Refrigeration Systems Controls

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Questions and Answers

What is the primary function of the inlet vane closed switch?

To prevent compressor operation if lube oil pressure is low

To indicate when chiller capacity control vanes are closed (correct)

To close the compressor motor circuit under certain conditions

To monitor water flow levels

What condition triggers the low oil pressure cut-off device?

When the condenser pressure is excessively high

When lube oil pressure falls below a safe minimum value (correct)

When the refrigerant temperature is too high

When lube oil pressure is above the maximum limit

What must occur for the condenser high pressure cut-off to be reset?

The compressor must be turned off completely

The system must maintain a constant flow of refrigerant

The temperature of water must be monitored

The switch must be manually reset by an operator (correct)

Under what condition does the refrigerant low temperature cut-off activate?

When the refrigerant temperature drops low enough to freeze water Signup and view all the answers

What is a function of the motor demand limiter in centrifugal chillers?

To limit maximum current flow to the compressor drive motor Signup and view all the answers

What role do face dampers play in the cooling process?

They maintain constant air quantity in the duct. Signup and view all the answers

What typically causes a high condenser pressure trip?

Inadequate condenser cooling water or airflow Signup and view all the answers

What effect does a high start temperature of the chilled water loop have on the electrical demand?

It can lead to very high electrical demand if no current flow restrictions are applied. Signup and view all the answers

Which combination of dampers helps regulate cooling capacity in a system?

Bypass damper with face damper connected to the same driver. Signup and view all the answers

How can the motor demand limiter minimize the risk of motor damage?

By gradually increasing the demand limiter settings. Signup and view all the answers

What method involves stopping and starting the compressor based on temperature?

Intermittent operation. Signup and view all the answers

Which of the following describes a condition that allows the centrifugal compressor to start?

When the chiller capacity control vanes are closed Signup and view all the answers

Which method allows for cooling capacity reduction without shutting down the compressor?

Compressor speed control. Signup and view all the answers

In what scenario does the low oil pressure cut-off require a manual reset?

After a low oil pressure condition occurs Signup and view all the answers

What does the operation of inlet vanes in a compressor do?

It restricts the flow of refrigerant, affecting current draw. Signup and view all the answers

What is the primary function of the bypass damper?

To open when the face damper closes. Signup and view all the answers

What is the primary purpose of a soft loading strategy in chillers?

To control the chilled water temperature increase gradually. Signup and view all the answers

What happens when the vibration on a motor or compressor bearing reaches its shutdown set point?

The compressor will automatically stop. Signup and view all the answers

Which of the following is NOT a method to control compressor capacity?

Constant power operation. Signup and view all the answers

What indicator serves to alert the operator of a high liquid level in a suction scrubber?

Float switch alarm. Signup and view all the answers

How do multistepped blowers enhance the refrigeration process?

By improving balance between fan airflow and cooling requirements. Signup and view all the answers

Why is the power consumption of a compressor motor significant?

It is directly proportional to the mass of refrigerant moved. Signup and view all the answers

What does cylinder unloading achieve in refrigeration compressors?

It prevents total compressor shutdown. Signup and view all the answers

Which of the following describes a consequence of fast unloading of a compressor?

It can cause low chilled water temperature trips. Signup and view all the answers

What is the primary reason for the lube oil temperature cut-off switch to prevent the compressor from starting?

To prevent excessive refrigerant dilution of the lube oil Signup and view all the answers

At what lube oil temperature does the inhibit switch prevent the compressor from starting?

Below 35°C Signup and view all the answers

What role does the crankcase heater play in the operation of a compressor?

To heat the lube oil to appropriate levels Signup and view all the answers

Which factor does NOT affect the refrigerating capacity of a system?

The ambient temperature of the room Signup and view all the answers

What may cause the compressor to lose its prime when starting?

Vaporized refrigerant forming bubbles in the lube oil Signup and view all the answers

What components might be used to monitor temperature in a compressor system?

Bimetal or thermistor temperature sensing elements Signup and view all the answers

How does the mass of refrigerant circulated affect the kW rating of a refrigeration system?

It dictates the refrigerating capacity of the system Signup and view all the answers

What function does the lube oil heater serve when the oil temperature is below 60°C?

It raises the oil temperature to prevent damage Signup and view all the answers

What is the primary purpose of using evaporator capacity control?

To manage the amount of vapour produced Signup and view all the answers

How does a sectional evaporator function in relation to cooling load?

Sections are shut off when the cooling load decreases Signup and view all the answers

What is a common issue with relying solely on face dampers for airflow control?

They can lead to unacceptable reductions in ventilation air Signup and view all the answers

What mechanism is typically used to control the operation of a sectional evaporator?

Solenoid valves in the liquid refrigerant line Signup and view all the answers

What component is responsible for adjusting the airflow in an evaporator equipped with dampers?

Positioner Signup and view all the answers

Why is it necessary to control both evaporator and compressor capacity simultaneously?

To maintain optimal efficiency and performance Signup and view all the answers

What is the role of a face damper in an evaporator system?

To vary the amount of air over the evaporator coils Signup and view all the answers

What happens when cooling load decreases in a system using sectional evaporators?

Some sections become inoperative to manage the load Signup and view all the answers

What determines the capacity of an identical refrigeration system?

The efficiency of the compressor Signup and view all the answers

What occurs when a hand-operated expansion valve allows too much refrigerant into the evaporator?

Evaporator icing may occur Signup and view all the answers

Why are thermostatic expansion valves referred to as 'constant superheat' valves?

They regulate superheat for better evaporator efficiency Signup and view all the answers

What is a disadvantage of hot gas bypass capacity control in refrigeration systems?

It can cause increased energy consumption Signup and view all the answers

When does a refrigeration compressor utilize cylinder unloading?

To reduce capacity under low load conditions Signup and view all the answers

Why are face dampers less effective for evaporator capacity control than bypass dampers?

Bypass dampers provide more precise control Signup and view all the answers

What is the primary function of external equalizer piping in thermostatic expansion valves?

To stabilize pressure readings Signup and view all the answers

What is one reason for using low-pressure float valves in certain evaporators?

To control the flow of refrigerant based on liquid levels Signup and view all the answers

Study Notes

Learning Outcomes and Objectives

Describe the purposes and operating principles of refrigeration system operational and safety controls.
Describe refrigeration system controls.
List the safety shutdown devices specific to centrifugal compressor water chillers.
Describe typical refrigeration system safety shutdown devices.
Describe the construction and operation of refrigerant metering devices.
Describe the different methods used to control evaporator capacity.
Describe the different methods used to control the capacity of refrigeration compressors.

Refrigeration System Controls

Refrigeration systems operate under varying conditions.
Load changes and atmospheric conditions influence system operation.
Refrigeration systems have three types of controls: operating, actuating, and limiting/safety controls.
Operating controls start, stop, or regulate compressor capacity.
Actuating controls either indirectly control operations or regulate the cycle.
Limiting/safety controls protect the system from exceeding its designed limits.

Chapter Introduction

Refrigeration systems need controls to operate safely and efficiently.
Controls respond to changing conditions, such as load changes, cooling requirements, and atmospheric conditions.
Refrigeration systems must operate at the required cooling load.
System capacity (kW or TR) control involves adjusting the mass of refrigerant circulated.
This can be done directly or indirectly by controlling compressor capacity or evaporator capacity.

Human Machine Interfaces (HMIs)

HMIs are often found on packaged refrigeration units.
HMIs provide operators with operational data and alarms.
Operators use HMIs to control set points and operational expectations.
Some may be standalone, not reporting to a central control system.
Some may be linked to plant supervisory systems (SCADA).
Manufacturer manuals and plant data are important for site-specific practices.

Safety Controls (Centrifugal Chiller Safety)

Centrifugal chillers have safety devices to protect the system during startup and operation.

Low Chilled Water Temperature Cut-Off Switch

Prevents damage from water freezing in the evaporator.
Water temperature sensor detects issues and shuts down the compressor if the temperature falls too low.

High-Pressure Cut-Off

Occurs when condenser pressure becomes too high.
Causes: Insufficient condenser cooling, lack of airflow, fouling, atmospheric conditions.
System safety features prevent damage due to excessively high pressures and low temperatures.

Low-Pressure Cut-Off

Low suction pressure and temperature could damage the system.
Low-pressure cut-off prevents such problems by shutting down the compressor.

Low Limit Thermostat

Used in systems where low temperatures could cause damage.
The pressure switch shuts off the compressor.

Low Oil Pressure Cut-Off

Protects from damage if oil level is too low.
Signals the operator to address the issue

Refrigerant Metering Devices

Metering devices control refrigerant flow to the evaporator.
Capillary tubes, fixed orifices, and expansion valves are metering devices.
Automatic control is possible in larger systems with the possible exception of very large systems.

Methods to Control Evaporator Capacity

Use sectional evaporators or evaporator dampers.
Evaporator capacity control affects the amount of vapor produced.
This is needed for efficient, continual operation.

Compressor Capacity

Various techniques can adjust compressor capacity in continuous operation modes.
These methods include cylinder unloading, bypass, hot gas bypass, variable speed drives, and variable inlet vane controls.
In intermittent operation, the compressor starts and stops to control cooling load.

Knowledge Exercises

Include detailed questions on the subject of refrigeration system controls.

Chapter Glossary

Includes a list of key terms and their definitions.

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Description

This quiz focuses on the operational and safety controls of refrigeration systems, including the purpose and functions of various controls. It covers the different types of controls, safety shutdown devices, and methods for regulating compressor capacity and evaporator operation. Enhance your understanding of refrigeration system management and safety measures through this assessment.