ASHRAE Journal - February 2011 - 72

emerging technologies	
Bin No. j 1 2 3 4 5 6 7 8 Bin Temperature Range (°F) 65 – 69 70 – 74 75 – 79 80 – 84 85 – 89 90 – 94 95 – 99 100 – 104 Representative Bin Temperature Tj (°F) 67 72 77 82 87 92 97 102 Cooling Load, Fraction of Rated Capacity 0.061 0.212 0.364 0.515 0.667 0.818 0.970 1.121 Fraction of Total Temperature Bin Hours nj/N 0.214 0.231 0.216 0.161 0.104 0.052 0.018 0.004

table 1:	Distribution	of	fractional	hours	in	temperature	bins	for	calculation	of	SEER	for	central	air-conditioning	units	with	a	twospeed	or	a	variable	speed	compressor.1 in widespread use for many years in Asia in ductless unitary AC (mini-splits) and in variable refrigerant flow AC systems. Typical speed ranges (and turndown ranges) are 3:1 and 4:1, often with a maximum speed above 60 Hz and 3,600 rpm. The limiting factor for the minimum speed is generally maintaining adequate lubrication, with oil pumping failing below a minimum speed. Over the operating speed range, efficiency tends to be maintained close to the design speed efficiency. Rapid loading/unloading of a scroll compressor operates by venting the axial force balancing pressure, enabling the orbiting and fixed scrolls to separate by several thousandths of an inch. Refrigerant vapor compression stops and the motor power drops to a low (but greater than zero) level. By varying the proportion of loaded vs. unloaded time, the capacity can be modulated continuously from 100% down to 10%. The nonzero power consumed when running unloaded is a loss that is relatively insignificant at moderate turndown, down to 50%. As the capacity is reduced further, the unloaded power becomes a more significant part of the total input. compression ratio, but since less capacity tends to be needed at lower outdoor temperatures, the impact on efficiency can be reasonably small. Tandem	Compressors A simple way to provide stepped capacity modulation in an air-conditioning system is to divide the required compressor capacity between two compressors, with either an equal or unequal split of capacities. Tandem configurations are arranged to provide balanced oil return to the two compressors and/or provide an oil equalization line between the two compressor oil sumps. The efficiency impact, which applies at both full capacity and half capacity is that in the residential capacity range, the compressor efficiency of larger compressors is higher by 5% to 10% than that of compressors with half the capacity. Two-Speed	Operation An alternative to electronically driven, continuously variable speed is two-speed operation. Half-speed efficiency is approximately 10% less than full-speed efficiency.

step-Wise capacity modulation
Several methods are used to provide step-wise, or staged, capacity modulation. Cylinder	Inactivation A two-cylinder reciprocating compressor is configured so that both pistons are driven at the full stroke when the rotation is in one direction. When the rotation direction is reversed, an eccentric bushing on one of the crank pins rotates 180° and reduces the stroke of one of the pistons to zero, reducing the capacity to approximately half (depending on the relative displacement of the two cylinders). The efficiency at reduced capacity decreases somewhat due to the fall-off of motor efficiency at reduced motor load and the continued dissipation of friction power in the bearings of the inactive cylinder. Capacity	Reduction	Port	in	a	Scroll	Compressor A port can be added partially into the spiral of the stationary scroll that vents a portion of the trapped refrigerant vapor back to suction before compression begins. This reduces the built-in
72	 ASHRAE	Journal	

energy saving Potential
Three primary factors account for the energy savings obtained with capacity modulation: significant reduction of losses associated with on-off cycling, more efficient use of the heat exchangers (the condenser and evaporator), and reduced blower energy consumption. On-Off	Cycling	Losses Several small losses are incurred when cycling the air-conditioning system on and off. Depending on the system configuration, these can include motor starting power, refrigerant charge bleeding from the high to low side, reevaporation of moisture from the evaporator coil surface, and the initial delivery of warm, moist air when restarting, before the evaporator cools back down to the steady-state operating temperature. Effective	Use	of	Heat	Exchangers Continuous operation of cooling equipment at reduced capacity, instead of on-off operation at full capacity, results in less
ashrae.org	 	 February	 2011



ASHRAE Journal - February 2011

Table of Contents for the Digital Edition of ASHRAE Journal - February 2011

ASHRAE Journal - February 2011
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Thermal Coupling of Cooling and Heating Systems
10 Common Problems in Energy Audits
Hall of Fame Feature: History of the Changing Concepts in Ventilation Requirements
A Guide to Wireless Technologies
Building Sciences
Solar NZEB Project
Emerging Technologies
People
Special Section
InfoCenter
Commissioning
Products
Washington Report
Classified Advertising
Advertisers Index
ASHRAE Journal - February 2011 - ASHRAE Journal - February 2011
ASHRAE Journal - February 2011 - Cover2
ASHRAE Journal - February 2011 - 1
ASHRAE Journal - February 2011 - 2
ASHRAE Journal - February 2011 - Contents
ASHRAE Journal - February 2011 - Commentary
ASHRAE Journal - February 2011 - 5
ASHRAE Journal - February 2011 - Industry News
ASHRAE Journal - February 2011 - 7
ASHRAE Journal - February 2011 - Letters
ASHRAE Journal - February 2011 - 9
ASHRAE Journal - February 2011 - 10
ASHRAE Journal - February 2011 - 11
ASHRAE Journal - February 2011 - 12
ASHRAE Journal - February 2011 - 13
ASHRAE Journal - February 2011 - 14
ASHRAE Journal - February 2011 - 15
ASHRAE Journal - February 2011 - Meetings and Shows
ASHRAE Journal - February 2011 - 17
ASHRAE Journal - February 2011 - Thermal Coupling of Cooling and Heating Systems
ASHRAE Journal - February 2011 - 19
ASHRAE Journal - February 2011 - 20
ASHRAE Journal - February 2011 - 21
ASHRAE Journal - February 2011 - 22
ASHRAE Journal - February 2011 - 23
ASHRAE Journal - February 2011 - 24
ASHRAE Journal - February 2011 - 25
ASHRAE Journal - February 2011 - 10 Common Problems in Energy Audits
ASHRAE Journal - February 2011 - 27
ASHRAE Journal - February 2011 - 28
ASHRAE Journal - February 2011 - 29
ASHRAE Journal - February 2011 - 30
ASHRAE Journal - February 2011 - 31
ASHRAE Journal - February 2011 - 32
ASHRAE Journal - February 2011 - 33
ASHRAE Journal - February 2011 - Hall of Fame Feature: History of the Changing Concepts in Ventilation Requirements
ASHRAE Journal - February 2011 - 35
ASHRAE Journal - February 2011 - 36
ASHRAE Journal - February 2011 - 37
ASHRAE Journal - February 2011 - 38
ASHRAE Journal - February 2011 - 39
ASHRAE Journal - February 2011 - 40
ASHRAE Journal - February 2011 - 41
ASHRAE Journal - February 2011 - 42
ASHRAE Journal - February 2011 - 43
ASHRAE Journal - February 2011 - A Guide to Wireless Technologies
ASHRAE Journal - February 2011 - 45
ASHRAE Journal - February 2011 - 46
ASHRAE Journal - February 2011 - 47
ASHRAE Journal - February 2011 - 48
ASHRAE Journal - February 2011 - 49
ASHRAE Journal - February 2011 - Building Sciences
ASHRAE Journal - February 2011 - 51
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ASHRAE Journal - February 2011 - 53
ASHRAE Journal - February 2011 - 54
ASHRAE Journal - February 2011 - 55
ASHRAE Journal - February 2011 - 56
ASHRAE Journal - February 2011 - 57
ASHRAE Journal - February 2011 - 58
ASHRAE Journal - February 2011 - 59
ASHRAE Journal - February 2011 - 60
ASHRAE Journal - February 2011 - 61
ASHRAE Journal - February 2011 - Solar NZEB Project
ASHRAE Journal - February 2011 - 63
ASHRAE Journal - February 2011 - 64
ASHRAE Journal - February 2011 - 65
ASHRAE Journal - February 2011 - 66
ASHRAE Journal - February 2011 - 67
ASHRAE Journal - February 2011 - 68
ASHRAE Journal - February 2011 - 69
ASHRAE Journal - February 2011 - Emerging Technologies
ASHRAE Journal - February 2011 - 71
ASHRAE Journal - February 2011 - 72
ASHRAE Journal - February 2011 - 73
ASHRAE Journal - February 2011 - 74
ASHRAE Journal - February 2011 - 75
ASHRAE Journal - February 2011 - People
ASHRAE Journal - February 2011 - 77
ASHRAE Journal - February 2011 - InfoCenter
ASHRAE Journal - February 2011 - 79
ASHRAE Journal - February 2011 - 80
ASHRAE Journal - February 2011 - 81
ASHRAE Journal - February 2011 - 82
ASHRAE Journal - February 2011 - 83
ASHRAE Journal - February 2011 - 84
ASHRAE Journal - February 2011 - 85
ASHRAE Journal - February 2011 - Commissioning
ASHRAE Journal - February 2011 - 87
ASHRAE Journal - February 2011 - 88
ASHRAE Journal - February 2011 - 89
ASHRAE Journal - February 2011 - 90
ASHRAE Journal - February 2011 - Products
ASHRAE Journal - February 2011 - Washington Report
ASHRAE Journal - February 2011 - Classified Advertising
ASHRAE Journal - February 2011 - 94
ASHRAE Journal - February 2011 - 95
ASHRAE Journal - February 2011 - Advertisers Index
ASHRAE Journal - February 2011 - Cover3
ASHRAE Journal - February 2011 - Cover4
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