Sensible heat is the heat you can sense or measure with a thermometer. The heat causes a change in the temperature of a substance without changing its phase (solid, liquid, or gas).
When you feel the temperature rise or fall in a room, it's primarily due to sensible heat transfer.
For example, when warm air blows into a cold room, the temperature increases, and you feel warmer.
Sensible heat is quantified in units of energy, such as British Thermal Units (BTUs) or joules, and it directly affects thermal comfort.
We are often concerned with calculating the sensible BTUs/hr required to heat or cool the air in sizing HVAC systems.
We can also use the sensible heat to calculate the CFM or air required to transfer these BTUs.
That's where the sensible heat equation comes in.
The following is an introduction and explanation of the sensible heat equation, one of the most commonly utilized equations in the HVAC industry.
The sensible heat equation is expressed as:
Qs = 1.08 × cfm x dT
Where:
Qs is the sensible heat transfer rate in British Thermal Units per hour (BTU/hr).
ΔT is the temperature difference between two air conditions in degrees Fahrenheit.
CFM is the flow rate of air in cubic feet per minute.
The 1.08 in this equation is not a constant. Instead, it is the product of the density(p) and specific heat (Cp) of the air at "standard air" conditions and the conversion factor of 60 minutes per hour.
1.08 = p × Cp × 60 min/hr = 0.075 lb/ft3 x 0.24 Btu/lb°F x 60 min/hour
Qs = (p × Cp × 60 min/hr) × cfm x dT = 1.08 × cfm x dT
"Standard air" has historically been defined by ASHRAE as having a density of 0.075 lb/ft3, which equates to air density at sea level (barometric pressure of 29.92 in. Hg).
Please note: Air conditions and elevations other than “standard air” will cause this factor to change.
As a side note, specific heat is the heat required to raise the temperature of the unit mass of a given substance by a given amount (usually one degree).
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