Heating, Ventilation, and Air Conditioning Flashcards
Heating Load
A buildings heating load is the maximum heat loss during the heating season. It consists of heat to make up for transmission and infiltration losses.
Maximum Heating Load
The maximum heating load occurs when the outside temperature is the lowest. It corresponds to the minimum furnace size, even though the lowest temperature occurs only a few times each year.
Average Heating Load
The average heating load is derived from the maximum heating load and is used to determine the annual fuel requirements.
Transmission Loss
Transmission loss is the heat lost through the walls, roof and floor.
Infiltration Loss
Infiltration loss is the heat required to warm ventilation and infiltration air.
Infiltration Air
Infiltration air is the leakage of outside air into the air conditioned space by means of cracks, windows, doors and other openings.
Design Conditions
Inside design temperatures for residences and office spaces range between 21.1 to 22.2 degrees Celsius.
Total Resistance
Is the total resistance to heat flow through the entire thickness of the material. It is the product of the resistivity and the material thickness. Conductance is the reciprocal of the total resistance.
Unit Resistance
The resistance per unit thickness of the material. Conductivity is the reciprocal of the unit resistance.
Overall Coefficient of Heat Transfer
Can be calculated for each transmission path from the conductivities and resistances of the individual components in that path, or it can be obtained from tabulations of typical wall/ceiling construction.
Ventilation Air
The new air that is deliberately drawn in from the outside and mixed with return air, to meet the necessary needs of the occupants in the conditioned space. It is provided to the occupied space primarily to remove heat and moisture generated in the space.
Humidification
Ventilation provides humidification to the occupied space. It can be provided by evaporating water in the occupied space or by injecting water or steam into the duct flow.
Metabolic Heat
Contains both sensible and latent heat
Sensible Heat
the pure thermal energy that increases the air’s dry bulb temperature.
Latent Heat
Moisture that increases the air’s humidity ratio.
Ventilation for Heat Removal
Ventilation requirements can be calculated from sensible heat and/or latent heat (moisture generation) rates. The sensible heat loads will usually be more significant than the latent load, and ventilation will be determined solely on that basis. When large moisture sources are present, however, the latent loads may control.
All Air Conditioning System
These systems maintain the temperature by distributing only air, and most systems rely on internal loads for heating, sending only cold air to the space.
Air and Water Conditioning System
Air and water are both distributed to the conditioned space.
All Water Conditioning System
The cooling and heating effects are provided solely by cooled and/or heated water pumped to the conditioned space.
Unitary Conditioning System
The fan, condenser, and cooling and heating coils are combined in a standalone unit for window and through the wall installation.
Cooling Load
The aspects of determining the cooling load is similar in some respects to the procedure of finding the heating load. The aspects of determining inside and outside air conditions, heat transfer from adjacent spaces, ventilation air requirements and internal heat gains are the same as for heating load calculations. However the cooling load is complicated considerably by many factors and its important to distinguish these factors into three terms, instantaneous heat absorption, instantaneous heat gain, and instantaneous cooling load.
Instantaneous Heat Absorption
Is the solar energy that is absorbed at a particular moment.
Instantaneous Heat Gain
Is the energy that enters the conditioned space at that moment. Due to solar lag, the heat gain is a complex combination of heat absorptions from previous hours
Instantaneous Cooling Load
Is essentially the convection portion of the instantaneous heat gain.
Source of Cooling
Once the cooling load is determined, the source and size of the cooling unit must be considered. Cooling normally comes from liquefied refrigerant passing through a cooling coil. Some or all of the airflow passes across the cooling coil. Alternatively, cold water may be used in the coil to cool the airflow. In such cases the water is cooled in a chiller running its own refrigeration cycle. The use of outside air in economizers is also another method and that air may still be conditioned by passing through coils, but less change will be required.
Economizer
An economizer is an electromechanical system that changes a portion of the cooling process in order to decrease cost, usually by taking advantage of cold ambient air.
Water Side Economizer
A water side economizer substitutes natural cooling from a cooling tower for the chiller’s more expensive refrigeration cycle when the ambient air temperature drops below the desired coil temperature.
Air Side Economizer
An air side economizer increases the amount of outside air that is brought into a space when the ambient air characteristics (temperature, humidity, or enthalpy) are better than the return airflow.
Instantaneous Cooling Load (Walls & Roof)
For exterior surfaces, the cooling load is calculated using values of CLTD. These values are dependent on the time of year, location and orientation, type, configuration, orientation of the surface, as well as other factors and are calculated using base conditions of these factors. Using the CLTD/SCL/CFL method, the instantaneous cooling load for conduction through opaque walls and roofs uses a corrected CLTD value. This occurs because base conditions generally don’t coincide precisely with actual conditions during the study period, so CLTD is corrected according to the indoor design temperature and mean outdoor temperature.
Instantaneous Cooling Load (Windows)
Using the CLTD/SCL/CLF method, the cooling load due to solar energy received through windows is calculated in two parts. The first is an immediate conductive part; the second is a radiant part. Appropriate tables are needed to evaluate the shading coefficient (SC) and the solar cooling load factor (SCL) for the radiant portions.
Cooling Load (Internal Heat Sources)
Latent Loads (including metabolic latent loads) are considered instantaneous cooling loads. Only a portion, given by the CLF, of the sensible heat sources show up as instantaneous cooling load. The CLF is a function of time, and depends on zone type, occupancy period, interior and exterior shading, and other factors. Although tables are necessary to evaluate the CLF, there are cases where the CLF is assumed to be 1.0. These include when the cooling system is shut down during the night, and when there is high occupant density, and when lights and other sources are operated for 24 hours a day.
Latent Loads
Increase cooling loads
Sensible Heat Ratio or Sensible Heat Factor
Is the sensible heat load divided by the total load (Sum of latent and sensible loads). Most air conditioning equipment is designed to operate at a sensible heat ratio in the range of 0.70 to 0.75. A sensible heat ratio of 0.77 matches the performance of typical residential vapor compression cooling systems.
Latent Factor
The latent factor is the reciprocal of the sensible heat ratio. A latent factor of 1.3 matches the performance of typical residential vapor compression cooling systems.
