Dry expansion evaporators use an expansion valve to control the flow of refrigerant into the evaporator as a liquid. By the end of the evaporator coil, the refrigerant has vaporized by absorbing heat from the substance being refrigerated. They have advantages like good control and low cost, but disadvantages like lower efficiency compared to flooded evaporators. Common materials include carbon steel, copper, and plastic. Typical applications include water chilling, liquid cooling, and household refrigerators.

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.