\/em>
Qs<\/sub> = 1.08 x CFM x \u0394T<\/em><\/em><\/p>\n\n\n\n
Qs<\/sub> = Sensible heat, btu\/hr
\u03c1 = Density of air, lb\/ft3<\/sup>
v = Velocity of air, ft\/hr
c = Specific heat of air, btu\/lb\/\u00b0F
\u0394T = Room temperature – supply air temperature, \u00b0F<\/em><\/p>\n<\/div><\/div>\n\n\n\nSensible Heat Calculation Example – Interior Wall<\/h3>\n\n\n\n
Qs<\/sub> = Sensible heat, btu\/hr
U = Coefficient of heat transfer (U-value), btu\/hr\/ft2<\/sup>\/\u00b0F
A = Surface area of the wall, ft2<\/sup>
to<\/sub> = Adjacent space average dry-bulb temperature, \u00b0F
ti<\/sub> = Indoor design dry-bulb temperature, \u00b0F<\/em><\/p>\n<\/div><\/div>\n\n\n\n
Qs<\/sub> = (0.124)(150)(90-75)
Qs<\/sub> = 279 btu\/hr<\/em><\/p>\n\n\n