1. . Level 3 Air Conditioning Inspections for Buildings
6. Evaporators Day 2
PRESENTED BY
Anthony Balaam

2. Evaporators Evaporator Selection
Different Evaporators suit different applications.
Different, if cooling air as opposed to cooling water.
Different Types are:-
1. Direct expansion (DX) air coolers
2. Shell and tube liquid coolers – direct expansion-type
3. Plate heat exchanger liquid coolers
4. Bauldelot liquid coolers
5. Oil control in evaporators
This area hear

3. Evaporators The Evaporating Process:-
Refrigerant passes through the ‘Expansion Device’.
Drops in ‘pressure’ and ‘temperature’.
Refrigerant enters the ‘Evaporator’, where it absorbs heat from the hotter air or water passing over its tubes.
Tubes absorb the heat flowing around the Evaporator.
Air or liquid is effectively chilled.
Some refrigerants may evaporate instantly during their expansion and may occur within the pipework leading to the Evaporator (flash gas) – Note - this does not provide any useful cooling.
Sensible heat
Refrigerant flow

4. Evaporators The Evaporating Process:-
Refrigerant boils off in the ‘Evaporator’, at its ‘evaporating temperature’.
Refrigerant turns from ‘a liquid’ to a ‘saturated vapour’ at the same temperature.
Once evaporation is complete, and the refrigerant is 100% vapour, the refrigerant temperature rises – called – ‘superheat.’
This is required for DX systems, so that no liquid returns to Compressor.
Superheating is not efficient use of the Evaporator.
More heat is absorbed during the boiling process (called ‘Latent Heat’- change of state)
Rather than by the vapour changing temperature (called ‘Sensible Heat’).
Superheat of 5˚K more than the evaporating temperature is the best compromise.

5. Evaporation Room Air 2 From the Expansion Device To the Compressor
Evaporator
pipework 2

6. DX Air Coolers 1. Direct Expansion (DX) Air Coolers:-
Typically used in cold rooms.
Usually enclosed within the Air Handling Units, Heat Recovery Ventilators, Fan Coil units or similar units for air-conditioning.
Same in construction as an Air Cooled Condenser.
Ducted system
Cold Room
Cold Room
Ducted Direct System
Direct System

7. DX Air Coolers Direct Expansion (DX) Air Coolers:-
Most use finned tubes and forced circulation of air.
Typically finned tube evaporators have paralleled circuits to:-
Maximise heat transfer
Ensure good oil return
Minimise pressure drop
Distributors are used to ensure the refrigerant flows evenly between the parallel circuits.
Air Flow

8. DX Air Coolers Batteries Air Sensors Different types of DX Air Coolers
Programmer

9. DX Air Coolers Direct Expansion (DX) Air Coolers:-
Saturated refrigerant is fed through the distributors.
Refrigerant undergoes 100% evaporation before reaching the outlet.
This is done by controlling the flow of the refrigerant, the ‘Expansion Device’ ensures the refrigerant leaving the Evaporator is superheated, as little as (5K to 7 K).
1. It ensures refrigerant effect is as high as possible.
2. It protects the ‘Compressor’ from liquid refrigerant returning via the ‘suction line’.
Small systems with capillary tubes have simpler evaporators.
Refrigerant charge is critical to ensure evaporation and to prevent liquid from flooding into the compressor.
Fan coil Unit

10. DX Air Coolers
DX Fan Coil Units :-

11. DX Air Coolers De-frosting of Evaporators:-
Ice can build up on DX coils when the temperature is below 0˚C, and can be removed by :- (Note – it decreases efficiency).
1. By Natural de-frosting - is when the air temperature is above 4˚C.
By Turn the cooling off and letting the fan run to defrost the blocked fins.
2. By Electric de-frosting – defrost heaters are embedded within the fin block.
Periodically switched on to defrost blocked fins while the compressor is off.
3. By Hot or cool vapour defrosting:-
Hot discharge vapour or warm saturated vapour from the top of the receiver is circulated through the evaporator to melt the ice.
4. Drain pan line allows any condensate to run off.

12. DX Air Coolers Energy Efficient De-frosting:-
Initiate only when necessary – on detection of loss of performance.
Use most efficient methods of applying heat.
Ensure defrost heat is evenly distributed over the fin block.
Stop the defrost cycle when the fin block is clear of ice.
Minimise the amount of defrost heat absorbed by the refrigerant or the surroundings.
Minimise frost build-up:-
Have the highest possible evaporating temperature.
Ensure low humidity around the evaporator.
Defrost on demand – using sensors.

13. Oil Control in Evaporators
Compressor lubricating oil flows around the system with the refrigerant.
It is important that the oil returns to the compressor.
However, it can drop out of the solution with the refrigerant in the Evaporator.
Oil will coat the tubes if it collects in the Evaporator and reduce heat transfer (it will act as an insulator).
In DX Evaporators an adequate refrigerant ‘velocity’ must be maintained to carry the oil through the tube assembly and suction line and thus return it to the compressor at all load variations/conditions.
With flooded Evaporators the oil can be removed if necessary – the method depending on the refrigerant type.

14. Efficiency Evaporator Efficiency Issues:-
In a DX air cooler the fin block should be kept clear of dirt and slime and adequately de-frosted if necessary.

15. Evaporator Efficiency Issues
Tubes in a Shell and Tube Evaporator should be cleaned to prevent fouling and corrosion (water treatment will be required).
Cooling medium flow should be maintained – pump and fan motors must work.
Oil should not be allowed to build up in the Evaporator.
The flow of refrigerant through the Evaporator should be correctly controlled to ensure full use of its capacity with minimum superheat.

16. Reference Material
“Heating, Ventilation, Air Conditioning and Refrigeration”, CIBSE Guide B, Chartered Institute of Building Services Engineers, 2005 “Energy Efficiency in Buildings”, CIBSE Guide F, Chartered Institute of Building Services Engineers, 2005 “CIBSE KS13: Refrigeration”, CIBSE Knowledge Series, Chartered Institute of Building Services Engineers, 2008 BSRIA Guide AG 15/2002 –” Illustrated Guide to Mechanical Building Services” Carbon Trust Good Practice Guide GPG280 – “Energy efficient refrigeration technology – the Fundamentals” ROGERS and MAYHEW: “Engineering Thermodynamics: Work and Heat Transfer” TROTT, A. R. (2000), “Refrigeration and Air-Conditioning (3rd ed.)” WANG, S. K.: “Handbook Of Air Conditioning And Refrigeration” JONES, W. P.: “Air Conditioning Applications and Design” “BS EN 378: Specification for Refrigeration Systems and Heat Pumps; Part 1: 2000: Basic Requirements, Definitions, Classification and Selection Criteria; Part 2: 2000: Design, Construction, Testing, Marking, and Documentation; Part 3: 2000: Installation Site and Personal Protection; Part 4: 2000: Operation, Maintenance, Repair and Recovery”, London: British Standard Institution, 2000

17. LEVEL 3 Air Conditioning ENERGY ASSESSORS TRAINING
ANY QUESTIONS OR FEEDBACK ON ANY SLIDE
Any questions or clarity needed over this topic and slides 17

18. Contacts:- STROMA Certification Ltd – Contacts Web Links
Contacts:-
STROMA Certification Ltd.
4 Pioneer Way, Castleford, WF10 5QU

19. End of this section