ASHRAE Journal - November 2013 - 19

Technical FeaTure
Fea

form of electricity to shaft work. Fortunately, for
the larger electric motors commonly found driving
compressors, the electrical efficiency is high (94%
to 95%) and remains relatively constant until very
low part-load conditions are reached (e.g., below
20%) when the motor efficiency will begin drop
dramatically.
For example, a premium efficiency
150 hp (112 kW) motor having a 95.1%
efficiency at full load will experience
a decrease in efficiency to 89.5% at
20% of full load. When this motor's
load drops to 5% of full load, its efficiency will decrease to 68%.
Figure 5 shows the efficiency of a
twin screw compressor over a range
of part-load conditions in singlestage duty for a range of suction
pressures and a constant discharge
pressure. Rather than expressing
the efficiency as the shaft bhp into
the compressor divided by its capacity in tons, the efficiency in this case
is expressed as the kW of electricity
required to drive the compressor for
each ton of refrigeration it develops. A motor efficiency of 94% is
included.
As noted previously, the efficiency
penalty for part-load operation
becomes more severe as the compression lift increases. For example,
the 0 psig suction case will experience a 50% penalty compared to fullload efficiency at a part-load ratio of
48% while the 25 psig (172 kPa) suction case can unload to 33% before
the comparable efficiency penalty is
reached.
In addition to electric motor inefficiencies, machines equipped with
variable speed drives will also have
electric losses associated with the
drive. Nominal drive losses at fullload conditions for larger horsepower motors are in the range of 3%
to 4% and those losses climb to 10%
and higher at low part loads.3

Compressor Sequencing and Control
All industrial refrigeration systems will be equipped
with more than one compressor for each suction pressure level. There are a number of factors that complicate
the development of compressors sequencing and capacity control for industrial refrigeration systems including
compressors with varying sizes (capacities), age (lower or

www.info.hotims.com/44638-34
N ovem ber 2013

ashrae.org

ASHRAE JouRnAl

19



ASHRAE Journal - November 2013

Table of Contents for the Digital Edition of ASHRAE Journal - November 2013

Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Sequencing & Control of Compressors
Fire Station Renovation
Sewage Plant Heats Village
Standing Columns
Engineer’s Notebook
Building Sciences
Special Supplement
BACnet Today & the Smart Grid
The Performance Gap
Refrigeration Applications
InfoCenter
Energy Modeling
Emerging Technologies
Data Centers
People
Products
Special Products
Geothermal for Big Box Retail
Classified Advertising
Advertisers Index
ASHRAE Journal - November 2013 - Intro
ASHRAE Journal - November 2013 - Cover1
ASHRAE Journal - November 2013 - Cover2
ASHRAE Journal - November 2013 - 1
ASHRAE Journal - November 2013 - 2
ASHRAE Journal - November 2013 - Contents
ASHRAE Journal - November 2013 - Commentary
ASHRAE Journal - November 2013 - 5
ASHRAE Journal - November 2013 - Industry News
ASHRAE Journal - November 2013 - 7
ASHRAE Journal - November 2013 - 8
ASHRAE Journal - November 2013 - 9
ASHRAE Journal - November 2013 - Letters
ASHRAE Journal - November 2013 - 11
ASHRAE Journal - November 2013 - Meetings and Shows
ASHRAE Journal - November 2013 - 13
ASHRAE Journal - November 2013 - Sequencing & Control of Compressors
ASHRAE Journal - November 2013 - 15
ASHRAE Journal - November 2013 - 16
ASHRAE Journal - November 2013 - 17
ASHRAE Journal - November 2013 - 18
ASHRAE Journal - November 2013 - 19
ASHRAE Journal - November 2013 - 20
ASHRAE Journal - November 2013 - 21
ASHRAE Journal - November 2013 - 22
ASHRAE Journal - November 2013 - 23
ASHRAE Journal - November 2013 - 24
ASHRAE Journal - November 2013 - 25
ASHRAE Journal - November 2013 - 26
ASHRAE Journal - November 2013 - 27
ASHRAE Journal - November 2013 - 28
ASHRAE Journal - November 2013 - 29
ASHRAE Journal - November 2013 - Fire Station Renovation
ASHRAE Journal - November 2013 - 31
ASHRAE Journal - November 2013 - 32
ASHRAE Journal - November 2013 - 33
ASHRAE Journal - November 2013 - 34
ASHRAE Journal - November 2013 - 35
ASHRAE Journal - November 2013 - 36
ASHRAE Journal - November 2013 - 37
ASHRAE Journal - November 2013 - 38
ASHRAE Journal - November 2013 - 39
ASHRAE Journal - November 2013 - Sewage Plant Heats Village
ASHRAE Journal - November 2013 - 41
ASHRAE Journal - November 2013 - 42
ASHRAE Journal - November 2013 - 43
ASHRAE Journal - November 2013 - 44
ASHRAE Journal - November 2013 - 45
ASHRAE Journal - November 2013 - 46
ASHRAE Journal - November 2013 - 47
ASHRAE Journal - November 2013 - Engineer’s Notebook
ASHRAE Journal - November 2013 - 49
ASHRAE Journal - November 2013 - 50
ASHRAE Journal - November 2013 - 51
ASHRAE Journal - November 2013 - 52
ASHRAE Journal - November 2013 - 53
ASHRAE Journal - November 2013 - 54
ASHRAE Journal - November 2013 - 55
ASHRAE Journal - November 2013 - Building Sciences
ASHRAE Journal - November 2013 - 57
ASHRAE Journal - November 2013 - 58
ASHRAE Journal - November 2013 - 59
ASHRAE Journal - November 2013 - 60
ASHRAE Journal - November 2013 - 61
ASHRAE Journal - November 2013 - 62
ASHRAE Journal - November 2013 - 63
ASHRAE Journal - November 2013 - 64
ASHRAE Journal - November 2013 - BACnet Today & the Smart Grid
ASHRAE Journal - November 2013 - BCover2
ASHRAE Journal - November 2013 - B1
ASHRAE Journal - November 2013 - B2
ASHRAE Journal - November 2013 - B3
ASHRAE Journal - November 2013 - B4
ASHRAE Journal - November 2013 - B5
ASHRAE Journal - November 2013 - B6
ASHRAE Journal - November 2013 - B7
ASHRAE Journal - November 2013 - B8
ASHRAE Journal - November 2013 - B9
ASHRAE Journal - November 2013 - B10
ASHRAE Journal - November 2013 - B11
ASHRAE Journal - November 2013 - B12
ASHRAE Journal - November 2013 - B13
ASHRAE Journal - November 2013 - B14
ASHRAE Journal - November 2013 - B15
ASHRAE Journal - November 2013 - B16
ASHRAE Journal - November 2013 - B17
ASHRAE Journal - November 2013 - B18
ASHRAE Journal - November 2013 - B19
ASHRAE Journal - November 2013 - B20
ASHRAE Journal - November 2013 - B21
ASHRAE Journal - November 2013 - B22
ASHRAE Journal - November 2013 - B23
ASHRAE Journal - November 2013 - B24
ASHRAE Journal - November 2013 - B25
ASHRAE Journal - November 2013 - B26
ASHRAE Journal - November 2013 - B27
ASHRAE Journal - November 2013 - B28
ASHRAE Journal - November 2013 - B29
ASHRAE Journal - November 2013 - B30
ASHRAE Journal - November 2013 - The Performance Gap
ASHRAE Journal - November 2013 - 66
ASHRAE Journal - November 2013 - 67
ASHRAE Journal - November 2013 - 68
ASHRAE Journal - November 2013 - 69
ASHRAE Journal - November 2013 - 70
ASHRAE Journal - November 2013 - 71
ASHRAE Journal - November 2013 - 72
ASHRAE Journal - November 2013 - Refrigeration Applications
ASHRAE Journal - November 2013 - InfoCenter
ASHRAE Journal - November 2013 - 75
ASHRAE Journal - November 2013 - 76
ASHRAE Journal - November 2013 - 77
ASHRAE Journal - November 2013 - 78
ASHRAE Journal - November 2013 - Energy Modeling
ASHRAE Journal - November 2013 - Emerging Technologies
ASHRAE Journal - November 2013 - 81
ASHRAE Journal - November 2013 - 82
ASHRAE Journal - November 2013 - 83
ASHRAE Journal - November 2013 - Data Centers
ASHRAE Journal - November 2013 - 85
ASHRAE Journal - November 2013 - 86
ASHRAE Journal - November 2013 - People
ASHRAE Journal - November 2013 - Products
ASHRAE Journal - November 2013 - Special Products
ASHRAE Journal - November 2013 - Geothermal for Big Box Retail
ASHRAE Journal - November 2013 - 91
ASHRAE Journal - November 2013 - 92
ASHRAE Journal - November 2013 - 93
ASHRAE Journal - November 2013 - 94
ASHRAE Journal - November 2013 - Classified Advertising
ASHRAE Journal - November 2013 - Advertisers Index
ASHRAE Journal - November 2013 - Cover3
ASHRAE Journal - November 2013 - Cover4
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