VFDs for Refrigeration
Evaporators
Todd B. Jekel, P.E., Ph.D.
IRC
1
Objective
Overview of Variable Frequency Drives
(VFDs)
„ Evaporator fan operation
„
Part-load
„
operational considerations
Comparison with fixed speed
Opportunities
Challenges
Notes:
•Other names for a VFD are
•Variable Speed Drive (VSD)
•Adjustable Speed Drive (ASD)
•Adjustable Frequency Drive (AFD)
2
Variable Frequency Drives
Good VFD Candidate
1,500
„
Good applications
Large
Hours per Year
Poor VFD Candidate
1,000
motors
High hours per
year operation
Frequent part-load operation
Variable torque processes
500
0
100
„
95
90
85
80
75
70
65
60
55
50
45
40
35
30
Percent of Design Load
As speed is reduced, so is torque
Fans and centrifugal pumps
Allows application without overheating the motor at low
speeds
3
Variable Frequency Drives
„
Motor requirements
Inverter-duty
„
may be necessary
VFD requirements & characteristics
must be within 50-100 ft of application†
May apply a single drive to more than one motor
Drive
„
„
Size drive for total connected horsepower
Individual motor over-current protection required
Startup
„
torque is reduced
Power factor
Near
†
unity (1) for VFDs w/harmonics-mitigating equip.
manufacturer dependent
4
Variable Frequency Drives
„
Applicable fan laws
N
N full −load
„
CFM
CFM full −load
hp
hp full −load
⎛ CFM
=⎜
⎜ CFM
full −load
⎝
⎞
⎟
⎟
⎠
3
Limitations
Typical
„
=
minimum motor speeds between 20-30-Hz
Impact on heat exchange
⎛ CFM
Capacity
=⎜
Capacity full −load ⎜⎝ CFM full −load
⎞
⎟
⎟
⎠
0.76
5
Fan Horsepower Impact of VFD
Rearranging results in
1
0.9
hp / hp full-load
„
0.8
hp
0.7
hp full −load
⎛ Capacity
=⎜
⎜ Capacity
full −load
⎝
⎞
⎟
⎟
⎠
3.95
= PLR 3.95
0.6
0.5
0.4
0.3
0.2
0.1
0
0.4
0.5
20-Hz
0.6
30-Hz
0.7
0.8
0.9
1
PLR
6
VFD Benefits
„
Reduced system power
Drastically
reduced evaporator fan
horsepower requirement at part-load
Lower refrigeration load from fans
„
(5-hp equals 1 ton of refrigeration)
Potentially fewer system transients
„ Increased motor life
„
Less
motor cycling
Inherently “soft-start”
7
VFD Benefits (continued)
„ Improved
power factor
(especially
on small horsepower motors)
„ Decreased
noise and “wind-chill”
„ Increased control, more stable
temperature control
Notes:
•Power factor on a standard efficiency motor less than 1-hp can be as low as 0.59.
•Power factor on a premium efficiency motor less than 1-hp can be as low as 0.78.
8
VFD Drawbacks
„
Drive losses (~4-6%, losses increase at low
loads)
Loss of evaporator “throw”
„ Typical systems have large number of
small evaporator fan motors (cost)
„ Additional equipment to maintain
„ Resonance of equipment (natural frequency)
„ Power quality
„ Siting of the drive
„
9
When to considered VFDs
„
„
Load requires close temperature control
Large fans and motors
Blast
freezers, penthouse freezer evaporators with
ducting, etc.
„
Low TD installations
Not
„
necessarily requiring low TD for space conditions
Significant and frequently occurring part-load
operation
Northern
„
climates
High electricity rates
10
Impact of Evaporator Liquid
Feed Configuration
„
Direct Expansion
Care
must be taken with sizing of thermal expansion
valve and distributor, and coil circuiting for low load
conditions
„
Gravity Flooded
Good
„
fit because liquid feed is proportional to load
Overfeed
Liquid
supply rate is independent of load
Suction riser should be sized to overfeed at part-load
conditions
11
How much can I save?
„
Evaporator fan horsepower usually a small
fraction of the system horsepower at full-load
Low
TD load requirements result in larger
contribution to the system horsepower & parasitic
refrigeration load
„
Part-load
Defined
as actual load divided by the installed
evaporator capacity
If no fan control, the fan horsepower contribution
to the system horsepower is constant
12
Analysis Assumptions
„
Evaporator
TD
„
= 12oF for cooler and 8oF for freezer
VFD costs
Assume
3-hp VFD for each evaporator
Installation 15 hours/VFD by electrician @
$55/hour
„
Energy costs
Blended
$0.05/kWh
13
Compressor + Evaporator
kW/ton
1.5
3
N evap =5
Fixed Speed / Variable Suction
Duty Cycling / Fixed Suction
1.3
TD design=11.7 [F]
Variable Speed / Fixed Suction
1.2
Variable Speed / Variable Suction
1.1
1
0.9
0.8
0.7
0.6
20
30
40
50
60
70
Fixed Speed / Fixed Suction
T space =35 [F]
80
90
Percent Load
100
Compressor + Evaporator kW/ton
Compressor + Evaporator kW/ton
Fixed Speed / Fixed Suction
1.4
T space =-20 [F]
Fixed Speed / Variable Suction
2.8
N evap =7
TD design=8.5 [F]
Duty Cycling / Fixed Suction
Variable Speed / Fixed Suction
2.6
Variable Speed / Variable Suction
2.4
2.2
2
1.8
1.6
20
30
40
50
60
70
80
90
100
Percent Load
Assumptions:
Cooler:
•Fully loaded single-stage screw compressor with thermosiphon oil cooling, average
discharge pressure of 165 psia (85oF), includes package losses
•Compressor motor efficiency of 93%
•Evaporator fan motor efficiency of 78% (0.75 hp, 3-phase, 460 volt, 1,140 rpm)
•VFD drive efficiency of 96% at full-load
Freezer:
•Fully loaded two-stage screw compressor with thermosiphon oil cooling, average
discharge pressure of 165 psia (85oF), includes package losses
•Compressor motor efficiency of 93%
•Evaporator fan motor efficiency of 78% (0.75 hp, 3-phase, 460 volt, 1,140 rpm)
•VFD drive efficiency of 96% at full-load
14
35F cooler
Hours per Year
1,000
500
0
100
95
90
85
80
75
70
65
60
55
50
45
40
Percent of Design Load
2,000
Hours per Year
Load Profiles
1,500
35
30
-20F Freezer
1,500
1,000
500
0
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
Percent of Design Load
Assumption:
•22.5 hours per day in refrigeration mode
15
VFD Cost
VFD Cost per Horsepower
$3,000
Source: Grainger (Wholesale Price)
Manuf acturer: Fuji Electric (GE)
Pow er supply: 3-phase, 460-V olt
A pplication: V ariable Torque
$2,500
$2,000
VFD Model
AF-300 P11, NEMA 1
AF-300 P11, NEMA 4
AF-300 C11
$1,500
$1,000
$500
$0.1
1
10
100
Horsepower
Notes:
•Prices taken from www.grainger.com in December, 2003.
•Less expensive VFDs are available if a suitable enclosure already exists or is part
of the new installation.
•NEMA 4 (waterproof) requirements results in higher cost, particularly for small
horsepower drives.
16
Economic Analysis
Cooler (35oF)
Freezer (-20oF)
From always on fan control to VFD
Savings per ton
Capital cost per
ton†
Installation cost per
ton
Simple payback
$45
$60
$65
$95
$40
$60
2.3 years
2.6 years
From cycling fan control to VFD
Savings per ton
Simple payback
†
$30
$40
3.8 years
3.6 years
Purchase of a single drive to operate all fan motors (4) on evaporator.
17
Closing Thoughts
„
Reasonably short payback (<3 years) compared
to always running the fan
Payback
will be shorter with evaporators requiring
larger horsepower drives
Longer if cannot use single drive per evaporator
„
Ask questions prior to implementation
If
retrofit
„
„
Is motor compatible with VFD?
Is resonance at lower fan speeds an issue?
Check
actual current draw on motors prior to sizing
drive
„
Fans require and motors can deliver more power at low
temperatures
18
Additional Resources
„
Northwest Energy Efficiency Alliance
Evaporator Fan VFD Initiative
Baseline
Market Evaluation Report, April 1999
Market Progress Evaluation Report No 2.,
November 2000
Market Progress Evaluation Report No 2.,
June 2002
Reports available at www.nwalliance.org
19
Controls
„
On-Off
Controller
turns liquid feed solenoid on or off to try to
maintain set point
„
Modulating (solenoid not necessarily controlled)
Proportional
„
The output of the control element is proportional to the
deviation from the set point
Proportional-Integral
„
(PI)
Adjusts control point based on accumulated deviation from
set point
Proportional-Integral-Derivative
„
(PID)
Adjusts control point based on accumulated deviation and on
rate of change
20
On-Off Control
39
Space Temperature, F
38
Solenoid
On
Solenoid
On
Solenoid
On
37
36
Deadband
Setpoint
35
34
Solenoid
Off
Solenoid
Off
Solenoid
Off
33
Time
21
Proportional Control
39
Space Temperature, F
38
37
60-Hz (maximum)
36
35
Setpoint
Offset
34
20-Hz (minimum)
Startup
33
Time
22
PID Control
39
Space Temperature, F
38
37
60-Hz (maximum)
36
35
Setpoint
34
20-Hz (minimum)
Startup
33
Time
23