1. Waterside
Energy Recovery
Requirements, Design and
Application
Mick Schwedler, PE
manager
Trane
applications engineering
© 2011 Ingersoll Rand

2. reasons to use
Energy Recovery
 Required by code or standard
 Availability of simultaneous heating
and cooling loads
 Economically justified
 Reduces heating load
 Reduces ancillary power
 Environmentally responsible
 Reduces emissions
 Eligible for “green” benefits (energy, water
usage)

3. ASHRAE 90.1-2010 (since 1999)
Waterside Energy Recovery
IF … Facility operates 24 hours per day
 Heat rejection exceeds 6 million Btu/hr
(~ 450 tons)
 Design service water heating load
exceeds 1 million Btu/hr
THEN Required energy recovery is
(smaller of):
 60% of design heat rejection
 Preheating water to 85°F

4. types of
Heat-Recovery Chillers
 Single condenser (“bundle”)
 Dual condenser
heat-
standard
 Equally sized condenser = recovery
condenser
bundles
 Auxiliary condenser
heat-
 Unequally sized standard
condenser > recovery
condenser
bundles

5. heat-recovery chillers
Single Condenser
helical-rotary (screw)
compressor
scroll compressor
centrifugal compressor

6. heat-recovery chillers
Dual Condenser
heat-recovery leaving capacity chiller
condenser water control? efficiency
Full capacity hot yes decreases
Partial capacity warm no increases
heat-recovery
condenser
standard
condenser
evaporator
water-cooled chiller with
centrifugal compressor

7. heat-recovery chillers
Comparison of Options
Chiller condenser option
Characteristic Dual Auxiliary “Heat pump”
Configuration Second, Second, No extra
full-size smaller condenser
condenser condenser
Application Large Preheating Large base-
heating loads heating loads
loads or continuous
operation
Leaving water Hot Warm Hot
Capacity control? Yes No Yes
Chiller efficiency Decreases Increases Acceptable

8. variable air volume
Tempering Supply Air
When required supply/primary airflow is
less than minimum setting:
 Reduce primary airflow to minimum
 Let space temperature drift downward
 Add heat to avoid overcooling
105°F (40.6°C) water often is sufficient
Supply air is always dehumidified

9. waterside heat recovery
Temperatures
Service-water 85°F to 95°F
preheating (29.4°C to 35°C)
Space heating 105°F to 110F
(40.6°C to 43.3C)
source: 2008 ASHRAE Systems
and Equipment Handbook

10. tempering VAV supply air
Heating Coil Selection
selection parameter 1-row coil 2-row coil
Entering water 113°F 105°F
Coil flow rate 4.33 gpm 1.75 gpm
Fluid delta-T 6.02°F 14.91°F
Coil fluid pressure drop 10.3 ft H2O 0.21 ft H2O
Air pressure drop:
design cooling airflow 0.45 in. wg 0.79 in. wg
minimum airflow (est) 0.04 in. wg 0.07 in. wg
Leaving-coil (primary) air 75°F 75°F
Effectively balances
heat-recovery temperatures
and system pressure drops

11. waterside heat recovery
Effect on Chillers
 Compressor work is proportional
to lift
 “Lift” is pressure difference between evaporator
and condenser
 Warmer condenser water (for heat recovery)
raises condenser pressure
 Changes in lift affect different compressors
differently
 Positive displacement
 Centrifugal (full load vs. part load)

12. positive-displacement water chiller
Refrigeration Cycle
heat recovery
5 condenser
2
4 3
pressure
expansion liquid/vapor
compressor
device separator
7 1
evaporator
6
enthalpy

13. positive-displacement water chiller
Capacity
100 compressor
type:
80 scroll
chiller capacity, %
screw
60
40
20
0
0° 10° 20° 30° 40°
temperature rise (leaving-condenser water)

14. positive-displacement water chiller
Efficiency
100 compressor
type:
power increase, % kW/ton
80 scroll
screw (S)
screw (M)
60
screw (L)
40
20
0
0° 10° 20° 30° 40°
temperature rise (leaving-condenser water)

15. centrifugal chiller performance
Power Increase
Heat recovery
40 Chiller 85-105 F 32
power increase, % kW/ton
impeller diameter, inches
compressor
change
30
30
power increase
20
28
10
impeller diameter
0 26
85 87 89 91 93 95 97 99
condenser water temperature, °F

16. centrifugal chiller comparison
Efficiency
Operating mode
Chiller type Cooling Heat recovery
Cooling only 0.57 kW/ton Not applicable
(6.2 COP)
Heat recovery 0.60 kW/ton 0.69 kW/ton
(5.9 COP) (5.1 COP)
Entering to leaving water temperatures:
Evaporator 54°F to 44°F 54°F to 44°F
(12.2°C to 6.7°C) (12.2°C to 6.7°C)
Condenser 85°F to 95°F 85°F to 105°F
(29.4°C to 35.0°C) (29.4°C to 40.6°C)

17. heat-recovery chiller control
Condensing Temperature
unloading with constant
% maximum pressure differential
leaving hot-water temperature
C
A
unloading with constant
B entering hot-water temperature
% load

18. heat-recovery chiller control
Condensing Temperature
Compressor type Acceptable basis of control
Positive displacement Entering-condenser water temperature
Leaving-condenser water temperature
• Provides less capacity
• Uses more power
Centrifugal Entering-condenser water temperature
• Reduces likelihood of surge

19. Energy Recovery Topics
 Airside  Waterside
 Outdoor air  Requirements
 Types
 Types
 Requirements
 Operation  System configurations
 Supply air tempering  Operation
 Requirements
 Operation

20. system configuration
Primary–Secondary
Available heat
off = 150 × (52.6 – 40)
52.6°F 40°F = 1890 MBh
Auxiliary heat required
750 gpm
= 2000 – 1890
52.6°F 40°F = 110 MBh
heat-recovery
300 gpm chiller production
52.6°F (supply)
40°F
distribution
225 gpm
(demand)
40°F
56°F 825 gpm

21. system configuration
Preferential Loading
Available heat
off = 150 × (56 – 40)
51.2°F 40°F = 2400 MBh
Rejected heat
750 gpm
= 2400 – 2000
production
51.2°F (supply) = 400 MBh
40°F
525 gpm distribution
225 gpm
(demand)
56.0°F 40°F
heat-recovery
300 gpm chiller
40°F
56°F 825 gpm

22. system configurations
Sidestream Loading
off Available heat
= 150 × (56 – 42.7)
50.2°F 40°F = 2000 MBh
900 gpm production No rejected heat
50.2°F (supply) No auxiliary heat
51.2°F 40°F
distribution
75 gpm
(demand)
42.7°F
56°F
heat-recovery
300 gpm chiller
40°F
56°F 825 gpm

23. system configurations
Sidestream Loading
Control strategies:
 Satisfy heating requirements
 Maintain leaving-condenser
water temperature
(positive-displacement compressors)

24. system configuration comparison
Heat Available/Required
System configuration
Primary–
Characteristic secondary Preferential Sidestream
Cooling load:
cooling-only units 393 tons 350 tons 383 tons
heat-recovery unit 157 tons 200 tons 167 tons
Heat-recovery 40°F 40°F 42.7°F
supply temperature
Available heat 1890 MBh 2400 MBh 2000 MBh
Auxiliary heat 110 MBh –400 MBh* 0 MBh
required
*Surplus recovered heat must be rejected

25. system configurations
Variable Primary Flow
Piping heat-recovery chiller
in sidestream position may
simplify control
bypass line
modulating control valve
VFD for minimum chiller flow
heat-recovery
chiller
control
valve

26. system configurations
Distributed Sidestream
Typical application:
 Remote heating requirement
 Chilled water load
 Small chiller (or water-to-water heat pump)
heating
load
heat-recovery chiller
chilled water
supply or return

27. airside system options
Load-shedding
economizer
heat-recovery chiller
heating
load
chilled water
supply or return
Control cooling load so
heat rejection equals
heating load
outdoor-air controller
temperature sensor

28. airside system options
Loading chiller with
exhaust airstream
Water from chiller
Water to chiller
EA RA
C
EA
space
T
H T C H
OA MA CA SA

29. single condenser
Heat-Recovery Control
controller
V1
cooling
tower
heating
T1
load
V2
heat condenser
exchanger
P P water-cooled
T2 chiller
controller P
evaporator
cooling
load

30. Dual-Condenser Chillers
heat-recovery
condenser
standard
condenser
piped to cooling tower
centrifugal
heat-recovery chiller

31. dual condenser
Heat-Recovery Control
controller
cooling T2
tower
P heating
V2 controller load
standard
condenser T1
heat-
P recovery P
condenser
auxiliary
water-cooled heat
P
evaporator chiller
Control based on
entering-condenser
water temperature
cooling
load

32. Analysis Tools
tool application
System Analyzer™ High-level scoping (< 1 hr)
TRACE™ Chiller Plant Simplified building entries
Analyzer
Full analysis of chilled water
plant, economic rates
EnergyPlus, HAP, TRACE Full energy simulation
Hour-by-hour calculations of
energy consumption, power
demand, related costs

33. Waterside Energy
Recovery Steps
 Simultaneous heating  Place the chiller(s) in
and cooling loads the appropriate system
location
 Chiller HR capacity =
Design heat recovery  Design the system with
load the proper connections
and controls
 Select lowest
temperature that  Train the building
meets requirements operators
 Select the proper  Operate the system
chiller type properly
 Analyze the system

34. waterside heat recovery
References
From Trane:
Waterside Heat Recovery in HVAC Systems
SYS-APM005-EN
1991 Engineers Newsletter: “Two Good
Old Ideas Combine to Form One New Great Idea”
http://www.trane.com/commercial/library/EN20-1.pdf
By others:
2008 ASHRAE Handbook: HVAC Systems and Equipment
chapter 8
2003 ASHRAE Journal: “Energy Efficiency for Semiconductor
Manufacturing Facilities”
Ralph M. Cohen, PE (August issue)

35. waterside heat recovery
References
2008 ASHRAE Handbook: HVAC Systems and Equipment
chapter 8
2003 ASHRAE Journal: “Energy Efficiency for Semiconductor
Manufacturing Facilities”
Ralph M. Cohen, PE (August issue)