Evaporators in Refrigeration Systems: Types, Design & Defrosting
EVAPORATORS
Introduction
Evaporators are heat exchangers designed to evaporate liquid refrigerant. The boiling temperature of the coolant is less than the temperature of the environment you want to cool. Refrigerant evaporates at low pressure, although higher than atmospheric pressure. The cooling efficiency of the liquid-vapor mixture depends on the liquid content in the mixture.
Types of Evaporators by Feeding Method
- Dry expansion
- Flooding
This classification is based on whether the facility contains a liquid separator.
1. Dry Expansion Evaporator (Without Liquid Separator)
In this type of evaporator, liquid feeding is done through a thermostatic expansion valve. The amount of liquid entering the evaporator is limited to the amount vaporized as it goes through the evaporator. For complete vaporization, a warming of 10°C is allowed at the end.
For a dry expansion evaporator, the ratio of liquid-wetted surface and the efficiency of the evaporator increases as the load increases. They have lower yields but are cheaper and simpler.
2. Flooded Evaporator (With Liquid Separator)
These evaporators are almost entirely filled with fluid during operation. The evaporator is fed with an overdose of liquid to keep the tube surfaces wet, serving also to remove oil.
Types of Evaporators by Construction
- Smooth tube evaporators
- Plate evaporators
- Finned evaporators
Evaporator surfaces in refrigeration units form secondary areas whose primary function is to capture heat from the surroundings and transport it to the refrigerant.
Smooth tube evaporators are used at temperatures below -1°C where frost accumulation on the evaporator surface is inevitable. Defrosting can be performed without interrupting the cooling process.
1. Smooth Pipe Evaporator
Flat tube evaporators are generally constructed from steel and copper. Smooth pipe coils are available in various shapes, sizes, and designs and are usually made to order for each installation. Common forms are zig-zag or oval. These evaporators are used for cooling liquids.
2. Plate Evaporators
Two sheets of metal are stamped and welded to provide a path for the refrigerant between them. They are used in refrigerators and freezers. They are easily cleaned and economical.
3. Finned Evaporators
Finned evaporators are flat tubes with incorporated metal plates or fins that act as heat collectors. These fins must be in good contact for optimal heat capture.
Finned evaporators have a larger surface area per unit length, allowing them to be built smaller.
Types of Evaporators by Air Circulation
- Natural convection evaporators
- Forced convection evaporators
Low air velocity decreases evaporator performance, causing slow product cooling and bacterial growth. However, excessive air velocity causes product dehydration.
The desired air movement speed depends mainly on:
- Humidity of the chamber
- Type of product
- Storage duration
These three factors are interrelated. Poor air circulation has the same effect on the product as high air humidity, while high airflow has the same effect as low humidity.
1. Natural Convection Evaporators
These are used in domestic refrigerators where high relative humidity is needed and product ventilation is not necessary. Their drawbacks are the low heat transmission rate, poor heat distribution, and defrosting difficulty.
Operation is based on density differences. Air velocity over the evaporator tubes in natural convection depends on the temperature difference between the evaporator and the chamber. The greater the temperature difference, the higher the air movement.
2. Forced Convection Evaporators
These are smooth tubes with fins and fitted with fans.
Advantages of Forced Convection Evaporators:
- More compact
- Smaller size
- Easy installation
- Adjustment of relative humidity
The temperature drop of air flowing through the evaporator should be half the difference between the heating and the vaporization of refrigerant. As a rule, the air velocity should be maintained between 1-2.5 m/s.
Types of Evaporators by Application
1. Closed Circuit Coolers
Closed circuit coolers include:
- Double-tube coolers
- Multitubular coolers: vertical and horizontal
Double-tube coolers consist of two tubes, one mounted inside the other, offering a high transmission coefficient. Pipe lengths are between 3 and 6 meters. They can work under dry and flooded conditions. Triple tube configurations are used in certain applications.
- Multitubular Coolers
Also known as shell and tube, there are two major groups based on their cooling function.
These evaporators are used to cool water, brine, milk, beer, and other low-viscosity liquids. These coolers are used exclusively for relatively high water temperatures. The evaporating temperature should be above the freezing point of water. The fluid velocity in the tubes cannot normally exceed 2 m/s to avoid pressure loss and corrosion problems.
Direct and Indirect Cooling Systems
Direct systems are in direct contact with the space or material to be cooled. Indirect systems use water, brine, or other suitable liquids cooled by direct expansion refrigerant in a liquid cooler. The secondary refrigerant can be in direct contact with the product or pass through an air cooler or other heat exchange surface.
Indirect cooling systems are used in installations where there is a larger distance between the equipment and the cooling area. However, leaks are more problematic in pipes carrying brine.
The most widely used secondary refrigerant is water due to its fluidity, high specific heat, and conductivity. It is also cheap and relatively non-corrosive. Brine is used when the working temperature is below the freezing point of water. Glycol solutions are non-corrosive and extremely stable, replacing brines in many applications.
Thermal Difference in the Evaporator
Depending on the selected thermal difference, the evaporator surface may be insufficient if the thermal difference is overestimated. This information is crucial when selecting or designing an evaporator.
Multiple definitions exist because neither the cooling fluid temperature nor the boiling temperature of the coolant remains constant as the fluids pass through the evaporator. The cooling fluid temperature decreases progressively as it passes through the evaporator. The temperature drop is not linear; it is higher across the first row of the coil and decreases as it passes through the other rows.
The actual evaporation temperature is the temperature at which evaporation takes place in the evaporator, but this temperature is not constant. As pressure varies, the evaporation temperature also varies. The definition of evaporation temperature is the saturation temperature of the refrigerant at the pressure at the evaporator outlet.
Frost Formation and Defrosting of Evaporators
1. Effects of Frost
- Insulation effect, leading to a lower heat transfer rate and a decrease in the refrigerating machine’s production, increasing machine uptime.
- Reduction of the actual air volume through the tubes and fins, as the air pressure drop increases and the flow decreases.
- Variation in moisture content, affecting proper product preservation.
Due to these reasons, the plant’s power consumption increases with increasing equipment uptime, requiring regular defrosting.
2. Disadvantages of Defrosting
- Disturbance of temperature and humidity in the cold store due to added heat and humidity and interruption of the cooling cycle.
- Defrost energy is wasted in the refrigerator warehouse.
- Fans require a delay before operation to prevent distributing excess moisture.
- Heat expands the air in the cold store, exerting pressure against the walls and ceiling.
Determining the duration and frequency of defrosting depends on the evaporator type, the nature of the facility, and the procedure used.
Defrosting systems can be classified into two major groups:
- External procedures: The melting of frost starts from the peripheral layer and should be total.
- Manual defrost (by scraping or brushing)
- Defrost by stopping the machine and natural warming of evaporators
- Defrost by stopping the machine and forced air circulation over the evaporator
- Defrost by sprinkling or spraying water, brine, or antifreeze
- Internal procedures: More modern and faster, the melting of frost starts from the inner layer in contact with the evaporator tubes without requiring a full melt as the frost breaks off.
- Defrost by electric heating of the evaporator
- Defrost by hot gases
3. External Procedures
These are slow and less common defrosting procedures.
- Manual defrost: Defrosting by scraping or brushing has been abandoned due to the significant manpower required and its limitation to smooth tube evaporators.
- Defrost by stopping the machine and natural warming of evaporators: This system can only be used if the chamber temperature is above zero degrees.
- Defrost by stopping the machine and forced air circulation over the evaporator: This is also a system for chambers where the temperature is above zero degrees.
- Defrost by spraying water or brine/antifreeze solutions: For cold temperature enclosures close to 0°C, defrosting can be done by atomizing water on the evaporator coils. If the temperature is below -2°C, brine or antifreeze solutions are used instead of water.
The defrost system by spraying water involves raining pressurized water on the ice accumulated on the evaporator. The ice melts and cleans the evaporator, preparing it for the next operation cycle. This procedure takes about 4-5 minutes. It is necessary to regulate the defrost operation so the machine does not restart until the evaporator has completely drained to prevent freezing during operation.
4. Internal Procedures
These are faster and more modern than external procedures and do not require a full melt of the accumulated frost. Falling chunks of frost on the drain tray may require heating the tray to melt them.
Easily convertible to automatic, these procedures are increasingly used for defrosting evaporators in negative temperature chambers and display cases for frozen products.
- Defrost by electric heating the evaporator: This is a comfortable, easy-to-install, and relatively easy-to-regulate method widely used for finned tube evaporators. It can be achieved using a low-resistance tube that melts the frost upon contact. The frost appears as circular plates and eventually melts into the drain tray at the bottom of the evaporator.
The defrost cycle is initiated by closing the solenoid valve in the liquid pipe and stopping the evaporator fan. This causes the pressure to drop rapidly, and the compressor stops, governed by the low-pressure switch. The resistance circuit then activates, melting the ice. Once the temperature in the evaporator reaches a certain point, ensuring no frost remains, the resistors are deactivated. Depending on the control used, the cycle can continue with a period of total stoppage to facilitate drainage of the melted frost. Then, the solenoid valve and the fan are activated, the compressor starts, and the installation resumes normal operation.
This procedure is widely used in negative temperature chambers that do not require very large exchange surfaces.
- Defrost by hot gas: This system has several variants, all using the hot gas discharged by the compressor as the heat source for defrosting the evaporator. This device, in addition to conventional refrigeration elements, should include:
- A hot gas pipe (1) linking the compressor discharge to the evaporator inlet valve after expansion.
- A check valve (2) preventing any unforeseen liquid feeding to the evaporator during defrost by discharge from the capacitor.
- A solenoid valve (4) in the liquid pipe before the expansion valve, interrupting the evaporator feed during defrost.
- A solenoid valve (3) that closes the hot gas pipe during normal operation.
A bypass pipe, equipped with a solenoid valve (3), is installed between the compressor and the evaporator. When the solenoid valve opens, hot gas discharged from the compressor enters the condenser and evaporator at a point immediately before the refrigerant control system.
One disadvantage of this system is that during defrosting, fluid accumulates in the evaporator, leaving little refrigerant for the compressor to recirculate. Thus, the system tends to exhaust the vapor before the evaporator is completely defrosted.
A common method of hot gas defrost is to use a supplemental evaporator coil line to re-evaporate the suction fluid. At regular intervals (3 to 6 hours), the defrost timing cycle begins by opening the solenoid valve in the hot gas line and closing the suction bypass. Simultaneously, the evaporator fans stop, and the re-evaporator fan starts. The system is reset to normal operation by closing the hot gas solenoid, opening the suction, stopping the re-evaporator fan, and starting the evaporator fans.
When two or more evaporators are connected to a capacitor, it is common to defrost them separately using this system.
- Defrost by reversing the cycle: This is the most effective hot gas defrost method, as it involves total gas condensation in the evaporator, and the formed liquid is re-evaporated in the condenser. During defrosting, the normal processes of the condenser and evaporator are reversed, hence the name “reverse cycle defrost.” The system’s high efficiency requires dissipating all the machine’s heat output in the evaporator during the defrost cycle, usually at a higher evaporating temperature than during normal operation.
This reversal can be achieved using a special device called a reversing valve. The high efficiency of the system requires that all the heat output of the machine be dissipated in the evaporator, which is under the defrost cycle, and usually at an evaporating temperature higher than its normal operation. This investment can be obtained using a special device called a reversing valve.