Unit 9 - Chapter 3 - Refrigeration System
Control and Operation Exam Questions
and Answers
Human Machine Interface (HMI) - Answer-Designed to allow operators to receive
operational data and alarms on a screen or a series of screens allowing to make
changes to set points or operational expectations.
Refrigeration plan capacity must always equal the __________. - Answer-Cooling Load.
Control systems respond to: - Answer-Temp, Pressure and Humidity.
Temperature-Actuated Control (Thermostat) - Answer-Electric controls can be used for
starting and stopping a compressor. A thermostat is placed to detect the refrigerated
medium. Deviations from set point operate electrical contacts that start or stop the
compressor.
Bimetal Thermostat - Answer-Can directly control the power supply to small compressor
motors. Larger motors with high current draw are indirectly controlled by a thermostat
that actuates a magnetic motor started. Can control the operation of an unloader
actuator
Filled System Remote Bulb Thermometer - Answer-When equipped with a switch can
directly control the compressor motor power supply. Once temp goes above set point,
thermometer expands and bellows expands. Bellows closes thermostat switch and
compressor starts.
What does the thermostat have to keep compressor from short cycling? - Answer-Cut in
is set a little lower than set point and cut out is set a little higher than set point. This is
called temp differential-difference between cut in and cut out temps.
Pressure Actuated Control for a compression refrigeration system? - Answer-Connected
to the suction line of the compressor which starts and stops it. Operates opposite to
thermostat. Opens when pressure goes below cut off setting and closes it when the
pressure rises to a preset differential. Uses bellows.
When can't suction pressure-actuated control be used? - Answer-When an automatic
expansion valve is used as a metering device. Auto expansion valves maintain a
constant evaporator pressure. Compressor would not cycle off. Nor can it be used with
a capillary tube or orifice-type metering device, since they do not prevent liquid
refrigerant flow from the high to low sides of the system when the compressor shuts
down. Compressor would start and stop too often causing mechanical failure and use
too much power.
, Humidity-Acutated Control (Humidistat) - Answer-Can be used both for humidity
measurement as well as starting and stopping refrigerant flow to an evaporator. Liquid
solenoid valve, coupled with a metering device would be used in conjunction with the
humidistat.
Low Chilled Water Temp Cut off Switch - Answer-Consists of a thermostat senses temp
of chilled water leaving the chiller. If water temp too low may freeze in evaporator. Temp
drops below 2-3 C, compressor will stop, when temp reaches 5-6 C compressor starts.
Chilled Water Flow Switch - Answer-Installed in the chilled water outlet line. Protects the
chiller from freezing due to low water flow. Opens compressor when water flow drops
below the safe minimum flow. Prevents compressor from starting if flow has not been
established.
Low Oil Pressure Cut-off - Answer-Prevents compressor from operating under low lube
oil pressure. Prevents compressor damage. Usually requires a manual reset.
Condenser High Pressure Cut-off - Answer-High limit switch which shuts down
compressor when condenser reaches an excessively high value. Manual reset.
Inadequate cooling water or airflow, or condenser fowling, is usually the cause of the
trip.
Refrigerant Low Temp Cut-off - Answer-Senses temp of refrigerant in chiller. If temp
drops low enough to freeze water, switch opens and shuts down the compressor.
Demand Limiter - Answer-Limits the max current flow to the compressor drive motor.
Overrides the water temp sensors if the motor load reaches the max amperage setting.
Motor Demand Limiter - Answer-Limit the current flow when machine first starts. Can be
adjusted to 40% until water brought down to temp. Increasing demand limiter in steps
through 60,80,100% helps reduce the max electrical demand and possibility of motor
damage due to high current flow.
Compressor Vibration Shutdown - Answer-If vibration on any bearing reaches a certain
set point, an alarm sounds to warn the operator of a vibration problem. If vibration
increases compressor will shut down at set point.
Suction Scrubber High Level Shutdown - Answer-If liquid level rises to a certain level,
an alarm will sound to alert the operator of a high level in scrubber. If level continues to
rise, a float switch will shut down the compressor to keep liquid from getting in the
compressor.
High Bearing Temp Shutdown - Answer-If temp rises to a certain point, a high bearing
temp alarm is activated. This allows operators to either take action to reduce temp or to
Control and Operation Exam Questions
and Answers
Human Machine Interface (HMI) - Answer-Designed to allow operators to receive
operational data and alarms on a screen or a series of screens allowing to make
changes to set points or operational expectations.
Refrigeration plan capacity must always equal the __________. - Answer-Cooling Load.
Control systems respond to: - Answer-Temp, Pressure and Humidity.
Temperature-Actuated Control (Thermostat) - Answer-Electric controls can be used for
starting and stopping a compressor. A thermostat is placed to detect the refrigerated
medium. Deviations from set point operate electrical contacts that start or stop the
compressor.
Bimetal Thermostat - Answer-Can directly control the power supply to small compressor
motors. Larger motors with high current draw are indirectly controlled by a thermostat
that actuates a magnetic motor started. Can control the operation of an unloader
actuator
Filled System Remote Bulb Thermometer - Answer-When equipped with a switch can
directly control the compressor motor power supply. Once temp goes above set point,
thermometer expands and bellows expands. Bellows closes thermostat switch and
compressor starts.
What does the thermostat have to keep compressor from short cycling? - Answer-Cut in
is set a little lower than set point and cut out is set a little higher than set point. This is
called temp differential-difference between cut in and cut out temps.
Pressure Actuated Control for a compression refrigeration system? - Answer-Connected
to the suction line of the compressor which starts and stops it. Operates opposite to
thermostat. Opens when pressure goes below cut off setting and closes it when the
pressure rises to a preset differential. Uses bellows.
When can't suction pressure-actuated control be used? - Answer-When an automatic
expansion valve is used as a metering device. Auto expansion valves maintain a
constant evaporator pressure. Compressor would not cycle off. Nor can it be used with
a capillary tube or orifice-type metering device, since they do not prevent liquid
refrigerant flow from the high to low sides of the system when the compressor shuts
down. Compressor would start and stop too often causing mechanical failure and use
too much power.
, Humidity-Acutated Control (Humidistat) - Answer-Can be used both for humidity
measurement as well as starting and stopping refrigerant flow to an evaporator. Liquid
solenoid valve, coupled with a metering device would be used in conjunction with the
humidistat.
Low Chilled Water Temp Cut off Switch - Answer-Consists of a thermostat senses temp
of chilled water leaving the chiller. If water temp too low may freeze in evaporator. Temp
drops below 2-3 C, compressor will stop, when temp reaches 5-6 C compressor starts.
Chilled Water Flow Switch - Answer-Installed in the chilled water outlet line. Protects the
chiller from freezing due to low water flow. Opens compressor when water flow drops
below the safe minimum flow. Prevents compressor from starting if flow has not been
established.
Low Oil Pressure Cut-off - Answer-Prevents compressor from operating under low lube
oil pressure. Prevents compressor damage. Usually requires a manual reset.
Condenser High Pressure Cut-off - Answer-High limit switch which shuts down
compressor when condenser reaches an excessively high value. Manual reset.
Inadequate cooling water or airflow, or condenser fowling, is usually the cause of the
trip.
Refrigerant Low Temp Cut-off - Answer-Senses temp of refrigerant in chiller. If temp
drops low enough to freeze water, switch opens and shuts down the compressor.
Demand Limiter - Answer-Limits the max current flow to the compressor drive motor.
Overrides the water temp sensors if the motor load reaches the max amperage setting.
Motor Demand Limiter - Answer-Limit the current flow when machine first starts. Can be
adjusted to 40% until water brought down to temp. Increasing demand limiter in steps
through 60,80,100% helps reduce the max electrical demand and possibility of motor
damage due to high current flow.
Compressor Vibration Shutdown - Answer-If vibration on any bearing reaches a certain
set point, an alarm sounds to warn the operator of a vibration problem. If vibration
increases compressor will shut down at set point.
Suction Scrubber High Level Shutdown - Answer-If liquid level rises to a certain level,
an alarm will sound to alert the operator of a high level in scrubber. If level continues to
rise, a float switch will shut down the compressor to keep liquid from getting in the
compressor.
High Bearing Temp Shutdown - Answer-If temp rises to a certain point, a high bearing
temp alarm is activated. This allows operators to either take action to reduce temp or to
