HVAC Engineer Pre-qualification - Answers

HVAC
1. What does H.V.A.C. stand for?
Heating, Ventilation & Air Conditioning

2. What three (3) specific conditions need to be kept constant in an air

conditioning system?
1. Temperature
2. Humidity
3. Pressure

3. List the benefits of HVAC electronic control.

1. Reduce energy costs by improving plant operating efficiencies.
2. Economical & worry-free control system & operating strategy.
3. Efficient & productive operating personel: diagnostics, analysis (e.g., trending),
response, tracability.
4. Plant delivers water/air temp. consistently and reliably; no unscheduled
equipment downtime.
5. Provides remote notification of system problems and alarms.
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6. Incorporate plant information into 3 -party programs.

4. Give two (2) examples of environmental temperature which affect thermal

comfort in heating?
1. Room air temperature
2. Radiation temperature (wall temperature)
reference: L&G Training doc.: Ventilating & Air-conditioning (introduction) (Basic
1
Training 2, E/50-010, pg. 5). P&C CD folder: Library\Training+Tests\Basic Training
2\AC Introduction\1-Comfort.

5. What are the two (2) main tasks of an air-conditioning system?

1. Air renewal (elimination or reduction of gases, vapours, odours)
2. Air treatment (filtering, blowing, heating, cooling, humification, dehumification)
reference: L&G Training doc.: Ventilating & Air-conditioning (introduction) (Basic
Training 2, E/50-010, pg. 13). P&C CD folder: Library\Training+Tests\Basic Training
2\AC Introduction\2-Plant Classification.

6. What four (4) thermodynamic processes can an air handler system control?
1. Cooling
2. Heating
3. Humification
4. Dehumification

7. When are ventilation or partial air conditioning systems required?

1. When the air in rooms is occupied by many people (e.g., theatres, assembly
halls, administrative and office buildings).
2. When strong odours are produced (e.g. restaurants).
3. When industrial processes cause pollution or require specific conditions (e.g.
humidity control in a textile testing lab).
The above air conditions can be renewed by simple forced air ventilation systems
(i.e., fan). Partial air conditioning systems provide ventilation and air treatment (i.e.,
filters, cooling, heating, and/or humidification). However, partial air conditioning plants

1
Project & Commissioning CD., Ver 4.2
 only treat the outside air. There are no return ducts from the zones back to the AC
plant.
reference: L&G Training doc.: Ventilating & Air-conditioning (introduction) (Basic
Training 2, E/50-010, pg. 10). P&C CD folder: Library\Training+Tests\Basic Training
2\AC Introduction\2-Plant Classification.

8. When is a full air conditioning system necessary?

When an air space requires 1) air renewal, 2) air treatment and 3) heat recovery. Air
renewal involves exhausting zone air and supplying fresh air. The task of air
treatment is to bring the air space temperature and humidity to the desired values and
to maintain them within the given tolerence independent of the conditions prevailing
inside and outside the zone.
Full air conditioning plants treat the total volume of air supply. Supply & Return air
ducts interconnect the AC plant and the zones. Returning zone air back to the AC
plant saves significant energy by reducing the need to treat 100% outside air (heat
recovery).
reference 1: L&G Training doc.: Ventilating & Air-conditioning (introduction) (Basic
Training 2, E/50-010, pg. 13). P&C CD folder: Library\Training+Tests\Basic Training
2\AC Introduction\2-Plant Classification.
reference 2: L&G Training doc.: Air-conditioning Plants (Basic Training 2, E/50-421,
pgs.6-8). P&C CD folder: Library\Training+Tests\Basic Training 2\AC Plant\1-Plant
Classification.

9. What is meant by “comfort”?

Comfort is characterized by the joint effect of the following components:
1. Room/Zone air temperature
Non-physical work (e.g., office activity):
Winter: 20-21C
Summer: 21-22C
Physical work: 10-18C
2. Radiation temperature (best comfort: wall temperature = room temp.)
3. Air humidity (35-65%rH)
4. Air velocity (0.15-0.25 m/s)
reference: L&G Training doc.: Ventilating & Air-conditioning (introduction) (Basic
Training 2, E/50-010, pg. 5). P&C CD folder: Library\Training+Tests\Basic Training
2\AC Introduction\1-Comfort.

10. What is the difference between wall surface/radiant temperature and room air
temperature?
Radiant temperature is the temperature of surfaces surrounding a room: walls, doors,
windows, ceiling, floor. Room air temperature is the temperature of the air.
Since surface temperatures are generally lower than the surface temperature of the
human body (25-27C), people will radiate heat to these colder surfaces. As a result
people will feel uncomfortable even though the air space temperature is normal (20-
22C).
reference: L&G Training doc.: Ventilating & Air-conditioning (introduction) (Basic
Training 2, E/50-010, pg. 5-6). P&C CD folder: Library\Training+Tests\Basic Training
2\AC Introduction\1-Comfort.

11. What is latent heat?

The amount of heat of moist or humid air. When the latent heat of an air space
changes only the water content changes (the process shown by the verticle line in the
psychometric chart).
12. What is sensible heat?
The amount of heat of dry air. When the sensible heat of an air space changes,
water is neither substracted nor added to it (the process shown by the horizontal line
in the psychometric chart).
reference: L&G Training doc.: Psychometric Chart & Conditions of Air (Basic Training
2, E/50-221, pgs. 1-12). P&C CD folder: Library\Training+Tests\Basic Training
2\Conditions of Air\ (sections 1 & 2).

13. How is heat dissipated?

By natural or forced ventilation. By cooling coil.
reference: L&G Training doc.: Ventilating & Air-conditioning (introduction) (Basic
Training 2, E/50-010, section 2). P&C CD folder: Library\Training+Tests\Basic
Training 2\AC Introduction\2-Plant Classification.

14. Explain how HVAC systems can protect building interiors? Site examples.
Temperature
If the temperature falls below freezing water pipes risk damage. Inventory material
and furnishings may be damage if the storage temperature limits are exceeded.

Humidity
If the humidity is too low (<35%) wood furniture risk cracking; carpets, furniture will
produce dust; plastics become electrically charged and pose a risk to electronic
devices.
If the humidity is too high (>70%) moisture in the air will condense on cold surfaces
causing moulding and mildew stains on furnishings containing organic matter.
reference: L&G Training doc.: Ventilating & Air-conditioning (introduction) (Basic
Training 2, E/50-010, pg. 5-6). P&C CD folder: Library\Training+Tests\Basic Training
2\AC Introduction\1-Comfort.

Ventilation System
Describe the following ventilation systems. Give example where they are used (e.g.,
hotel) and its purpose.
1. Extract Ventilation.
In extract ventilation air is sucked from a room by a fan and blown out to the
atmosphere either directly or through ducts. The air from the neighbouring rooms or
outside air enters the room through doors, windows and other openings. Also known
as a venting plant. Venting plants create an underpressure in the room which
prevents the infiltration of stale air into neighbouring rooms. Extraction ventilation is
used to ventilate areas in which the air is polluted by gases, vapours, odours or
rooms with high temperature. Examples include kitchens, bathrooms, laboratories,
transformer stations, battery rooms, paint stations, parking garages.

2. Supply Ventilation.
In supply ventilation air is suck from the outside and blown into the room. The air
leaves the room through doors, windows, leaks or other openings and enters the
neighbouring rooms or flows out to the atmosphere through a duct. Also known as a
pressure ventilation plant. Pressure plants create an overpressure in the room which
prevents the infiltration of undesirable air (e.g., dust, odours, germs) from
neighbouring rooms. Examples include aseptic rooms, clean rooms, office spaces.

3. Supply & Extract Ventilation

Also know as compound ventilation. Compound ventilation is required for large areas
where it is necessary to supply fresh air and to remove stale air. The output of the
supply and extract fans will be different only if an over- or under-pressure has to be
maintained in the room. Examples include meeting halls, theaters, cinemas,
restaurants.
4. Supply & extract ventilation with return air.
This is compound ventilation with a return air duct. The return air duct provides heat
recovery (limited use in the summer time).
reference: L&G Training doc.: Ventilating & Air-conditioning (introduction) (Basic
Training 2, E/50-010, section 2). P&C CD folder: Library\Training+Tests\Basic
Training 2\AC Introduction\2-Plant Classification.

5. Partial air conditioning system.

see HVAC, answer #7.

6. Partial air conditioning system with humidification.

see HVAC, answer #7.

7. Full air conditioning system.

see HVAC, answer #8.

Control & Interlock in HVAC systems

1. What is closed loop control? Site example.
Closed loop control uses feedback to assure the process condition equals the desired
condition (setpoint). One example is as we drive our car we detect that we are
drifting out of the center of the lane (setpoint). We make a slight adjustment of the
steering wheel (output), then observer the effect (feedback). If we don’t return to the
center lane, we make a further adjustment. Another example is controlling the
supply air temperature. The SAT temperature sensor connected to a temperature
controller forms a closed loop control system. If the outside air temperature (OAT)
suddently becomes colder the SAT controller detects the deviation from setpoint
(e.g., 16C) and opens the heating valve to increase the supply air temperature.

2. What is open loop control?

Open loop control does not use feedback to control a process.

3. What are the difference between Open loop & Closed loop control? Site
examples.
Feedback is the key difference. An example of open loop control is operating an
electric stove. The dial sets the approximate temperature, but there is no feedback
loop to correct for load changes or disturbances (typically not required). An example
of closed loop control is operating an electric oven. The dial sets the temperature
and a feedback temperature sensor and controller regulates the oven heater element
to precisely match the oven temperature to the dial. Any disturbances (e.g., opening
the oven door) will be dectected by the controller and corrected.

4. What is meant by “control process”?

A process is a series of operations directed toward a specific result. For an office
environment, an air conditioning process renews and treats the air and often provides
heat recovery. The result is comfortable working conditions for people with the least
amount of energy expended.
An automated system is used automatically control a process. The basic elements of
an automated control system consist of sensors, controllers and final drive elements
(e.g., actuators). Controlling a process falls into two (2) general categories:
1. Operating – maintain space condition (e.g., temp., humidity, pressure, etc.);
reduce energy (e.g., heat recovery, setpoint reset curve)
2. Safety – maintain equipment at safe levels (process limits).

5. A control system consists of: sensor, controller & controlled device. Define the
following control system elements:
a) Reference variables – variable parameters used in control strategies (e.g., reset
curve parameters). Reference variables are generally not physical variables (i.e.,
analog input); they reside in computer memory.
b) Setpoint – the value of the desired condition of a process (e.g., room temp.)
c) Measured value – the value of a physical variable (e.g., temperature).
 d) Control deviation – the difference between the PV (process variable) and the
setpoint.
e) Output signal – the control signal to the final drive element (e.g., damper)
f) Correcting range – the range of control deviation that governs 0-100% output.
Also known as the proportional band.
g) Controlled variable - process variable (PV). The value of a physical variable
(e.g., temperature) that represents the condition of a process in which a final
control element influences (e.g., cooling valve).

Control Equipment
Sensors
1. Give examples of temperature sensors in view of its construction.
• Duct Temperature Sensor:
1. temperature measuring element at tip of probe(resistive)
2. removable sensor probe for easy service/cleaning (unnecessary to dismantle
housing or disconnect wiring)
3. plastic pipe between housing and measuring element tip minimizes thermal
conduction (optimum measurement quality)
4. wiring terminal block
5. robust construction
reference: FK-T1K1 (L&S 1775)

• Wall Temperature Sensor:

1. base with terminal block; suitable for wall mounting on flush-mounting boxes
2. printed circuit board that includes temperature sensor + circuitry.
3. attractive cover
4. option for controls: setpoint adj., display, operation modes (e.g., occupied,
unoccupied, energy hold-off)
5. wiring terminal block
reference: FR-T1/A (L&S 1736); Construction & dimensions (L&S 1719)

• Immersion Temperature Sensor base with terminal block

1. immersion probe with temperature measuring element
2. stainless steel immersion pocket; able to service temp. element without
draining the hydraulic system.
reference: FT-T1K1 (L&S 1796)

Controllers
1. Controllers can regulate temperature, pressure, volume, humidity & water level.
Define the following:
a) Pneumatic controller.
a pneumatic controller uses air pressure as the signaling & controlling
medium (e.g., 3-15psi).
b) Electro-pneumatic controller
a electro-pheumatic controller uses electricity as the signaling medium (e.g.,
4-20mA) and pneumatic energy as the controlling medium (e.g., 3-15psi).
c) Electro-hydraulic controller
similar to the electro-pneumatic controller, but uses hydraulic energy as the
controlling medium.
d) Electro-magnetic controller
similar to the to electro-pheumatic controller, but uses magnetic energy as
the controlling medium.

2. What is the control action?

The output of a controller that drives the final control element (e.g, valve or damper
actuator, motor contactor).
3. Define ON/OFF controllers?
2-state control. The final control element is either turned ON (100%) or OFF (0%)
according to the control deviation and the controller action (e.g, direct or reverse
acting).

4. Define proportional controllers?

The controller output is proportional to the control deviation (difference of the process
variable and the setpoint).

Computers
1. Define a digital computer.
A digital computer performs calculations directly with binary digits rather than through
measurable physical quantities (i.e., analog computer).

2. Draw a block diagram of a microcomputer? Define the components.

RAM CPU ROM Memory

Memory

Driver/Controller
U.A.R.T. Parallel I/O
(floppy disk, H.D., printer)
D.A.C
Video Monitor

Driver Serial I/O

(K.B., mouse, remote I/O)

CPU – Central Processing Unit. Performs memory & I/O read/writes and executes
instructions per the control program (located in RAM memory).

RAM – Random Access Memory. Volitale memory; data will be lost if the power is
removed. Often known a “scratch pad” memory. Used to store the program,
operating system and temporary calculations.

ROM – Read Only Memory. Permanate memory; data will be saved if the power is
removed. Other ROM versions that are erasable: EPROM (UV erasable), EEPROM
(electrically erasable). Used to store operating system, boot program, BIOS (basic
input/output system)

U.A.R.T. – Universal Asynchronous Reciever Transmitter. Converts parallel data into

serial data.

Driver – Provides electrical drive for I/O interface.

Controller – Links CPU bus to hardware device.

D.A.C. – Digital-to-Analog Converter. Converts digital data to an analog signal.

3. What is meant ROM? RAM?

ROM - Read Only Memory; see question #2
RAM – Random Access Memory; see question #2.
4. What is DOS?
DOS = Disk Operating System.

5. What is the importance of DOS in a microcomputer? What role does it play?

DOS primarily manages the hardware resources of the computer:
1) processor
2) memory
3) storage (floppy disk, harddrive)
4) I/O devices (monitor, K.B.)
5) communication devices (i.e., modems)
6) data

6. Site examples of storage devices?

1. floppy diskette
2. harddrive
3. ZIP drive
4. CD-ROM
5. PCMCIA FlashDisk

Electrical
With reference to the attached electrical wiring diagram.
KF = fire alarm relay
KS = smoke alarm relay
S1 = common for aux. contact of damper
S2 = connected to S1 when damper deenergized
S3 = connected to S1 when damper energized

1. If power is applied to the transformer (T1) thru terminal block L1 & N (column 2)
what will happen? (assume no smoke or fire alarm).
1. 24VAC will appear on terminal 7 (column 11) which commands the damper to
open.
2. When the damper is open 24VAC will appear on terminal 10 (column 13) which
energizes relay K1.
3. Relay K1 turns on the supply fan.

2. If L.B. button (column 17) is pressed, what will happen?

1. Relay KL (column 17) is energized.
2. The Fire Alarm, Smoke Alarm & Damper Open/Close lamps turn ON. L.B. button
is for lamp test.

3. If terminals 1 & 2 of FACP are closed (column 5), what will happen?
After FACP is cleared, what must you do to restart the system?
1. 24VAC will appear on terminal 2 (column 5).
2. Relay KF (column 5) is energized & latched (if terminals 1 & 2 open, relay KF will
remain energized).
3. The Fire Alarm lamp will turn ON.
4. Terminal 8 (column 11) falls to 0VAC which causes the fresh air damper to close.
5. Terminal 10 (column 13) falls to 0VAC which causes relay K1 to de-energize.
6. Relay K1 turns OFF the supply fan.
7. To reset the system (after FACP is cleared) the R.B. button (column 14) must be
pressed.
4. If terminals of 5 & 6 of Smoke Detector are closed (column 9), what will
happen?
After the Smoke alarm is cleared, what must you do to restart the system?
1. 24VAC will appear on terminal 5 (column 9).
2. Relay KS (column 9) is energized and latched (if terminal 5 & 6 open, relay KS
will remain energized).
3. The Smoke Alarm lamp will turn ON.
4. Terminal 8 (column 11) falls to 0VAC which causes the fresh air damper to close.
5. Terminal 10 (column 13) falls to 0VAC which causes relay K1 to de-energize.
6. Relay K1 turns OFF the supply fan.
7. To reset the system (after the smoke detector is cleared) the R.B. button (column
14) must be pressed.

Electronics / Digital Logic Circuitry

1. Draw the following Logic gates:
a) NOT
b) AND
c) OR
d) NAND
e) NOR
f) XOR

2. Make a truth table of the above gates. Assume 2 inputs.

3. Define Boolean algebra? What is the difference between Boolean algebra and the
standard high school algebra?
Boolean algebra solves the logical result of 2-state (binary) combinational gates.
H.S. algebra uses symbols (as letters) to determine the relationship between
numbers (base 10).

4. Reduce the equation to simplest form by karnaugh maps or Boolean algebra:

ABCD + ABCD + ABCD + ABCD + AB CD + A BCD + A BC D + ABCD

5. With reference to the above logic gates, draw a diagram which produces a continous
square wave output.

6. Draw a logic diagram that will produce a single shot output. Assume an input from
OFF state to ON.