Mechanical Systems Flashcards
British thermal unit (Btu)
the amount of heat required to raise the temperature of 1 lbm water by 1 degree F
U (coefficient of heat transmission)
the overall rate of heat flow through any combination of materials, including air spaces and air layers on the interior and exterior of a building assembly;; it is the reciprocal of the sum of all the resistances in the building assembly
C (conductance)
the number of BTUs per hour that pass through 1 ft^2 of homogeneous material of a given thickness when the temperature differential is 1 degree F
k (conductivity)
the number of BTUs per hour that pass through 1 ft^2 of homogeneous material 1 inch thick when the temperature differential is 1 degree F
dew point
the temperature at which water vapor in the air becomes saturated and begins to condense into drops of water
dry-bulb temperature
the temperature of the air-water mixture as measured with a standard dry-bulb thermometer
enthalpy
the total heat in a substance, including latent heat and sensible heat
latent heat
heat that causes a change of state of a substance, such as the heat required to change water into steam;; the amount of heat required to change the state of a substance is much greater than the heat required to raise the temperature of the substance (sensible heat). The average value of latent heat per pound of moisture is 1061 Btu.
resistance
the number of hours needed for 1 Btu to pass through 1 ft^2 of material or assembly of a given thickness when the temperature differential is 1 degree F;; it is the reciprocal of conductance.
sensible heat
heat that causes a change in temperature of a substance but not a change of state;; the sensible heat needed to raise the temperature of 1 lbm of water from 50 deg F to 100 deg F is 50 Btu. In contrast, the latent heat needed to change liquid water at 212 deg F to steam at 212 deg F is 1061 Btu.
specific heat
the number of BTUs required to raise the temperature of a specific material by 1 deg F;; specific heat is a measure of a material’s capacity to store heat as compared with the storage capacity of water.
wet-bulb temperature
the temperature of the air as measured with a sling psychrometer;; the wet-bulb temperature is a more critical measure of heat in high humidity because it is an indicator of physical stress caused when the human body is near the upper limits of temperature regulation by perspiration.
mean radiant temperature (MRT)
the uniform temperature of an imaginary surrounding enclosure in which radiant transfer from the human body would equal the radiant heat transfer in the actual nonuniform enclosure;; MRT is a calculated variable and cannot be directly measured
cross-ventilation vs. stack ventilation
Natural ventilation. Cross-ventilation is driven by windows and relies on narrow plans with large openings on either side; it is naturally compatible with daylighting. Stack ventilation depends on very low openings to admit outside air and high openings to exhaust it; it’s generally weaker than cross-ventilation except when there is no wind.
actuator
a device in a building control system that receives commands from a controller and activates a piece of equipment
annual fuel utilization efficiency (AFUE)
the ratio of annual fuel output energy to annual input energy;; this includes nonseasonal pilot light input losses
coefficient of performance (COP)
a unites number that is a rating of the efficiency of heating or cooling equipment;; it is derived by dividing the steady-state rate of energy output (or the rate of heat removal, in the case of cooling equipment) of the equipment by the steady-state rate of energy input to the equipment. The output and input values must be in equivalent units, such as watts out to watts in.
controller
a device that measures, analyzes, and initiates actions in a building control system
deadband
in a building control system, the range of temperatures within which neither heating nor cooling is needed
energy efficiency ratio (EER)
the ratio of net cooling capacity in Btu/hr to the total rate of electrical input in watts under designated operating conditions
energy management system (EMS)
a computer- based system used to monitor and control facility energy use;; an EMS is typically part of a building automation system
ground-coupled cooling
a method of cooling a building by direct contact with the earth or by circulating air through underground tunnels to cool it
heating seasonal performance factor (HSPF)
a measure of the performance of a heat pump operating in the heating cycle
home energy rating system (HERS)
a standardized system for rating the energy efficiency of residential buildings using the HERS Council Guidelines and the Mortgage Industry HERS Accreditation Procedures;; a HERS score is a numeric value between 0 and 100 indicating the relative energy efficiency of a given home as compared with the HERS Energy-Efficient Reference Home.
