Today I'm going to discuss two aspects of Comfort from heating and cooling systems Temperature and Humidity. This is the first of a series of articles on this subject.

 

Heating, ventilating, and air conditioning (HVAC) systems used in residential, commercial, and industrial buildings provide comfort to occupants. A feeling of comfort results when temperature, humidity, circulation, filtration, and ventilation of the air are controlled. HVAC systems are designed to fulfill comfort requirements with maximum efficiency.

HVAC systems contain mechanical, electrical, and chemical components. Mechanical components include all moving parts. Electrical components include blowers, compressors, and controls. Chemical components include fuels and refrigerants. Precautions should be taken and safety rules followed when working with HVAC systems.

COMFORT

Comfort is the condition that occurs when a person cannot sense a difference between themselves and the surrounding air. Comfort occurs when no differences exist or undesirable conditions have been corrected. The five requirements for comfort are proper temperature, humidity, circulation, filtration, and ventilation. See Figure 1-1. Discomfort is the condition that occurs when a person can sense a difference between themselves and the surrounding air. It can occur when any of the five requirements for comfort are not met.

Temperature

Controlling the temperature of the human body is an important physiological function. Physiological functions are the natural physical and chemical functions of an organism. The body produces energy by digesting food. Some energy is used for normal living processes, some is stored, and some is used as thermal energy (heat). Physiological systems regulate body temperature to maintain comfort.

Normal body temperature is 98.6°F. The body has natural heating and cooling systems to maintain this temperature. These systems control heat output by responding to the conditions of the air according to the internal temperature of the body. The body responds by controlling blood flow at the surface of the skin, radiating heat from body surfaces, or using evaporation of perspiration from skin. Evaporation is the process that occurs when a liquid changes to a vapor by absorbing heat. When the body is clothed, the body's temperature control system provides comfort at an air temperature of approximately 75°F. If the air temperature varies much above or below 75°F, the body begins to feel uncomfortably warm or uncomfortably cool.

Signals (electrical impulses) from different points in the body are sent through a network of nerves to the hypothalamus, a gland in the brain. The hypothalamus regulates body temperature by controlling blood flow to capillaries (tiny blood vessels) located in the skin. Capillaries regulate perspiration flow to the surface of the skin. If the body temperature rises, blood flow to the skin increases. Blood carries heat to the skin, where it is given off to the air. A person who becomes overheated also becomes flushed because the blood flow to the skin increases. If signals indicate the body is cooling off, the system reduces the cooling effect by allowing less blood flow to the skin. In cold weather fingers and toes become cold before the rest of the body because blood flow to these areas is reduced. Cold areas also appear pale because of the reduced blood flow. Temperature control in buildings is provided by warm air from heating equipment or cool air from air conditioning equipment. Heating equipment supplies the proper amount of heat in cold weather to offset heat loss from a building.

Heating equipment is rated in Btu per hour (Btu/hr.), A Btu (British thermal unit) is the quantity of heat required to raise the temperature of 1 lb. of water 1°F. Air conditioning equipment supplies the proper amount of cooling in hot weather to offset heat gain to a building. Air conditioning equipment is rated in Btu per hour or ton of cooling. A ton of cooling is the amount of heat required to melt a ton of ice (2000 lb.) over a 24-hour period. One ton of cooling equals 288,000 Btu per 24-hour period or 12,000 Btu/hr. (12,000 x 24 = 288,000).

The operation of heating and air conditioning equipment is a function of temperature control in the HVAC system. For maximum comfort, temperature control equipment maintains air temperature in a building within 1°F or 2°F of the temperature necessary for comfort.

Humidity

Humidity is the amount of moisture (water vapor) in air. Humidity is always present in air. A low humidity level indicates dry air that contains little moisture. A high humidity level indicates damp air that contains a significant amount of moisture.

Relative Humidity. Relative humidity is the amount of moisture in air compared to the amount of moisture the air would hold at the same temperature if it were saturated (full of water). Relative humidity is always expressed as a percentage.  For example, air at 50% of relative humidity holds one-half of the amount of moisture it would hold at the same temperature if it were saturated.  Air at 60% relative humidity holds 60% of the amount of moisture it would hold at the same temperature if it were saturated.  The amount of moisture required to saturate the air changes as the dry bulb temperature changes.  In addition, the relative humidity and the capacity to hold moisture change as the dry bulb temperature changes.  Humidity is important in determining comfort.

Humidity affects comfort because it determines how slowly or rapidly perspiration evaporates from the body. The flow of perspiration is controlled by the cooling system in the body, which regulates body temperature. Evaporation of perspiration cools the body. The higher the relative humidity, the slower the evaporation rate. The lower the relative humidity, the faster the evaporation rate. For example, with no temperature change and an increase in relative humidity, a person feels warmer because of the slower evaporation rate. With no temperature change and a decrease in relative humidity, a person feels cooler because of the faster evaporation rate. A steam room is an example of an area with high temperature and high relative humidity. These conditions cause the body to have a high perspiration rate with a slow evaporation rate. A steam room is an Heating, ventilating, and air conditioning (HVAC) systems used in residential, commercial, and industrial buildings provide comfort to occupants. A feeling of comfort results when temperature, humidity, circulation, filtration, and ventilation of the air are controlled. HVAC systems are designed to fulfill comfort requirements with maximum efficiency.