Refrigeration System & Heat Exchangers

What does the compressor do?

Raises the pressure of the refrigerant vapor and moves refrigerant through the system.

What state of refrigerant enters the compressor?

Low-pressure vapor.

What does the condenser do?

Rejects heat from the refrigerant, causing it to condense from vapor into liquid.

What happens to refrigerant pressure in the condenser?

It remains on the high-pressure side.

What state of refrigerant leaves the condenser?

High-pressure liquid.

What does the TXV (Thermostatic Expansion Valve) do?

Throttles refrigerant flow to maintain superheated vapor leaving the evaporator.

What does the TXV sense?

Refrigerant temperature leaving the evaporator.

What does the TXV control?

Superheat.

What does the TXV NOT control?

Box temperature.

Why does the TXV maintain superheat?

To ensure liquid refrigerant does not return to the compressor.

What does the evaporator do?

Absorbs heat, causing the refrigerant to evaporate.

What happens to refrigerant in the evaporator?

Low-pressure liquid/refrigerant mixture absorbs heat and becomes vapor.

What is the evaporator's job in terms of the refrigerated space?

Removes heat from the space/box.

What does the solenoid valve sense?

Box temperature.

What does the solenoid valve do?

Opens or closes in response to box temperature.

What happens when the solenoid valve is open?

Refrigerant can flow to the TXV.

What happens when the solenoid valve closes?

Refrigerant cannot flow to the TXV.

Does the solenoid valve control superheat?

No.

Does the solenoid valve control the refrigerant's superheat?

No. The TXV controls superheat.

What does the LP cut-out sense?

Compressor suction pressure.

What does the LP cut-out do?

Starts and stops the compressor during normal operating conditions.

What happens when suction pressure gets high enough during startup?

The LP cut-out senses the high pressure and turns the compressor on.

What happens when suction pressure becomes too low during shutdown?

The LP cut-out senses the lack of pressure and shuts the compressor off.

What does the HP cut-out sense?

Compressor discharge pressure.

What does the HP cut-out do?

Starts and stops the compressor under emergency conditions.

What is the difference between LP and HP cut-outs?

LP senses suction pressure; HP senses discharge pressure.

What is the box temperature?

The temperature of the refrigerated space/box.

What component responds to box temperature?

The solenoid valve.

What happens when box temperature is above the set point?

The solenoid opens.

What happens when box temperature reaches the set point?

The solenoid closes.

What is sensible heat?

Heat that is absorbed or released with no change of state.

How can sensible heat be measured?

By a change in temperature.

What is latent heat?

Heat absorbed or released during a change of state.

What happens to temperature during latent heat?

The material changes state while absorbing or releasing heat.

What is enthalpy?

The total heat content of a thermodynamic system under constant pressure.

What are the four main components of the refrigeration cycle?

Compressor, condenser, TXV, and evaporator.

What is the basic order of the refrigeration cycle?

Compressor → Condenser → TXV → Evaporator → Compressor.

Which components are on the high-pressure side?

Compressor discharge, condenser, and liquid line leading toward the TXV.

Which components are on the low-pressure side?

Evaporator and compressor suction side.

What is the startup sequence?

Box temperature rises above set point → solenoid opens → refrigerant flows to TXV → TXV opens → refrigerant flows to evaporator → pressure rises → LP cut-out turns compressor on.

What is the shutdown sequence?

Box reaches set point → solenoid closes → refrigerant stops flowing to TXV → refrigerant stops flowing to evaporator → pressure drops → LP cut-out shuts compressor off.

What does a heat exchanger do?

Transfers heat from one fluid to another.

Which direction does heat naturally flow?

From hot to cold.

What are the two types of heat exchanger contact?

Direct contact and indirect contact.

What is an indirect-contact heat exchanger?

Fluids transfer heat through a plate, tube, or other separating material.

What is a direct-contact heat exchanger?

The fluids come into direct contact with each other.

What is parallel flow?

Both fluids flow in the same direction.

What is counter flow?

The two fluids flow in opposite directions.

Which flow arrangement is most common?

Counter flow.

What is cross flow?

The fluids flow at right angles to each other.

Where is cross flow commonly used?

Condensers.

What does a shell do in a shell-and-tube heat exchanger?

Acts as the outside pressure vessel.

What does the tube sheet do?

Supports the tubes.

What do the tubes do?

Provide the internal passages through which fluid flows.

What do baffles do?

Redirect fluid over the tubes, lengthen the flow path, improve heat transfer, and support the tubes.

What is a floating tube sheet?

A tube sheet that supports the tubes while allowing expansion from temperature changes.

What do zinc anodes do in a seawater-cooled heat exchanger?

Protect the heat exchanger from corrosion.

What are the main advantages of a plate heat exchanger?

Good heat-transfer precision, true counter-current flow, easy capacity expansion, and less fouling/stress/wear/corrosion.

What is one major disadvantage of a plate heat exchanger?

Poor sealing can cause leakage.

What limits the operating temperature of a plate heat exchanger?

The gasket material.