9/5/2013
HEAT PIPE CONTROLS FOR EXHAUST AIR
HEAT RECOVERY APPLICATIONS
Tom Brooke MBA, PE, CEM, Member ASHRAE
INTRODUCTION
Heat Pipes belong to the class of HVAC air-to-air heat exchangers that transfer only sensible (heat)
energy. By design, they trade off the ability to transfer latent energy (moisture) to be the simplest, most
reliable, most economical, and easiest to maintain of all heat exchangers, and they also have the least
ability of transferring any contamination between air streams of all heat exchangers used in residential,
commercial and industrial HVAC systems. But they share the same industry standards for their testing1,
use2 and certification3 as energy wheels and plate heat exchangers which also transfer latent energy.
This paper provides an intermediate level of instruction on all aspects of commercial heat pipe HVAC
systems used to recover and exchange energy from a building’s return air (RA) at Point C to precondition
a building’s incoming outside air (OA) at Point A in Figures 1 and 2. The heat pipes straddle two different
airstreams (sometimes called a parallel application4) and the psychrometrics show heating of the winter
OA and subsequent cooling of the EA. Of course, it is also active in summer by cooling the OA from
heating the EA.
In this example, note that as the Return
condensation per hour per 1000 CFM
from the RA/EA airstream. It is evident
Point A
Outside Air
40.0 ºF DB
25.2 ºF DP
12.6 Btu/lb.
Point D
Exhaust Air
50.7 ºF DB
50.7 ºF DP
20.7 Btu/lb.
HEAT PIPES
Air (RA) gives up heat to the incoming
OA, it is also cooled below its dew-point
(DP). Therefore, there are 2.1 lbs. of
63.7 ºF DB
25.2 ºF DP
18.3 Btu/lb.
72.0 ºF DB
52.2 ºF DP
26.4 Btu/lb.
Point B
Supply Air
Point C
Return Air
Figure 1
1
ASHRAE Standard 84 “Method of Testing Air-to-Air Heat Exchangers”
AHRI Standard 1060 “Performance Rating of Air-to-Air Heat Exchangers for Energy Recovery Ventilation
Equipment”
3
AHRI OM 1060 “Operations Manual for Air-to-Air Energy Recovery Ventilation Equipment”
4
The use of the word “parallel” in this context follows AHRI convention that “parallel” airflows provide
conditioning of the OA from the separate EA while “series” airflows provide conditioning of the air downstream of
the cooling coil from the same air upstream of the cooling coil. It has no connection to its meaning in the context
of parallel, counter flow or cross flow air streams.
2
1
3065
65
60
PSYCHROMETRIC
CHART
65
25
POINT C
RA
Normal Temperature
I-P Units
POINT D
EA
SEA LEVEL
BAROMETRIC PRESSURE: 29.921 in. HG
20 50
55
BU
L
BT
60
EM
55PER
50
5
5
This paper does not include a
%
45
90
50
description of the controls for
%
80
15 40
70%
45
the pumped heat pipes because
POINT B
POINT A
60%
SA
40
OA
they’re completely different
50 %
35
%
0
4
from all other heat pipes and
30 %
10
deserve their own discussion.
20%
IDITY
10% RELATIVE HUM
Otherwise, the reader is
35
40
45
50
55
60
65
DRY BULB TEMPERATURE - °F
expected to already understand
the basic physical and
psychrometric aspects of heat
pipes used for Exhaust Air (EA)
Figure 2
energy recovery. Considerable
further basic technical information may
be found at http://heatpipe.com/HomePage/mktg_materials/Papers.html.
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
60
WE
T
DEW POINT TEMPERATURE - °F
that drain pans (to be covered
in a future paper) and controls
to prevent frosting and freezing
(see page 4) must be addressed
in the system design. However,
unlike other types of air-to-air
heat exchangers, there is no
chance that this moisture
removal by heat pipes carries a
contamination risk.
45
40
35
AT
UR
E-
°F
30
25
20
10
0
70
75
80
GENERAL HEAT PIPE CONTROL CONSIDERATIONS
The heat pipes’ simplicity extends to their controls which can be categorized into either air-side type or
fluid-side type, the fluid being the two phase heat transfer fluid at the heart of all heat pipes. As shown
in Figure 3, air-side control is used for both frost/freezing control, and ventilating/economizer control;
however, fluid-side control is only appropriate for frosting/freezing control. The reader should also
consult the latest locally approved building codes5 for the minimum requirements of these controls.
Air-side control consists of using standard HVAC system modulating actuators and control dampers. The
actuators may be pneumatic, electric or electronic and the dampers are usually opposed blade type
because of their control characteristics, and may be standard, low leakage, industrial grade, etc. When
used, face and bypass dampers are usually oppositely linked. The actuators and dampers may be either
5
Particularly relevant is ASHRAE Standard 90.1-2010 “Energy Standard for Buildings Except Low-Rise Residential
Buildings”, including paragraphs 6.4.3 and 6.5.1.1.
2
factory furnished and installed or field furnished and installed in smaller systems; they are usually field
furnished and installed in larger systems.
Heat
Pipe
Size
All
Large
Control
Type
AirSide
FluidSide
Range
0100%
0100%
Control
Application
Frosting/
Freezing
Ventilation/
Economizer
Frosting/
Freezing
Description
Increments
Dampers
Medium
Depends on
Preheater
Stages
Preheater
Dampers
Medium
Stepper
Valves
Fine
Figure 3
The last column of Figure 3 highlights the different levels of incremental control that would be
appropriate for different types of facilities. But there are two other important decision criteria for the
system designer considering air-side controls. The first is to consider the volume of space needed for the
air bypass. While 5,000 CFM only requires 2.5 ft2 of cross sectional duct area6, easily found in most
projects, 40,000 CFM requires 20.0 ft2 of cross sectional duct area, not so easily available. When also
considering the necessary duct lengths and turning radii, few mechanical spaces have that much volume
available without expressly allowing for it.
The second criteria that should be taken into account is the control system’s response when the face
velocity is reduced across a heat pipe (say, from a bypass damper opening) and its Effectiveness
therefore increases, and vice versa. That is, the percentage of available heat transferred from the high
temperature source to the lower temperature sink increases (e.g., in winter, a greater percentage of
available heat transfers from the warmer exhaust air stream to the colder outside air stream). The
winter increase in temperature across the heat pipe could then be noted by monitoring temperature
sensors as abnormally high; however, recall that the absolute value of the BTUH transferred does
actually decrease because of the lower CFM. The point is that there can be a natural variability in the air
temperature change across a heat pipe.
Fluid-side control inherently eliminates both of the above air-side control considerations. Fluid-side
control means the refrigerant flow is controlled by valve(s) positioned in the heat pipes’ refrigerant
circuits. Depending on its size, application, and need for “fineness” of control, a given EA energy
recovery heat pipe system may have up to six refrigerant circuits in it, and each circuit may have a
stepper valve. For example, it’s not unusual for a heat pipe system to only have valves on half the
circuits because of either the budget or there simply is no need to go beyond that breath of control.
They are also available in a variety of voltages and frequencies, do not draw current except when the
valve position changes and are often controlled by microprocessors in banks of two.
6
While design duct velocities range from 500 to 3,000, 2,000 fpm is used here.
3
The reader may still occasionally find reference to tilt control of heat pipes. That goes back to early heat
pipe designs that, no matter how large they were, had to be physically repositioned (tilted) to enable
either summer or winter operation; this of course causes air leakage at the two transitions over time.
Beyond the on/off aspect, further tilting has minimal effect on the capacity and as a result, tilting is now
seldom used. Modern heat pipe systems do not have to use tilting for either modulation or on-off
control.
SPECIFIC CONTROL FOR FROSTING/FREEZING PROTECTION
Figures 1 and 2 depict a moderate winter condition, and the EA heat pipe will not have frost or freezing.
But frost and freezing will occur anytime the EA is cooled below its dew point and subjected to below
freezing temperatures. Not only does that reduce the heat transfer and EA airflow, but in extreme cases
can even completely ice over and block airflow through the RA/EA section of the heat pipe. In
commercial heat pipe applications where constant ventilating OA is required, there are three methods
of control used to prevent this, but all inherently reduce the energy available for recovery when
operating:
A.
Reduce the Effectiveness of the air-to-air heat exchanger. For heat pipes, use valves for fluidside control and the stepper valves are ideally suited for this in larger systems. They have the
finest (smallest) control increment and are controlled by an analog output (AO) signal to
maintain the set-point (the actual set-point is discussed below). Each stepper valve may be
controlled individually or they may be controlled in banks of two.
B. Add a heat source (preheater) upstream of either the OA heat pipe or the EA heat pipe. If the
former, the heat source should only be enabled enough to exactly allow there to be enough
heat transferred to the EA stream so it won’t freeze. Or, if the latter, heat (even direct fire) is
added directly to the EA so freezing doesn’t occur at the heat pipe. Whether the heat source is
single stage, multi-stage or variable dictates the use of a digital output (DO) or AO signal
respectively to maintain the set point. It may appear somewhat backwards to add heat while full
heat recovery is operating, but energy recovery does take a back seat to operational
considerations like coil freezing.
4
T
T
HEAT PIPES
C. Bypass Dampers are installed
Outside Air
around the OA heat pipe as in
Figure 4. As more air is bypassed
around the OA heat pipe, the
Exhaust Air
heat sink of the OA heat pipe is
reduced and less heat can
transfer from the EA heat pipe
to the OA heat pipe. This keeps
the EA at a higher temperature and frosting and
Bypass
Damper
Supply Air
Return Air
Figure 4
freezing are prevented. Dropping below the EA set-point triggers an AO signal to the bypass
damper actuator.
The control strategy should be one which minimizes the reduction of recovered energy, incorporating a
defined risk of frosting. Higher set-points that initiate the freezing strategy sooner (as the temperature
drops) do not save as much energy as a lower set-point. Thus, one has to balance the desire for energy
savings against the risk of the EA heat pipe section freezing up. The author believes most HVAC
designers would want to absolutely prevent any chance of freezing (with perhaps another degree or two
included for further safety) and give up extreme energy savings. In any event, the absolute lowest
recommended set point is the dry bulb temperature at which frosting begins to occur in the heat pipe
core. Since the relative humidity of the EA affects that temperature, there is, in one sense, some
automatic conservatism “built in” to a dry bulb temperature set point. Another approach is to consider
manually adjusting the set-point until frosting is observed during near design winter conditions and
extrapolating from that. Another consideration to save energy is to use air blenders to extend the set
point as low as possible.
As more of a supplement and/or backup strategy, the final alternative to consider is to use a differential
pressure sensor across the heat pipe instead of using temperature sensors. An improvement over the
single point sensor is to use upstream and downstream grids of pitot tubes. As frost builds up in the heat
pipe fins, the pressure drop will increase triggering the freeze control strategy. The advantage of this
control is that it only responds to actual frost buildup, eliminating if you will the exhaust humidity
variable, other variables, and all the safety factors. Thus, it can be expected to provide the highest
energy savings. However, it can’t be used on systems with variable flow EA and the standard pressure
drop must be established individually for each specific application.7
On the other hand, recognize that there is a unique BIN distribution of hours for every location. Baton
Rouge, LA has .3% of its annual hours at the 25/30 BIN and below while Bismarck, ND has 30%. So,
depending on location, it may or may not be worthwhile to stretch for the greatest energy savings. In
any event, since “for frosting or icing to occur, an airstream must be cooled to a dew point below 32ºF”8,
a 32ºF set point may be considered the starting point. Determining how much below requires some
careful analysis and balancing the frosting risk against the desire and potential for energy savings.
So, given a set-point and a design Effectiveness of the heat pipe, which of the freeze control strategies is
best? Phillips et al9 analyzed these three strategies, and others more for residential applications that
stopped the ventilating OA; the latter are not discussed here since they aren’t appropriate for
commercial buildings and ASHRAE Standard 62.1. They found that in moderate climates with less than
7,500 Degree Days ºF which includes much of the US, the freeze control strategy has little impact on
7
Airxchange. 2005. Frost control strategies for Airxchange enthalpy wheels.
2012 ASHRAE Handbook – HVAC Systems and Equipment. p26.7.
9
Phillips, E.G., D.R. Fisher, R.E. Chant, and B.C. Bradley. 1992. Freeze-control strategy and air-to-air energy
recovery performance. ASHRAE Journal 34(12):44-49.
8
5
energy savings. However, parts of Canada have twice that figure and in those colder climates, the energy
savings from highest to lowest are:
A.
B.
C.
D.
Variable preheat
Valve control; Bypass dampers (same results)
Staged preheat
Fixed preheat
At this point, a “caution” must be mentioned for the case of bypass dampers being used in very cold
climates. Because the heat pipe’s Effectiveness increases at lower face velocities, the operating face of
the outside air heat pipe (even though reduced in total BTUH transfer) may still receive enough heat so
that the exhaust air heat pipe will freeze even with the bypass wide open (and no face dampers). The
exact details of when that may happen are an intricate interplay of many variables including: exhaust air
temperature and humidity, the Effectiveness of the heat pipe, the pressure drop characteristics of the
bypass damper, the temperature of the outside air, and the relative airflow amounts of the outside air
and exhaust air. Depending on the criticality and airflows of the application, installing interlocked face
dampers would eliminate that potential freezing problem. For further assistance on this subject, please
contact your local authorized Heat Pipe Technology sales representative.
SPECIFIC CONTROL FOR VENTILATION/ECONOMIZER OPERATION
Supply Air
Fan Motor
Cooling
Outside Air
Preheat
10
E
Heat Pipes
The other need for control in EA
energy recovery systems is
during ventilation and/or
economizer operations. While
they are linked, specifically the
former refers to the commercial
building code requirement10 that
a certain minimum amount of
OA must be introduced into a
facility with human occupancy.
The latter recognizes that for
both the minimum OA and an
incremental OA amount up to
the system maximum
unconditioned air above the
minimum amount may be
introduced into a facility to save
central plant energy as long as
the unconditioned air falls within
E
Return Air
Exhaust Air
Damper
s
Figure 5
ASHRAE Standard 62.1 – 2010 “Ventilation for Acceptable Indoor Air Quality”
6
certain dry bulb and dew point temperature extremes. It is apparent then that, given only the envelope
of satisfactory ambient conditions, the uncontrolled use of any air-to-air energy recovery device will
certainly not provide the energy savings of which the device is capable. This paper will provide the
proper control methodology for all recovery devices that, by design, only transfer sensible energy from
the RA/EA to the incoming OA.
Figures 5 and 6 show the
correct location of the EA to
OA energy recovery heat pipes
in both a simple 100% OA
systems and a simple Mixed
Air (MA) System respectively.
An underlying requirement for
both systems is that the
minimum OA per ASHRAE
Standard 62.1 (or other site
specific criteria requiring more
OA) be met at all times by the
correct settings for all
balancing dampers (not
shown).
Supply Air
Fan Motor
Cooling
Preheat
Outside Air
Dampers
Heat Pipes
E
Return Air
Exhaust Air
Figure 6
35
70
75
70
OA Example 1
3065
PSYCHROMETRIC
CHART
70
OA Example 2
WE
T
BT
Normal Temperature
I-P Units
60
SEA LEVEL
65
25
BAROMETRIC PRESSURE: 29.921 in. HG
55
20 50
90
%
65
BU
L
EM
P
ER
AT
UR60
E°F
55
RA Summer
60
SA Summer
50
55
%
80
If the OA is at the
condition in Example 1,
heat should flow through
the heat pipes in the 100%
OA system to heat the RA
and be exhausted to
ambient. Since maximum
0%
OA7Example 4
15
45
50
25%
OA Example 3
604%
5
50%
40
35
40%
30
15%
25
20
30%
20%
10% RELATIVE HUM
10
0
IDITY
45
50
55
60
Chart by: HANDS DOWN SOFTWARE, www.handsdownsoftware.com
65
70
75
DRY BULB TEMPERATURE - °F
Figure 7
7
80
85
DEW POINT TEMPERATURE - °F
The psychrometric chart
in Figure 7 shows both
types of systems, their
summer RA and SA state
conditions, and four
example OA conditions.
Similar logic to that below
should be used for other
summer and winter
ambient conditions.
E
90
heat removal from the OA is desired, the heat pipe bypass dampers should be closed. In the MA system,
since it is desired to use minimum compressor energy to cool the OA, the RA damper should be at its
maximum open position.
A cautionary note: in the latter system, it may appear that as the OA dry bulb further decreases, at some
point its value as reduced by the heat pipes will become lower than the RA. While that’s true, the heat
pipe by design only transfers sensible heat so the absolute value of the OA humidity will not be reduced.
The ensuing heat pipe reduced OA enthalpy is virtually certain to be above the RA enthalpy, which again
points to a maximum damper open position for maximum RA.
If the OA is at the condition in Example 2, the heat pipe bypass dampers in the 100% OA system should
be open to save fan energy and prevent the OA temperature (and enthalpy) from increasing. In the MA
system, it’s more probable that the OA enthalpy increased by the heat pipe will be less than the RA
enthalpy. While the OA dry bulb will be increased by the heat pipe, its final enthalpy should be the
decision criteria. Enthalpy sensors or separate linked dry bulb and humidity sensors should monitor both
air streams and control the dampers to use the airstream with the lowest enthalpy.
If the OA is at the condition in Example 3, it is a certainty that the OA enthalpy in the 100% OA system
will be less than the RA enthalpy, and heat pipe operation should be prevented. Thus, the heat pipe
bypass dampers should be opened and the dampers in the MA system should be set to maximum closed
position for full economizer operation. With both Examples 2 and 3 in the MA system, the thought
process should be to always compare the heat pipe reduced OA enthalpy to the RA enthalpy and choose
the lower as long as it is above the summer SA enthalpy.
If the OA is at the condition in Example 4, the sequence of operations for both systems will shift to
winter mode and the SA set point will increase. But in the 100% OA system we still want to take
advantage of the heat pipe raising the OA temperature, so the heat pipe bypass dampers should be
closed. In the MA system, we need to minimize the OA airflow because it has a lower dry bulb (and
enthalpy). That means we open the dampers to maximum position.
CONCLUSIONS AND SUMMARY
It is clear that air-to-air sensible energy recovery has a well-deserved place in the many HVAC comfort
and process applications that either can not allow any chance of cross contamination from a facility’s
exhaust air stream to its incoming fresh outside air, must have ultrahigh reliability, and/or require
absolute minimum maintenance. This paper provides detailed control guidance for both the freezing
and ventilation requirements of those systems.
8