Level 3 Air Conditioning Inspections for Buildings

1. . Level 3 Air Conditioning Inspections for Buildings 7. Compressors (Day 2) PRESENTED BY Anthony Balaam aircon@stroma.com

2. Compressors Basics of Compressors • Is to draw ‘low pressure’ refrigerant vapour from the ‘Evaporator’ and compress it to a ‘higher pressure’. • It allows the refrigerant vapourto becondensed back into a liquidby some convenient low-cost source of cooling (ambient air or water). • Compressor housings can be either: 1. hermetic/ 2. semi-hermetic or 3.open-type. • Main types of compressors are:- • 1.Reciprocating Compressors • 2.Screw Compressors • 3.Scroll Compressors • 4.Centrifugal Compressors This area hear High pressure Low pressure

3. Housings 1.Hermetically Sealed:- • Built into the same housing as the compressor parts. • Welded shell with no access to internal parts for servicing or repair. 2.Semi-Hermetically Sealed:- • Assembled with removable covers – limited access for onsite servicing. • Above about8kW motor power, these types are usuallysuction gascooled. • i.e. Refrigerant cools the motor before compression. • Reduces compressor capacity. • Externallycooled types – vapourpasses directly into thecylinders. • Usually about 8% more efficient than the above.

4. Housings 3.Open Type:- • Drive Shaft passes through a rotating vapour seal. • Motor is connected to the external drive shaft – by direct coupling/belts. • Motor must be accurately sized for compressor’sduty and starting torque. • Running a motor below the design duty reduces its power factor and efficiency. • When comparing efficiency with previous types:- • Motor’s efficiency and drive losses need to be taken into account.

5. Compressors 1.Reciprocating Compressors(most common type – variety of sizes) • Suction vapour is compressed by the pistons in close fitting bores. • Suction and discharge valves are either:- • Simple flapper reeds – on smaller compressors. • or Ring valves – on larger machines. • Bearings are lubricated by the refrigeration oil from the crankcase.

6. Reciprocating Chiller Simple flapper reeds

7. Capacity Control Reciprocating Compressors – Capacity Control 1 Larger compressors (>7.5kW nominal motor size) • 1. Multi-modular (compressor staging) • Several compressors make up a chiller (see picture) – each being a stage of capacity. • Chiller control system needs to be compatible with compressor safety controls – should be set to avoid frequent cycling. Sage 1,2,3,4 from left to right

8. Capacity Control Reciprocating Compressors – Capacity Control 2 Larger compressors (>7.5kW nominal motor size) • 2. Cylinder unloading (blocked suction unloading) • Achieved by maintaining oneormore cylindersin theraised positionbyhydraulic pressure. • Allows refrigerant gasto pass back and forth withoutcheck – reducing mass flow through the compressor. • Flow needs to be maintained to minimise overheating and ensureadequate oilreturn. • Insufficient oil return will trip oil safety switch. • Long hours of operation with cylinder unloading should be avoided.

9. Cylinder Unloading

10. Capacity Control Reciprocating Compressors – Capacity Control 3 Larger compressors (>7.5kW nominal motor size) • 3. Speed variation (variable speed drives) • Output is directly proportional to speedof the shaft’s rotation. • Can be changed by varying the speedof the prime mover. • Needs to maintain the minimum speedforeffective lubrication. • Two speed compressors now replaced by inverter drives. • Damage to windings may occur during operation due to:- • fluctuationsinelectricitysupply; • or compressor powerrequirement during speed changes.

11. Capacity Control Reciprocating Compressors – Capacity Control 4 Larger compressors (>7.5kW nominal motor size) • 4. Hot gas bypass (were we discharge vapour recirculation) • Load on the compressor is maintained whilst the ‘Evaporator capacity’is varied. • Most effective arrangement is for the gas to bypass the Condenser and inject therefrigerant into the system ‘downstream’of the‘Expansion Valve’and ‘upstream’of the‘Evaporator’. • No energy economies at part load. • Results in high discharge pressures and should be avoided. • Extensive operationcan damage the compressor.

12. Hot Gas Bypass Shell and Tube Evaporator Cool water to AHU Hot water from the AHU Upstream of the Evaporator Hot Gas Bypass Line Downstream of the Expansion valve Condenser Expansion Valve Compressor

13. Compressors Reciprocating Compressors – Capacity Control 5 Larger compressors (>7.5kW nominal motor size) • 5. Evaporator pressure regulator • It’s a means of maintaining evaporator suction pressure by throttlingthe flow of gas to thesuctionof thecompressor. • Energy efficiency is impaired. • This method should be avoided.

14. Compressors Rotary Compressor Another piston type of compressor. Was developed for smaller applications. Stage 1 Stage 2 Stage 3

15. Compressors 2. Screw Compressors • Two main types:- • Single screw machine • Oil injectedtwin rotor machine

16. Screw Compressors

17. Compressors Screw Compressors • Positive displacement machine with very few moving parts. • Operates at higher pressureratios than reciprocating machine. • Suited for arduous duties calling for extended running under heavy loads. • Operates on a specific internal pressure ratio, determined by its construction and geometry.

18. Screw Compression

19. Compressors Screw Compressors • For optimum efficiency:- Internal pressure ratio = external pressure ratio (in terms of the system) • Where:- • Internal pressure ratio: is determined by the volume ratio of the compressor. • External pressure ratio: exists across the refrigeration system. Modern screw compressors have the ability to adjust their internal geometry to match the varying head pressure.

20. Compressors Screw Compressors • Normally cooled byoil that is injected into the machine to seal the running clearances between the rotorsandcasing, absorbing a significant amount of the compression heat. • For optimum efficiency the oiliscooled externally to the machine in a shell and tube heat exchanger which is cooled by either:- • Refrigerant • Water • Thermo-syphon cooling • Cooling by direct injection of refrigerant into the compressor reduces available capacity – hence reducing efficiency.

21. Capacity Control Screw Compressors - Capacity Control 1 • Sliding Valve • Which adjusts the compressor displacement: • Retards the point at which compression begins. • Reduces the size of the discharge port to obtain volume ratio. • Typically allows 10-100% capacity control. • Below60%full load – efficiency very low.

22. Capacity Control

23. Capacity Control Screw Compressors – Capacity Control 2 • Inverter Driven Motors • Multiple Compressors

24. Compressors 3.Scroll Compressors (hermetic type) • Two scroll components: one fixed, one orbiting. • Compliant type – tolerant to some liquid return and particle contamination. • Quieter and vibrate less than reciprocating type.

25. Compressors

26. Scroll Chiller

27. Compressors Scroll Compressors – Capacity Control 1 • Two speed motors • Ideal for multiple compressors • Variable speed drives • Can be used, however, not ideal – effects lubricationof thesystem. • Not compatible with method ofradial compliance, which prevents damagebysmall quantities of liquid refrigerantorsolid particles. • Electronic modulation system • Momentarily separates the scrolls axially. • Can give between10-100%capacityvariation. • Does noteffect lubrication as shaft continues to rotate.

28. Hot Gas Bypass Condenser Hot Gas bypass Compressor Evaporator

29. Performance Compressor Performance Data • Capacity and power requirements vary with evaporating and condensingtemperatures. • Data is specific to rated suction and liquid conditions. • When comparing compressors, this has to be done at the same conditions. • For accurate comparison this should be done as the most prevalentconditions as opposed to design conditions.

30. Performance COP = Coefficient of Performance

31. Efficiency Compressor Efficiency Issues • Variessignificantly withtype. • Some compressors require ancillaries which absorb power. • For large loads, having multi-modular staging is good for sharing the load. • If these are unevenly sized, the degree of controlisincreased. • Frequent starting and stopping as a result of matching the capacity of an oversized compressor to a load can erode efficiency and reduce reliability.

32. Efficiency Compressor Efficiency Issues • Operation on in-built capacity control should be avoided or minimised, achieved by:- • Avoid the use of a single large compressor. • Selecting a combination of compressor sizes which avoids the need for operation of one or more of the compressors on capacity control. • For multiple compressors – use a control strategy which minimises the operation of the compressors on part-load (especially avoid 2 @ 50%, as opposed to 1 @ 100%).

33. Efficiency

34. Compressors

35. Reference Material “Heating, Ventilation, Air Conditioning and Refrigeration”, CIBSE Guide B, Chartered Institute of Building Services Engineers, 2005 “Energy Efficiency in Buildings”, CIBSE Guide B, 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

36. LEVEL 3 Air ConditioningENERGY ASSESSORS TRAINING ANY QUESTIONS OR FEEDBACK ON ANY SLIDE

37. STROMA Certification Ltd – Contacts Web Links www.stroma.com/certification Contacts:- STROMA Certification Ltd. 4 Pioneer Way, Castleford, WF10 5QU 0845 621 11 11 training@stroma.com

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