WAHP Parametric Study in Air Conditioning Systems

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Theoretical Parametric Study of Wrap-Around Heat Pipe (WAHP) in Air
Conditioning Systems
Article · December 2018
DOI: 10.1142/S2010132519500044
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Mridul Sarkar
Integrated Environmental Solutions, Pune, India
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International Journal of Air-Conditioning and Refrigeration
World Scientific Publishing Company
1
Theoretical parametric study of Wrap-Around Heat Pipe (WAHP) in air
conditioning systems
Mridul Sarkar
Integrated Environmental Solutions India Pvt. Ltd.
Pune-411021, India.
[e-mail]; mr[email protected]
Warp around heat pipes (WAHP) belong to a special class of recuperative heat exchangers that transfer
heat from inlet to outlet locations via thermal gradient, without using any energy. In the present work,
effects of various mechanical parameters on the performance of a WAHP dehumidifier system that are
based on the underlying principles of heat and mass conservation are presented primarily from a
theoretical point of view. A simplified methodology pertaining to wet cooling coils is applied here for
defining the case of moisture condensation during precooling process at WAHP evaporator. Inlet air
temperature, inlet humidity ratio, air mass flow rate, dehumidifier outlet temperature and effectiveness
are the main operational parameters considered in this study. On the other hand, comparative
performance study of the WAHP system is done through other set of thermodynamic parameters like
the supply air temperature, supply humidity ratio, specific coil load and recovered enthalpy. The subtle
variations in these factors against the operational parameters not only help in stipulating functional
characteristics of the WAHP, but also allow HVAC designers to make informed decisions for system
design and performance without relying entirely on manufacturer’s equipment data.
Keywords: Wrap-around heat pipe (WAHP), dehumidification, humidity, coil load, recovered
enthalpy, effectiveness
1. Introduction
Due to global rise in energy prices and demand, it
becomes very important that the energy sources be used
and managed in an efficient and prudent way. Almost,
10-30% of annual energy consumption in building
sector is due to air conditioning equipment1. In recent
years, serious strides are taken in the field of energy
recovery for cooling and dehumidification applications,
which is a major requirement from HVAC systems
particularly for hot, humid and temperate climates. The
usage of heat pipes in air conditioning equipment for
air-to-air heat recovery and efficient dehumidification
is becoming more popular in recent time due to its ease
of integration, less maintenance and no supplementary
energy requirement for operation2-3. In fact, thermal
conductivity of heat pipes is reported to be several times
higher than many conductors of comparable
dimensions4-5.
Performance investigations of heat pipe heat
exchangers (HPHX) and WAHP applied in HVAC
systems for heat recovery are important topics of
research. El-Baky et al. performed experimental
investigation6 on the effect of return to fresh air mass
flow ratios and fresh air temperatures on the
effectiveness of HPHX system. Experimental study on
a 2-Row copper HPHX7 charged with R-134a
refrigerant by Yau et al. is aimed towards the
investigation of the influence of evaporator inlet
temperatures and face velocity on heat pipe
performance. From the results, they concluded that the
sensible effectiveness of HPHX actually decreased as
the evaporator face velocity is increased. However, in
the temperature range considered for the study, the
sensible effectiveness stayed almost constant. Noie-

2 Mridul Sarkar
Baghban et al. presented theoretical and experimental
investigations of a methanol-based HPHX system8 for
hospital surgery rooms. Yau showed the impact of heat
pipes on the energy efficiency of dehumidification
systems9 through transient simulation model of an
HVAC system installed with two 8-row HPHX for an
operating theater in tropical climate of Malaysia.
Ahmadzadehtalatapeh investigated the performance of
an air conditioning system with a HPHX10 and verified
that it met the comfort criteria recommended by
ASHRAE through TRNSYS simulation.
Experimental study of an air handling unit with 7-
looed WAHP11 by Jouhara et al. is aimed towards the
investigation of the effect of heat loads and face
velocities on the overall resistance of the heat pipe
loops. They also concluded that the overall
effectiveness of WAHP decreases as the face velocity
increases. Ezzuddin et al. presented an experimental
investigation of WAHP charged with R-134a
refrigerant12 and two-pass evaporator and condenser
sections to characterize the thermal performance of the
system in terms of overall thermal resistance.
Many researchers have also evaluated the economic
potential of heat pipes for building air conditioning.
Jouhara conducted a detailed study on the energy
performance13 of WAHP and reported an annual saving
of 134 MWh for a ventilation system supplying 3m3/s
of outdoor air. Zhang et al. presented simulation
results14 showing the energy conservation potential of
heat pipes for dedicated outdoor air handling units
serving office buildings in Hong Kong.
From the literature review, it is quite evident that
the application of heat pipes for air conditioning and
building HVAC services is an active area of research
and development. Despite all of these, lack of
simplified methodologies for analyzing the
performance of heat pipes forces designers and
engineers to depend on various manufacturer selection
software and catalogs. The present work is aimed
towards bridging this knowledge gap. The primary two-
fold objective of the present work is outlined as follows:
• Establishing basic formulations for defining
psychrometric process through a WAHP enhanced
Fig. 1. Schematic of heat pipe systems used in HVAC: (a) Wrap-Around Heat Pipe (WAHP), (b) Heat Pipe Heat Exchanger (HPHX).

Theoretical parametric study of Wrap-Around Heat Pipe (WAHP) in air conditioning systems 3
dehumidifier system, including the limiting case of
moisture condensation during precooling.
• Depicting theoretical variation of characteristic
parameters of a WAHP against the operating
parameters like inlet air temperature, inlet humidity
ratio, dehumidifier coil outlet temperature, air mass
flow rate and WAHP effectiveness.
2. Basic Heat Pipe systems
A heat pipe system does not utilize any mechanical
component or consume electricity to transfer energy
from one point to another. Heat pipes used for air
conditioning applications are closed loop systems,
where a difference in density and temperature between
the two phases of refrigerant fluid drives its movement
inside the tubes and creates a pulsating effect. The
working fluid evaporates by absorbing heat at one point
and rejects heat at the other by condensing back to
liquid as per Ref.2. Figure 1 shows the two basic
configurations of heat pipes used prevalently in air
conditioning systems. In wrap-around heat pipe
(WAHP) system, front and rear sides of the
dehumidifier coil are covered by the evaporator and
condenser sections, respectively. The evaporator
section precools the incoming air before the
dehumidifier coil cools it further. The cooled and
dehumidified air is subsequently reheated as it passes
through the condenser section and supplied to the space.
In heat pipe heat exchanger (HPHX) system, the supply
and exhaust streams pass through the evaporator and
condenser sections, respectively, which allows air-to-
air heat recovery between the two streams. The
connecting tubes enable transport of the refrigerant
fluid between the evaporator and condenser sections of
the heat pipe system during the whole cycle. The
operating temperature range of WAHP and HPHX for
air conditioning systems depends upon the required
supply conditions for intended space application and
refrigerant used in heat pipes. Literature sources [Ref.
6, 8] suggest 15-550C to be appropriate temperature
range for heat pipes in air conditioning applications.
3. Basic underlying equations
The sensible effectiveness of WAHP from fig.1 is
given by:
Energy balance across the evaporator and condenser in
terms of air enthalpy is expressed as:
If recovered heat at the evaporator is entirely sensible,
then the effectiveness is also defined by:
which gives:
The evaporator exit temperature from Eq. (4) is
compared with the dew point temperature (DPT) at inlet
(point 1) to check whether condensation occurs at the
evaporator. The DPT can be expressed in terms of
humidity ratio and absolute pressure as per Ref.15:
Where,
Based on the comparison between evaporator exit
temperature and the inlet dew point temperature, two
(1)
(3)
(4)
43
13
WAHP tt
tt
1 2 4 3
m q q m q q  
(2)
12
13
WAHP tt
tt
2 1 1 3WAHP
t t t t
22
0 0 0 0
0adp
tB kT B kT 4A AT BT T
2A
(5)
plv
t
0
0 plv 0
C
A2R
P
k ln ln
P
l C T
BR

4 Mridul Sarkar
different cases can be shown as per the proceeding
subsections.
3.1.1. Case 1: No condensation
If the temperature at evaporator exit (t2) is greater
than or equal to the inlet DPT, moisture condensation
does not occur at the evaporator and unsaturated air
passes through the coil. So in case 1:
In this case, the absolute humidity at point 2 will be:
On a psychrometric chart, the cooling-dehumidification
process through the coil can be simply depicted by a
straight line from the coil inlet to outlet conditions that
intersects the saturation curve at coil ADP (apparatus
dew point) on extending further16. In terms of the ADP
and coil bypass factor (BF), coil outlet temperature is:
Similarly the absolute humidity ratio at coil exit is:
Since, ADP condition corresponds to the lowest
saturation limit of air passing through the coil, the vapor
pressure at this temperature (tadp) is determined from
the modified Clausius-Clapeyron equation17-18 as:
The unit for temperature in Eq. (10) is Kelvin. Hence,
here ‘T’ implies T0+t. The saturation humidity ratio
corresponding to this temperature is given by:
Humidity ratio at the coil exit is obtained by
substituting the value of ωadp into Eq. (9). It should be
noted that the relative humidity of air at coil exit can not
exceed 100%. If air reaches the saturation condition in-
between coil inlet and exit temperatures, it follows the
100% RH curve on a psychrometric chart for rest of the
process. In that case, ω3 will be simply equal to
saturation humidity ratio corresponding to the coil exit
temperature. Since, heat addition to the air stream at
condenser is entirely sensible, humidity ratio remains
unchanged after leaving the condenser:
However, due to an increase in temperature through the
condenser, relative humidity of the supply air reduces.
The condenser exit (supply) temperature is obtained by:
3.1.2. Case 2: With Condensation
If the evaporator exit temperature calculated from
Eq. (4) is lower than the inlet DPT, condensation of
water vapor occurs during precooling. Since heat
absorbed at the evaporator is not entirely sensible in this
case, the effectiveness given by Eq. (3) is not valid.
From conservation of energy, heat released at the
condenser section is expressed in terms of air enthalpy
difference by:
(6)
(7)
(8)
(13)
21
t DPT
21
32
1
adp
t t BF t BF    
31
1
adp BF BF      
(9)
(10)
adp
adp
ad T
tT
pP
PP
(11)
43
4 3 1 3WAHP
t t t t
(12)
43ca
Q m q q
plv
adp
C
R
00
0 plv 0
a
dp
dp
0a
TT
PPT
l C T 11
exp
R T T