Heating, Ventilating and Air Conditioning:

Introduction
Text book
Jacob Perkins ice making machine
Schematic and T-s diagram for the ideal vapor
compression refrigeration cycle
Schematic and T-s diagram for the actual vapor
compression refrigeration cycle
Ammonia
absorption
refrigeration
cycle
The objective of a refrigerator is to remove heat
(QL) from the cold medium.

The objective of a heat pump is to supply heat

(QH) to a warm medium.
AIR CONDITIONING

 Improved human comfort: healthier, more productive lives

 Goods can be produced better, faster and more economically

 Earlier, was limited to cooling / improving indoor environment

during warm months

 Now it refers to controlling temperature, moisture content,

cleanliness, air quality and air circulation as required by occupants, a
process or a product in the space
ASHRAE: American Society of Heating, Refrigerating and Air-Conditioning
Engineers, Incorporated

ASHRAE Handbook:

 Fundamentals
 Refrigeration
 HVAC Systems and Equipment
 HVAC Applications
ENERGY vs POWER
 Rate at which energy is produced or consumed.

 The electrical power (kW) required by and HVAC system or component

depends on size (capacity or load or demand).

 Energy (kW-hr) used by an HVAC system depends on:

 size,
 fraction of capacity or load at which it is operating and
 the amount of time that it runs.

 Cost of running HVAC systems is often the largest part of the utility bills for a
building.
HEATING
 Transfer of energy to a space or to the air in a space by temperature
difference between the source and the space or air

 To bring a space up to a higher temperature than previously existed, or

 To replace energy being lost to colder surroundings by a space

Achieved by:
 Direct radiation, or
 Free convection, or
 Direct heating of forced circulated air, or
 Transfer of electricity/heated water to the vicinity of the space to heat the
circulated air
The flow of energy in space heating
Method of space heating
 Transfer and diffuse warm air into space, mixing with cooler air
 Equal amount of mixed air removed from space helping carry away some
pollutants
 Some of it exhausted, some mixed with outside air, brought back to heating
device

 Air provides both energy and ventilation: all-air system

 Coil placement:
 in ductwork, or
 in conditioned space or
 in air handler kept in central mechanical room
Sensible heat transfer: Heat transfer resulting in a rise in air temperature

Rate of sensible heat transfer:

Q
q s  m c p (te  ti )  c p (te  ti )
v

Specific volume and volume flow rate are specified at the inlet conditions

Specific heat of air is assumed to be an average value

Change of temperature (K) and state of water with enthalpy
HUMIDIFYING

 Humidification: transfer of water vapor to atmospheric air

 Is accompanied by heat transfer
 Concentration of water in the air-water vapor mixture increases

Accomplished by:
 Adding water vapor to the circulating air stream, or
 Spraying fine droplets of water that evaporate into the circulating air
stream
 Latent heat transfer:
Heat transfer not resulting in a rise in air temperature

Rate of latent heat transfer:

ql  h fg m w
h fg Enthalpy of vaporization J/kg

m w Rate at which water is vaporized kg/s

COOLING

 Transfer of energy from a space or air supplied to a space to make up

for the energy being gained by the space

Accomplished by:
 Circulating air over a surface maintained at a low temperature
 Water/volatile refrigerant is the cooling medium

 Usually signifies sensible heat transfer

The flow of energy in space cooling
Air handler of the draw-through type with cooling and heating coils in series
A blow-through air handler A draw-through air handler
Air handler of the blow-through type with cooling and heating coils in parallel
DEHUMIDIFYING

 Dehumidification: transfer of water vapor from atmospheric air

 Concentration of water in the air-water vapor mixture decreases
 Latent cooling: energy involved in moisture removal only

Accomplished by:
 Circulating air over a surface maintained at a sufficiently low
temperature to cause condensation of water vapor, or
 Spraying cold water into the air stream
CLEANING

 Cleaning of air by filtering:

 Solid particles captured in a porous medium
 Electrostatic cleaners to remover very small particles
 Water sprays may also be used

 Removing contaminant gases from air

 By absorption or physical adsorption
AIR MOTION

 Should be strong enough to create uniform comfort conditions in the

space but should be gentle enough to be unnoticed

Achieved by:
 Proper placement of air inlets to the space
 By using air-distributing devices
SEASONAL OPERATION

 Winter: cooling and dehumidifying sections are inactive

 Summers: heating and humidifying sections are inactive

 In large commercial installations all functions may be under control

throughout the year

 Cleaning of air and air circulation are used continuously except when
the space is not occupied
CONTROLS AND INSTRUMENTATION
 Most of the time, HVAC systems operate at part load conditions
 The loads vary with time, so controls are required to modulate the output

 Controls: pneumatic, electric, electronic or self-contained (no external

power required)

 Some HVAC systems have combination systems

 DDC: direct digital control

CONTROLS AND INSTRUMENTATION

Better control results in:

 Additional monitoring capability

 Energy management systems (EMS)
 Building automation systems (BAS)
 Better determination of unsafe operating conditions
 Better control of the spread of contamination or fire
 Minimized human intervention in system operation results in reduced
possibility of human error
CONTROLS AND INSTRUMENTATION

For interoperability among different vendors’ products using a computer

network, there must be a set of rules (protocol) for data exchange:

 BACnet: Building Automation and Control networks”: developed by ASHRAE

 Other “open” protocols: LonMark and ModBus

 Open networks: HVAC networks designed to permit the use of components

from a wide variety of manufacturers.

 Gateway: a device needed between two systems operating on different

protocols to allow them to communicate
Closed-loop system: the change in the
controlled device (the control valve) results
in a change in the downstream air
temperature (the controlled variable),
which in turn is detected by the sensor.

Feedback: the process by which the change

in output is sensed.

Open-loop or feedforward system: the

sensor is not directly affected by the action
of the controlled device.
Elementary air-temperature control system
 Two-position (or on-off) control action

To prevent rapid cycling, there must be a difference between the setting at which the
controller changes to one position and the setting at which it changes to the other.

In some instances time delay may be necessary to avoid rapid cycling.

The difference between the actual operating differential and the set, or control
differential can be reduced by artificially shortening the on or off time in anticipation
of the system response.

For example, a thermostat in the heating mode may have a small internal heater
activated during the on period, causing the off signal to occur sooner than it would
otherwise.

With this device installed, the thermostat is said to have an anticipator or heat
anticipation.
Floating control action
Modulating control action
Typical equipment characteristic for thermostat control of room temperature
A stable system under proportional control
An unstable system under proportional control