Air conditioners often display both the total cooling capacity and the sensible cooling capacity. Many people wonder what is sensible cooling capacity so am I. Hence, I did a little study.
Sensible cooling capacity determines the temperature reduction capability of the air conditioner. Usually, it is about 80% of the total cooling capacity of the air conditioner. Meanwhile, the remaining is the latent cooling capacity which determines the moisture extraction capability of air conditioners.
Sensible heat and latent heat are basic but technical subjects in HVAC. Sometimes, we can understand better by looking at examples used in HVAC.
What is Sensible Heat and Latent Heat?
To understand why the cooling capacity of air conditioners is made of two different types of cooling capacities, we need to understand what are sensible heat and latent heat.
As the name suggests, sensible heat is the heat that we can feel or able to sense (sense-able). Conventionally, it is measured as temperature.
The greater the sensible heat, the higher the temperature.
In other words, the greater the sensible heat in a room, the higher the sensible cooling capacity is needed for air conditioners to cool the room appropriately.
Therefore, it is a must for air conditioners to have sensible cooling capacity but not always the case for latent cooling capacity.
One of the synonyms for the word latent is hidden. Therefore, latent heat can also be read as hidden heat. As the name suggests, it is the heat that we can’t feel or sense.
When we boil a bowl of water to its boiling point, the temperature of the water stop raising. It is because heat energy is used to change the water from liquid form to gas form and that heat energy is latent heat.
Conversely, if we want to extract moisture from the air, meaning to change the water from gas form back to liquid form, we need to absorb a sufficient amount of the latent heat from the water.
Latent heat can be further explained by a typical PH diagram that represents the refrigeration cycle of air conditioners as below:
If you look closely, the pressure of the refrigerant remains constant during the process of condensation and evaporation. Since pressure is directly proportional to temperature, the temperature of the refrigerant also remains constant during the two processes.
Again, this is because the refrigerant is changing from gas form to liquid form during condensation and the opposite during evaporation. Although heat energy is given during the two processes, the energy is used for phase-change rather than raising the temperature of the refrigerant.
Air conditioners have a certain percentage of latent cooling capacity which represents their latent heat absorption capability or dehumidification capability.
In certain applications, air conditioners don’t require to have latent cooling capacity because dehumidification is not required. Hence, the cooling capacity of the air conditioners is 100% for sensible heat.
As you might already know by now, the higher the moisture content in the air, the greater the amount of latent heat is needed to be removed for proper dehumidification.
In other words, the higher the humidity level, the higher the latent cooling capacity is needed to ensure that the air conditioner can bring the humidity level down to appropriate levels.
Examples of Sensible Heat and Latent Heat in HVAC
Sensible heat is always present in HVAC but not latent heat. If dehumidification is not needed, air conditioners only need to deal with sensible heat.
Precision Air Conditioning
Precision air conditioners are sometimes known as data centre air conditioners or server room air conditioners. They deliver precise temperature control for temperature-sensitive hardware.
In a typical server room, there are two types of air conditioning systems; a) precision air conditioning system and b) comfort air conditioning system.
Comfort air conditioning system is the normal air conditioning system we used in homes such as the split system, VRF system and central air conditioning system. They have both the sensible cooling capacity as well as the latent cooling capacity.
Many server rooms use split air conditioners to control the temperature and humidity within the rooms. What’s the difference is the total cooling capacity of the split air conditioners is much lower than conventional residential or commercial applications because the majority of the sensible load is handled by precision air conditioners.
On the other hand, precision air conditioners are usually installed right below the server hardware. They are usually small in cooling capacity due to the absence of latent load because computer hardware doesn’t produce any moisture. So, precision air conditioners have 100% sensible cooling capacity.
When air conditioners operate in dry mode, it is said to reduce power consumption because the air conditioners are concentrating on latent heat absorption (dehumidification) which uses a smaller portion of the air conditioner’s total cooling capacity.
Dehumidifiers fundamentally work the same as air conditioners except their capacity is rated for the latent load. In other words, dehumidifiers have 100% latent cooling capacity.
Evaporative coolers require us to pour water once in a while for cooling. They use evaporative cooling to reduce the room temperature in exchange for a higher level of humidity. Since evaporative coolers don’t extract moisture from the air, we can say that they are having 100% sensible cooling capacity.
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Infiltration or the introduction of unwanted outdoor air into a building is often one of the causes of high humidity issues and thus, mold and fungus growth issues.
Although the introduced outdoor air increases both the sensible load and latent load, air conditioners tend to have more difficulties in dealing with latent load because most of them only have 20% of their total cooling capacity as latent cooling capacity.
Cooling Load Calculation
Many cooling load calculations involve the separation of sensible and latent heat. For instance, the following elements involve only the sensible heat:
- Solar heat gain from windows and glasses
- Adjacent heat gain from doors and walls
- Heat gain from floors and ceilings
- Heat gain from electrical appliances
Also, one of the cooling load calculation methods suggested by ASHRAE is multiplying the total sensible load by a latent load factor to account for latent heat.
In addition, some cooling load calculations use an additional 400 btu/hr per person to size the cooling capacity of air conditioners. Such an additional capacity is already accounted for both the sensible and latent heat.
Generally speaking, any object, hardware or tools that generate heat as a result of their operation, the heat generated is sensible heat. If the heat is produced by humans or other living creatures, it is a combination of sensible and latent heat.
Outdoor air contains moisture. Hence, if unwanted outdoor air is introduced into a building, both the sensible load and latent load are increased.
Keep in mind that latent load is harder to deal with because most air conditioners only have 20% of their total cooling capacity as latent cooling capacity.
Also, if you are thinking of increasing the total cooling capacity and hence, the latent cooling capacity to improve the humidity level, be careful not to be overly done on the sensible cooling capacity.
Lastly, if air conditioners are required to reduce the humidity level, they must have latent cooling capacity. Otherwise, they are fine with 100% sensible cooling capacity.
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