Sensible Cooling: Definition & Load

Instructor: Jennifer Perone

Ms. Perone has taught College Engineering, Ethics, Psychology, Perception, Statistics, Experimental Design & Analysis, Physics and secondary STEM topics for more than 15 years!

Sensible Cooling Load is one of the two types of cooling loads that comprise the design cooling load. This article describes what sensible cooling load is, how it is different from latent cooling load and how these calculations are used to calculate the design cooling load of HVAC systems.

Sensible Cooling Load

Sensible cooling load refers to one of two types of cooling loads that make up the totality of the design cooling load. The design cooling load is the amount of heat energy that must be removed from a building to maintain the indoor design temperature, even during the most extreme outdoor conditions. Let's say that you live in Florida and it's 98 degrees F (~37 degrees C) outside with 100 percent humidity. Those are pretty unbearable conditions and they can put a strain on HVAC systems, trying to maintain a cool 77 degrees F (25 degrees C) with a much lower humidity. 'Any type of 'cooling' is accomplished not by cooling, but by moving heat energy around the system.

Factors influencing Sensible Cooling Load

There are several factors that influence sensible cooling load, among them are types of openings, such as glass windows and doors, sunlight intrusion, exterior walls, partitions, floors over open crawl spaces, air infiltration between cracks, ceilings under an attic, people in the building, equipment and appliances operated, lights, types of roofing, ductwork and insulation throughout the home.

Out of these factors, the following are the ones you can't control: external temperature and humidity, home size, direction your home faces. If your home faces the south or west, your home with have more heat infiltration from sunlight. The following factors are those which can be controlled: air leaks, insulation levels, heat infiltration through windows, heat producing appliances in your home. You can seal up leaks and cracks, provide more insulation and limit the running of heat producing appliances to the evening hours.

Equation

You can calculate sensible heating or cooling load in SI units with the following equation:

hs = cp * rho * q * dt

where hs = sensible heat (kW), cp = specific heat of air (1.006 kJ/kg degrees C), rho = density of air (1.202 kg/m3), q = air flow (cms), dt = temperature difference (degrees C).

As previously stated, sensible cooling load is one of two types of cooling loads that define the design cooling load. The other cooling load is the latent cooling load. Sensible cooling load refers to the dry bulb temperature of the building and latent cooling load refers to the wet bulb temperature of the building. What are dry bulb and wet bulb temperatures and how do they differ?

Dry Bulb Temperature versus Wet Bulb Temperature

The Dry Bulb Temperature is the normal air temperature and can be measured with a normal thermometer that is shielded from sunlight and moisture. It is called dry bulb temperature because it is not affected by moisture in the air. Obtaining the Wet Bulb Temperature is a bit more complicated, but it can be measured by placing a clean piece of wet muslin over the bulb of a thermometer.

Wet Bulb Temperature, Dew Point and Humidity

The wet bulb temperature indicates the amount of moisture in the air and the rate of evaporation from the wet bulb thermometer depends on the humidity of the air. Evaporation is decreased when the air is more humid. The more humid the air, the more demands placed on the HVAC system to provide a comfortable cooling level. While most of us are not as familiar with the wet bulb and dry bulb temperatures, we all have a general knowledge of dew point.

For reference, the wet bulb temperature is always found in between the dry bulb temperature and the dew point. If you recall, the dew point is the temperature at which the water vapor condenses out of the air or the air becomes saturated. You've probably had the experience of feeling the dew or moisture precipitates or condenses out of the air between sundown and dawn. When you first wake up early in the morning, there is dew all over the ground. As the sun rises, the temperature also rises and evaporates this water back into the air, increasing humidity throughout the day. If you have a cold bottle you take from the refrigerator, you'll see condensation forming at room temperature of 77 degrees F (25 degrees C) and you'll see condensation even more quickly if you bring the bottle outside if the outside temperature is higher than inside at 86 degrees F (30 degrees C, for example). If the outside temperature is the same at that of the refrigerator, no condensation will form. This is because the relative humidity is low in both places.

Sensible cooling and latent cooling

As stated at the beginning of the article, there are two types of cooling that occur: 1) sensible cooling and 2) latent cooling. Sensible cooling removes the temperature heat or the heat that we sense out of the air. Latent cooling involves dehumidification of the air.

All of these calculations and concepts are linked to HVAC systems and comfort levels. Summertime cooling involves controlling humidity (latent cooling) over temperature (sensible cooling). For example, most people prefer their thermostats set to 72 to 78 degrees Fahrenheit (~22 to 26 degrees Celsius) during the warm weather months and 67 to 72 degrees Fahrenheit (~19 to 22 degrees Celsius) during cold weather months.

Sensible Heat Ratio

The sensible heat ratio can be expressed as SHR = hs/ht,

Where hs = sensible heat, ht = total heat (sensible + latent).

The SHR is used to calculate sensible cooling load for various environments, with those requiring more sensitive conditions resulting in a higher ratio.

Discomfort Index

The Discomfort Index is expressed as DI with one of two equations:

1) DI = 0.4 (td + tw) + 15, where td = dry bulb temperature and tw = wet bulb temperature OR

2) DI = 0.55 td + 0.2 tdp + 17.5, where td = dry bulb temperature and tdp = dew point temperature.

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