Chiller Type

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FACTORS FOR CONSIDERATION IN

CHILLER-COMPRESSOR SELECTION

Stephen W. Duda, PELEED AP

Director, Mech Engineering

Ross & Baruzzini, Inc.

[email protected]

St. Louis, MO

Indianapolis, IN

Miami, FL

ASHRAE 2008 Winter Meeting / New York, NY

January 22, 2008 / Seminar #63

TC 8.02 Centrifugal Machines


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THE PREMISE

•• Suppose you need to select and specify a new Chiller forSuppose you need to select and specify a new Chiller forcommercial comfort cooling at typical ARI conditions.commercial comfort cooling at typical ARI conditions.

•• Advancements in technology, refrigerants, andAdvancements in technology, refrigerants, and

manufacturer offerings all affect which compressionmanufacturer offerings all affect which compression

technology is best suited for a given application.technology is best suited for a given application.

•• So, which compressor technology should you select?So, which compressor technology should you select?

•• Scroll?Scroll?•• Reciprocating?Reciprocating?

•• Rotary Screw?Rotary Screw?

•• Centrifugal?Centrifugal?


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• Availability• Noise

• Weight

• Footprint

• Energy Consumption

• Capacity Variability & Control

• Maintenance & Reliability

• Refrigerant Considerations

• Price

SOME FACTORS TO CONSIDER


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AVAILABILITY

(3.5 kW)(3.5 kW) (35 kW)(35 kW) (350 kW)(350 kW) (3500 kW)(3500 kW)

Approximate Liquid Chiller Availability

Range by Compressor Type

1 10 100 1000 10000

Reciprocating

Scroll

Screw

Centrifugal

Cooling Capacity (TonsR)


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• Above approximately 800 tons (2800 kW), centrifugal

chillers are the only option in a single machine selection.

• While at least one manufacturer markets a centrifugal

chiller down to 125 tons (440 kW), many say that 200 to

250 tons (700 to 880 kW) is the practical lower limit forcentrifugal machines.

• The chiller sales representatives interviewed for this

seminar tend to recommend against reciprocatingcompressors in favor of scroll compressors.

• For the purpose of this seminar, a water-cooled chiller

selection of about 240 tons (844 kW) will be followed for

comparison of options.

AVAILABILITY


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• Sound pressure levels for centrifugals, scrolls, and screwmachines are comparable.

• The presenter found a greater variation in sound pressure

levels between manufacturers than between compressor

types within a manufacturer.

• Centrifugal chiller noise is somewhat speed dependent

(some gear-drive machines rotate at a very high rpm).

• The typical screw chiller has a pure tone noise that some

may find bothersome, particularly if located near occupied

spaces or sleeping quarters.

NOISE


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• 240-ton (844 kW) Screw Chiller typical operating weightrange 13,000 to 16,000 lbs (6,000 to 7,200 kg).

• 240-ton (844 kW) Centrifugal Chiller typical operating

weight range 11,000 to 20,000 lbs (5,000 to 9,100 kg).

• A pair of 120-ton (422 kW) Scroll Chillers net typical

operating weight is 10,000 to 12,000 lbs (4,500 to 5,400 kg).

• The more efficient chillers often weigh more, due to

extended shell and tube length.

• Weight may be a consideration if located on an upper

floor of a building, especially in a retrofit case.

WEIGHT


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• A screw chiller appears to require 5% to 20% less floorspace than a comparable centrifugal chiller.

• Among centrifugal chillers, a greater variation in floorspace consumed is found between manufacturers -- This

is related to speed of rotation of the compressor’s impelleras slower compressors are larger in diameter.

• Many scroll chillers have the advantage of fitting througha 3-foot (1 m) door. However, capacity limitations often

require multiple scroll machines where a single screw orcentrifugal could be applied, resulting in greater net floorspace consumed.

FOOTPRINT


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• 240-ton Screw Chiller typical energy performance range is

0.61 to 0.68 kW/ton peak and 0.52 to 0.56 kW/ton IPLV.

844-kW Screw Chiller typical energy performance range is

5.1 to 5.8 COP peak and 6.2 to 6.8 COP IPLV.

• 240-ton Centrifugal Chiller typical energy performance rangeis 0.51 to 0.60 kW/ton peak and 0.49 to 0.55 kW/ton IPLV.

844-kW Centrifugal Chiller typical energy performance range is

5.8 to 6.9 COP peak and 6.4 to 7.2 COP IPLV.

• A pair of 120-ton Scroll Chillers typical energy performanceis 0.79 kW/ton peak and 0.60 kW/ton IPLV.

2 ea 422-kW Scroll Chillers typical energy performance is about

4.5 COP peak and 5.9 COP IPLV.

ENERGY CONSUMPTION


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• Previous figures do not include the impact of a VariableFrequency Drive (VFD).

• Remember to consider differences in pumping energy

and/or cooling tower energy.

• Energy performance often sets centrifugal chillers apart

from the other compressor technologies.

• Some chillers are more efficient at peak load, while others

perform better at part load, so an accurate load model is

necessary to make a fully informed choice.

ENERGY CONSUMPTION


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• Most centrifugal chillers are available with optional VFD.• At least one screw chiller is available with optional VFD.

• Scroll and reciprocating chillers vary capacity by staging

multiple compressors for incremental capacity control.

• Centrifugal compressors commonly use adjustable

prerotation vanes for capacity control.

• Screw compressors commonly use a slide valve to adjust

the effective length of the compression path.

• Screw compressors do not surge and suffer less capacity

reduction at high condensing temperatures.

CAPACITY VARIABILITY / CONTROL


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• Manufacturers’ quotations for a service contract are notsignificantly different between screw, scroll, and

centrifugal chillers of equal capacity.

• Multiple compressors associated with scroll chillers predict more failures, but less loss of capacity per failure.

• While some centrifugal chillers have been in service for

over 40 years, screw and scroll compressors are asomewhat newer technology. Long-term longevity

comparisons are difficult.

MAINTENANCE & RELIABILITY


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• Screw chillers historically used R-22 and are now

primarily available with R-134a.

• Centrifugal chillers are available with R-134a or R-123.

• Scroll compressors historically used R-22 and are now

coming out in R-407C or R-410A.

REFRIGERANT CONSIDERATIONS


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• 240-ton (844 kW) Screw Chiller typical budget price range

$70,000 to $75,000.

• 240-ton (844 kW) Centrifugal Chiller typical budget price

range $107,000 to $130,000.

• A pair of 120-ton (422 kW) Scroll Chillers typical budget

price range $65,000 to $75,000 (installation additional).

• Higher first cost of centrifugal machines may be offset in

the long term by lower energy consumption, leading to a

Life Cycle Cost analysis . . .

PRICE


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• Compares first & recurring costs

• First Costs:

– Purchase and installation

– Auxiliaries – piping, power supply, etc.• Recurring Costs:

– Energy – Including pumping energy, water use, etc.

– Maintenance – routine, major, and repairs

• Convert recurring costs to a first cost equivalent for

apples-to-apples comparison.

LIFE CYCLE COST ANALYSIS


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• You can do better than simple payback!

• Formulas: Find P given F, A, i & n where

• P is present value

• F is future value• A is recurring value each unit of time

• i is interest rate per unit time

• n is quantity of time units

• P = F (1 / (1 + i)n))

• P = A (((1 + i)n -1) / ( i (1 + i)n))



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• Very Basic Example:

•System A: $75,000 first cost

•System B: $100,000 first cost

•System A: $17,500 annual recurring costs•System B: $14,000 annual recurring costs

•Interest rate is 6.5% per year

•n is number of years

• Run a basic 10-year example.



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• Very Basic Example Results for 10-Year Period

• LCC = P + A (((1 + i)n -1) / ( i (1 + i)n))

• System A:

75000 + 17500 (((1 + 0.065)

10

-1) / ( 0.065 (1 + .065)

10

)) =$200,805

• System B:

100000 + 14000 (((1 + 0.065)10 -1) / ( 0.065 (1 + .065)10)) =

$200,644

• “Simple” payback would have shown a 7-year payback ($3,500

annual energy difference versus $25,000 first cost offset.)

• For more complex analyses, set up a spreadsheet.



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FOR MORE INFORMATION

•• Chillers and/or Compressors: 2004 ASHRAE Handbook ofChillers and/or Compressors: 2004 ASHRAE Handbook ofHVAC Systems and Equipment, Chapters 34 & 38.HVAC Systems and Equipment, Chapters 34 & 38.

•• Watch for the 2008 Edition of the above.Watch for the 2008 Edition of the above.

•• TCs 8.01 and 8.02 have been working hard to update and improveTCs 8.01 and 8.02 have been working hard to update and improveChapters 34 & 38 for 2008.Chapters 34 & 38 for 2008.

•• Life Cycle Cost Methodology: 2007 ASHRAE HandbookLife Cycle Cost Methodology: 2007 ASHRAE Handbook

of HVAC Applications, Chapter 36.of HVAC Applications, Chapter 36.


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THANK YOU

•• Thank you for attending Seminar #63.Thank you for attending Seminar #63.

•• Questions?Questions?

Documents

Chiller Type