Sensible and Latent Heat

The distinction between dry air and the moisture content of the air and between dry-bulb and wet-bulb temperatures is extended to the two types of heat carried by the entering air and the air already contained in the space: sensible and latent heat.

Sensible Heat:  is the amount of heat in air that can be measured by an ordinary thermometer. The daily weather report gives us sensible heat temperatures, but it does not represent the total heat we experience. It constitutes a portion of the heat resulting from air infiltration, ventilation and internal heat sources such as people, electric lights and electric motors. Sensible heat also results form heat leakage or solar radiation.

Latent Heat: is the amount of heat contained in the water vapor (moisture) in the air. It constitutes a portion of the heat resulting from infiltration and ventilation and any internal sources capable of adding water vapor to the air. The amount of latent heat in the air can be determined by using a psychometric chart.  The amount of excess latent heat so determined will indicate the amount of moisture that must be removed from the air in order to obtain comfortable conditions.

Above is an example of a Psychrometric Chart

Both sensible and latent heat gain are expressed in Btu. When the total of the two are added together, their sum represents the total heat gain in Btu that must be removed from the air each hour by the air conditioner.

Sensible heat gain is represented by a change in the dry-bulb temperature readings, where as latent heat gain is represented by a change in the web-bulb temperature.

This post was published by Tucson air conditioning service but was written by James Brumbaugh in the book "HVAC Fundamentals: Air Conditioning, Heat Pumps and Distribution Pumps (Volume 3)"

Understanding Humidity in Air Conditioning

Humidity: is a general term used to refer to the water vapor (moisture) content of air. When this term is used, it is usually in reference to the sensation (or lack of) of moisture in the air. For purposes of heating and cooling conditions, the more narrowly defined terms of absolute humidy, relative humidity, and specific humidity are used.

Water vapor is actually steam at low termperatures and consequently, low pressure; hence its properties are those of steam. According to Dalton's law, in any mechanical mixture of gases each has has a partial pressure of its own, which is entirely independent of the partial pressures of the other gases of the mixture.

In all air conditioning calculations it should be understood that the dry air and water vapor composing the atmosphere are separate entities, each with its own characteristics. Water vapor is not dissolved in the air in the sense that it loses its own individuality and merely serves to moisten the air.

Cold air is saturated when it contains very small quantities of water vapor, wheras warm air is not saturated until it contains Fahrenheit is saturated when it contains but one-half of one grain when it contains 8 grains of vapor per cubic foot, while at 83 degrees, 12 grains per cubic foot are required to saturate.

Absolute Humidity: is the actual mass of water vapor in one cubic foot of air (the weight of water vapor per unit volume of air) and is experessed in grains or pounds per cubic food (1 lb = 7000 grains) or grams per cubic centimeter. Absolute humidity is equivalent to the dentistry of the air.

Specific Humidity: is the weight of water vapor per pound of dry air. Do not confuse specific humidity with relative humidity. Ther latter term indicates the percentage of water vapor, the former the weight.

Relative Humidity: is the ratio of absolute humidity to the maximum possible density of water vapor on the air at the same temperature. In other words, it is a percentage or ratio of water vapor in the mixture of dry air and water vapor at a certain termperature relatvie to the maximum quantity that the volume of air could possibly carry at that temperature. The relative humidity at any given temperature can be ovtained by first using a sling psychromter to determine the amount of moise actually present in the air and then dividing this figure by the amount of moisture that the air can hold at the temperature, and multiplying the result by 100 in order to obtain the percentage factor.

Drying Effect of Air: varies approximately inversely with its relative humidity. In other words, the drying effect decreases as the relative humidity increases. It should be noted that it is relative humidity that determines the drying effect of air and this effect depends both on the temperature and the water content of the air since relative humidity depends on both these factors.

Dew Point: is the temperature of saturation for a given atmospheric pressure. In other words, for a given atmospheric pressure, it is the temperature at which moisture behins to condense in the form of tiny droplets or dew.

Humidification: may be defined as the addition of moisture to the air. The conditioning machine that functions to add moisture to the air is called a humidifier. A humidifier is commondly a low pressure, low-temperature boiler in which the water is avaporated and vapor thus formed is caused to mixed with air.

As an Tucson air conditoining technician, humidity plays into a very small part of our job as the climate in southern Arizona has a low humidity. Our air conditioners can have more effect on our dry air than they could in a more humid climate such as Kansas or Minnesota. Our air conditioners do not have to work as hard. Today for instance, our humidity is 18% in Tucson and in Kansas it is 42%.

(Article content was taken from James Brumbaugh's book "HVAC Fundamentals: Air conditioning, Heat Pumps, and Distribution Systems (Volume 3)"

Preventing Ladder Accidents Video

Here is a video about how ladder safety by Elvis Benoit. He talks about properly placing the ladder to the roof and how to get on and off the ladder. Ladder safety is no joke! Statistics show that most falls off of a roof (a fall of eleven feet or more) 50 percent of victims will die.

The Consumer Product Safety Commission reports that more than 90,000 people received emergency room treatment for ladder accidents in 2010. With these high numbers of ladder accidents, we could all use a ladder safety refresher.



Whether you are cleaning your gutters, trying to do your own roof repairs or checking out the work done by your Highlands Ranch roofers, you need to remember these ladder safety tips.