Coils in the Refrigeration System

1. Coils in the Refrigeration System North Seattle Community College HVAC Program Instructor – Mark T. Weber, M.Ed.

2. The Coil • In a refrigeration system, the coil’s job is to transfer heat. • It transfers heat by being either being warmer than the air (Condenser) or colder than the air (Evaporator) • Air coils can be either the evaporator or the condenser. • The evaporator absorbs heat • The condenser rejects heat.

3. The Coil • Because heat travels from hot to cold, a coil must be at a temperature that is lower or higher than the medium being cooled or heated. If the coil is being used as an evaporator, it must be cooler. If it is used as a condenser, it must be hotter. • Removes both latent and sensible heat from the air • Latent heat is in the form of moisture • Sensible heat reduces air temperature

5. Indoor coils Outdoor coils The most commonly seen coils in residential split systems

6. Heat Exchange Characteristics of a Coil • Common materials used are copper, aluminum, steel, brass and stainless steel • Corrosion factor and application determines the materials used • Rapid heat transfer rate between two liquids – ground source heat pumps • Slower heat transfer rate between two vapors

7. Heat Exchange Characteristics of a Coil (cont’d.) • Film factor • In the case of an air-coil (air passing by the coil) a small film of air will “stick” to the surface of the coil. • The film acts as an insulator • The faster the air passes by (velocity), the smaller the film. • The slower the velocity, the larger the film thus it is a larger insulator.

8. Heat Exchange Characteristics of a Coil (cont’d.) • Temperature difference (Delta T) between two mediums • Large temperature difference = higher heat transfer rate • Small temperature difference = lower heat transfer rate 50 degrees 30 degrees 70 degrees 70 degrees 100 BTUH 200 BTUH 0 BTUH

9. Types of Coils • Natural convection (draft) coils • Made up of bare tubes or pipes • Physically large; very low air velocity • Usually used in a walk-in cooler or large industrial applications

10. Types of Coils (cont’d.) • Mechanical draft coils • Use blowers or fans to move air across the coil • Improved heat transfer rate • Smaller than natural draft evaporators

11. Types of Coils (cont’d.) • Stamped plate coils • Creates a large heat transfer surface; metal plates stamped with grooves • Finned tube coils: increased surface area • Multiple circuit coils • Parallel circuits are created in the coil • Reduced pressure drop across the coil

12. Pressure Drop In Coils • The pressure drop through the coil should be as low as possible • Each circuit should be fed with the same amount of refrigerant (also liquid/vapor percentages) • Reasons for uneven feeding include blocked distributors, dirty coil, uneven air distribution and different length coil circuits

13. Summary • Heat flows naturally from Hot to Cold • The coil is maintained at a temperature that is lower or higher than the medium being cooled or heated, depending • The coil absorbs or rejects sensible and latent heat • Removal of Sensible heat lowers the temperature • Removal of Latent heat lowers the humidity

14. Summary (cont’d.) • Coils are commonly constructed of copper, steel, aluminum, stainless steel, and brass • Large temperature differences between the coil and the air result in a higher heat transfer rate • Common types include the natural draft and forced draft

15. For more information please contact Mark T. Weber, M.Ed., CMHE At North Seattle Community College WWW.NorthSeattle.edu Mark.weber@seattlecolleges.edu