Basic principle and working of dry expansion evaporators.

1. DRY EXPANSION
EVAPORATORS
Basic principle and working

2. Introduction
• In an evaporator, the refrigerant boils or evaporates and
in doing so absorbs heat from the substance being
refrigerated.
• The name evaporator refers to the evaporation process
occurring in the heat exchanger.
• Evaporators are manufactured in a wide variety of types,
shapes, sizes and designs, and are classified in the
following way:-
o type of construction
o method of refrigerant feed
o operating conditions
o method of air (or liquid) circulation
o type of controls
o application

3. Dry expansion evaporator
• It is called so because the refrigerant entering the evaporator
as a liquid vaporizes by the time it reaches the end of the
evaporator coil.
• In case of the dry expansion type of chillers or evaporators
the flow of the refrigerant to the evaporators is controlled by
the expansion valve.
• In case of the flooded type, the evaporator is filled with the
refrigerant and constant level of the refrigerant is maintained
inside it.
• A dry type evaporator concept is often used as shell-and-tube
type and fin-and-tube type constructions.

4. Dry expansion evaporator
• There is always at least 20% vapour present within the
evaporator pipework.
• The amount of liquid present in dry expansion evaporators will
depend upon the refrigeration load.
• At light load conditions, the amount of liquid will be small, and
in high load conditions, the amount of liquid will be large.
• The larger the wetted surface, the greater the efficiency of dry
expansion evaporators.

5. Dry vs Flooded type
Schematic of a dry type evaporator

6. Dry vs Flooded type
• Flooded-flow refrigeration circuit shown in green. The red line
symbolizes evaporation and compression in a DX evaporator
designed for equal duty.

7. Dry vs Flooded type
• (a) Temperature profile through a superheated
evaporator (DX). (b) Temperature profile through a non-
superheated evaporator (Flooded).

8. Advantages
• Most of the throttle devices can adapt to load changes.
• Good control, and easy construction.
• Low cost.
• Because of the large flow rate, the refrigerant can be directly
returned to the compressor by the velocity of the gas
refrigerant itself.
• Requires a lower amount of refrigerant to function when
compared to flooded type.
• Has a lesser chance of flood back to the compressor.

9. Disadvantages
• The superheated refrigerant vapor accounts for about 20% of
the heat transfer area of the evaporator, but it can only
provide a small amount of enthalpy of refrigeration.
• Comparatively lower COPs.
• The system runs with higher pressures.
• Slower pull-down with heavy loads.
• Can only be used with moderate capacity refrigerators.

10. Nomenclature
• The nomenclature of a typical industrial grade shell-and-
tube dry expansion evaporator.

11. Materials
• The standard construction of the dry shell & tube
evaporators consists of the following materials:
o Carbon steel for headers, tube-sheets, shell, refrigerant and water
connections.
o Copper for exchanger tubes.
o Plastic material for baffles.
o Asbestos free and O-Ring gaskets.
o Alloys steel bolts and nuts.
o Alternative materials can also be used.

12. Applications
• The main applications of dry-expansion evaporators are:
o Water chilling in air conditioning plants.
o The liquid or brine solutions cooling in refrigeration plants.
o Water heating in heat pump systems.
o Household and domestic refrigerators.

13. A drawing of a typical shell and tube, dry type, 1 circuit evaporator
manufactured by ONDA.