Strojniški vestnik - Journal of Mechanical Engineering 67(2021)6, 302-310 Received for review: 2021-03-17
© 2021 Journal of Mechanical Engineering. All rights reserved.  Received revised form: 2021-05-05
DOI:10.5545/sv-jme.2021.7168 Original Scientific Paper Accepted for publication: 2021-05-19
*Corr. Author’s Address: Vilnius Gediminas Technical University, Lithuania, kestutis.ciuprinskas@vilniustech.lt 302
Minimization of the Lifecycle Cost of a Rotary Heat Exchanger 
Used in Building Ventilation Systems in Cold Climates
Sokolnikas, I. – Čiuprinskas, K. – Čiuprinskienė, J.
Ignas Sokolnikas
1
 – Kęstutis Čiuprinskas
2,*
 – Jolanta Čiuprinskienė
2
1 
Salda UAB, Lithuania 
2 
Vilnius Gediminas Technical University, Lithuania
This article presents an analysis of rotary heat exchangers (RHE) used as heat recovery units in building ventilation systems in cold climates. 
Usually, heat exchangers with the highest heat transfer efficiency are the preferable option for this purpose. However, such exchangers usually 
have the highest media pressure drop, thus requiring the highest amount of energy for media transportation. In this study, the problem is 
solved by analysing the lifecycle cost (LCC) of the RHE including both the recovered heat and the electricity consumed in the fans of the air 
handling unit (AHU). The purpose of the investigation was to determine the optimal set of geometrical characteristics such as the exchanger’s 
length, foil thickness, the height and width of the air channel. Two hundred and seventy different combinations were examined using analytical 
dependencies and ANSYS simulations. The results are compared with experimental data obtained earlier at the KOMFOVENT laboratory. The 
results show that the best overall energy efficiency is obtained in heat exchangers that do not offer the best heat recovery efficiency, and 
LCC differences in the same climatic and economic conditions can go as high as 31 %, mainly due to the geometrical parameters of the heat 
exchanger.
Keywords: rotary heat exchanger, heat recovery, ventilation system, temperature efficiency, pressure loss, ANSYS, lifecycle cost
Highlights
• 	 The energy efficiency of a heat exchanger should comprise not only the efficiency of heat recovery but also the energy needs for 
media transportation through the heat exchanger.
• 	 Heat exchangers with the highest heat recovery efficiency usually have the highest media pressure drop and, as such, the 
highest energy requirements for media transportation.
• 	 During the lifecycle of the heat exchanger, the best overall energy efficiency is obtained in heat exchangers with sub-par heat 
recovery efficiency.
• 	 During the lifecycle of the heat exchanger, the differences in the overall energy efficiency can reach 31 % due to the geometrical 
parameters of the heat exchanger.
• 	 The best energy performance was obtained with the rotary heat exchanger that was the longest and had the smallest thickness 
of foil.
0  INTRODUCTION
Nowadays, mechanical ventilation systems are an 
indispensable part of building engineering systems. 
Ventilation systems consume about 10 % of energy 
in commercial buildings [1]. Ventilation systems also 
increase heating and cooling system operating costs, 
which constitute 25 % and 9 % of building energy, 
respectively [1]. The main element of a ventilation 
system that saves the majority of thermal energy is the 
heat exchanger located in the air handling unit (AHU). 
Rotary and plate heat exchangers are most commonly 
used for this purpose.
In Lithuania, as well as in other cold climate 
countries, rotary heat exchangers (RHE) are the 
preferred type due to lower frosting in low outdoor 
temperatures, humidity recovery, compactness, and 
no drainage requirement. Rotary heat exchangers 
usually are classified into three types: condensation, 
enthalpy, and sorption heat exchangers. In this 
research, a condensation-type rotary heat exchanger is 
investigated. 
The initial aim of the literature review was to 
discover some constructional recommendations in 
pursuance of maximal overall energy efficiency 
of the RHE, including heat transfer efficiency and 
the energy required to overcome the aerodynamic 
resistance of the RHE. As expected, no such specific 
recommendations were found. The majority of studies 
focus on the heat and moisture transfer efficiency 
of heat exchangers and ignore the pressure loss. 
The primary goal of research [2] was to develop a 
mathematical and numerical model to predict the 
energy wheel’s effectiveness and to take into account 
the condensation and evaporation processes. Paper 
[3] presents the modelling results of the recovery unit 
working in real conditions with regard to changes 
in temperature, air velocity, and rotor speed. In [4], 
the heat and mass transfer in a desiccant wheel was 
modelled into a set of linear differential equations 
under the linearization assumptions on the temperature 

Strojniški vestnik - Journal of Mechanical Engineering 67(2021)6, 302-310
303 Minimization of the Lifecycle Cost of a Rotary Heat Exchanger Used in Building Ventilation Systems in Cold Climates
and humidity profiles and the psychrometric relation. 
In [5], the optimum operational conditions of the 
rotary regenerator were obtained using genetic 
algorithm optimization technique subject to a list of 
constraints. The objective function in the optimization 
technique was the thermal effectiveness, while the 
design parameters (decision variables, the optimum 
operating conditions of the rotary regenerator were 
obtained using a genetic algorithm. The objective 
function in the optimization technique was the 
thermal effectiveness, while the design parameters 
(decision variables) were volumetric flow rates of 
cold and hot air streams, matrix rotational speed, and 
the exchanger’s frontal area (heat transfer surface 
area). The purpose of the study in [6] was to propose 
a mathematical model for the heat and mass transfer 
characteristics (moisture removal capacity and the 
moisture removal regeneration) of a zeolite-coated 
heat exchanger. The lack of such recommendations 
has also been observed by RHE manufacturers [7]. 
In a study of geometrical parameters on heat and 
mass transfer processes [8], pressure drop over the 
RHE is used as an indicator of the frosting progress 
only; energy consumed by fans is not included in 
the evaluation. In some research projects, energy 
consumed in AHU fans is taken into account [9] and 
[10] and even is referred to as a “significant part” of 
the total primary energy used [10]; however, it still 
omitted from the overall energy efficiency evaluation. 
AHU fan energy is included in power usage 
effectiveness (PUE) used as an energy efficiency 
indicator of data centre facilities, and the study in [11] 
is quite informative in this context; however, it only 
gives a distinct presentation of electrical power, but 
not energy use, and no energy increase due to the use 
of the RHE is given.
Regardless of the different field of application, 
one of the best formulations of the problem is given 
in an RHE optimization study [12]. Recovered heat 
and energy used in fans are defined as “conflicting 
objectives”, and a solution is given as a Pareto 
optimum. In the field of air conditioning, the 
connection between heat transfer efficiency and 
pressure loss is probably given the most attention in-
depth in two studies [13] and [14]. In [13], the optimal 
geometrical parameters of the RHE are determined 
using the Pareto front concept; in [14] the boundaries 
of a more generalized porosity factor are defined 
for the typical operational conditions of the RHE. 
Unfortunately, neither of the studies analyse the 
ratio between the recovered heat and the consumed 
electrical energy.
Simplified analytical models are used in 
most of the sources reviewed, though it is well 
known that solving non-linear partial differential 
equations is generally needed in the case of heat and 
mass transfer. In [4], the advantages of analytical 
versus computational methods are presented, and 
an analytical solution using the linearization of 
differential equations of heat and mass transfer in 
a desiccant wheel is proposed. In contrast, some 
authors state that it is impossible to accurately 
calculate the temperature efficiency of an RHE by 
applying the analytical method, because the process 
of heat exchange is not stationary [15] and [16]. The 
application of numerical models produces results that 
are closer to experimental results yet are not always 
very reliable [2] and [17].
As expected, there is no clear dependence 
of temperature efficiency or pressure loss on one 
parameter. Due to the aforementioned reasons, there 
are no distinct guidelines for designing an RHE, or 
they are too abstract. Although some sources [13], 
[14] and [18] suggest that the RHE with the highest 
temperature efficiency ratio would not be expected to 
be the most energy-efficient due to the high-pressure 
losses of such a heat exchanger. Therefore, this 
research focuses on the optimal ratio of temperature 
efficiency to pressure losses.
The objective of this research was to find the 
optimal set of geometrical RHE parameters, taking 
into consideration the heat recovered by the heat 
exchanger and the energy used for air transportation 
through the exchanger itself (i.e., the difference in the 
amount of electricity used in the fans with and without 
the RHE). These energy amounts are assessed during 
the lifecycle of the RHE. Additional outcomes from 
this research are the range of variation of lifecycle 
costs (LCC) and the possibility to use the results 
obtained in formulating the selection criteria of the 
RHE.
1  METHODS
The computational fluid dynamics (CFD) model 
was adopted to determine the main variable of the 
problem: the temperature efficiency ratio of the RHE. 
The pressure drop was initially calculated using the 
same CFD model, which was later replaced with an 
analytical model due to its shorter calculation time. 
The CFD and analytical calculation results were 
validated by experimental tests results. The total 
amount of heat recovered, electricity consumed as 
well as energy and materials costs, were defined for 
Lithuanian weather and economic conditions.

Strojniški vestnik - Journal of Mechanical Engineering 67(2021)6, 302-310
304 So k o lnik as,	I.	–	Čiuprinsk as,	K.	–	Čiuprinskienė,	J.
The set of the most significant parameters, such 
as the foil wave height and wavelength, the length of 
the RHE, the air velocity through the RHE, the foil 
thickness and the rotational speed of the RHE was 
determined on the basis of the literature review.
1.1  Geometrical Model
Constructively, an RHE is a drum (cylinder) of 
aluminium foil arranged in concentric rings. Both in 
the CFD model and in the experimental tests, the wave 
height and the wavelength of the RHE are determined 
using the Eurovent certification methodology [7]. 
The wavelength (b) was calculated using Eq. (1); the 
measurement scheme is shown in Fig. 1.
Fig. 1.  Measuring the wavelength of the rotary heat exchanger
 b
Dc D
n

  arcsin /
, (1)
where b is the calculated wavelength; D is the diameter 
of the rotor [mm]; c is the length of the measured 
segment [mm]; n is the number of the waves in the 
measured segment.
The wave height was calculated by measuring 
the height of 10 to 20 waves, which was then divided 
by the number of waves. The wave height is shown 
below (Fig. 2):
Fig. 2.  Measuring the wave height (a) of the rotary heat exchanger
The temperature efficiency of the rotary heat 
exchanger was calculated using Eq. (2) [7]:
 
t
tt
tt



22 21
11 21
, (2)
where t
22
 is the temperature of the supplied air [°C];  
t
21 
is the outdoor air temperature [°C]; t
11
 is the indoor 
air temperature [°C].
1.2  CFD Model
The temperature efficiency of the RHE is determined 
using the ANSYS FLUENT 18.0 finite volume-based 
software [19]. The CFD model of an RHE consists 
of the RHE itself and four airflow sections. The 
RHE model is divided into a finite volume network 
consisting of 2,039 nodes and 8,087 finite volumes. 
The finite volume network is built from tetrahedrons. 
The model and the finite volume network thereof are 
shown in Fig. 3.
Fig. 3.  The finite volume network of the rotary heat exchanger 
model
To describe the conditions of the problem, the 
k-epsilon viscosity model is selected. The change of 
the physical parameters of air over time is described in 
accordance with the polynomial functions. To describe 
the physical parameters of aluminium, the same data 
as in experimental tests are used (see below). Then 
the boundary conditions (the outdoor and indoor air 
temperatures, as well as air velocities) are determined.
The characteristics of the rotary heat exchanger 
are introduced by applying the function of porous 
material. In this case, the porosity of the material σ, 
the ratio between the surface area and volume A/V and 
the heat transfer rate h
T
 are introduced [13].
  

A
AA
c
Mc
, (3)
 A
ab
c


2
, (4)
 
  aas 2, (5)
   bbs 2, (6)
 AP
s
M
=
2
, (7)

Strojniški vestnik - Journal of Mechanical Engineering 67(2021)6, 302-310
305 Minimization of the Lifecycle Cost of a Rotary Heat Exchanger Used in Building Ventilation Systems in Cold Climates
 Pb
ba
b
a
b
a




























2
22
3
2
4
2
22
2
2



, (8)
 
A
V
PL
AAL
cM

 
,
 (9)
 h
Nu k
D
T
a
eq


, (10)
   
Nu
a
b
a
b
a
 






















1 1791 12 7701 3 1901
1 9975
2
.. .
.
 


















b
a
b
34
0 4966 ., (11)
     
Da
a
b
a
b
a
b
eq
 




















1 0542 0 4670 0 1180
0 1794
2
.. .
.
 
















34
0 0436 .,
a
b
 (12)
where σ is the porosity of the RHE; s is the foil 
thickness, [m]; a' is the internal wave height of the 
channel, [m]; b' is the internal width of the channel, 
[m]; A
M
 is the total area of foil cross-section, [m
2
]; A
c
 
is the total area of air channel cross-sections, [m
2
]; P 
is the total perimeter of channels, [m
2
]; L is the length 
of the RHE, [m]; A is the surface area of foil, [m
2
]; V 
is the volume of the RHE, [m
3
]; h
T
 is the heat transfer 
coefficient, [Wm
–2
K
–1
]; Nu is the Nusselt number; k
a
  is the thermal conductivity of air, [Wm
–1
K
–1
]; D
eq
 is 
the equivalent diameter of the channel, [m].
First, the problem is approached in a stationary 
mode until the air velocity and temperature stabilizes 
while the rotor is not running. Then the non-stationary 
mode is activated, and the non-stationary problem of 
heat transfer is solved. The selected time-step is 0.25 
s, while the maximum number of iterations during 
this time-step is 10. The process is halted when the 
variation in the temperature of the supplied air drops 
below 0.3 °C. This condition is reached within 150 s 
to 200 s of the process simulation, while calculations 
take 30 min to 40 min.
1.3  Calculation of Pressure Loss
Unlike temperature efficiency, the pressure loss of an 
RHE can be calculated using the analytical method. 
Compared to numerical modelling methods, this 
method saves a significant amount of calculation time. 
The calculations were carried out using the following 
equations [13]:
 pu f
L
D
u
c
eq
  
1
2
4
2
22
, (13)
 Re
uD
eq


, (14)
 
u
v
f


,
 (15)
 
fRe
a
b
a
b
























9 5687 10 0772 0 8619
0 8314
2
.. .
.
a a
b
a
b
a
b 




























345
0 2907 0 0338 .. , (16)
where ξ
c
 is the factor for air expansion and 
compression at the entrance and exit of the rotor 
(0.2); ρ is the density of air under standard conditions  
(1.2 kgm
–3
);  is the air velocity in the channel; f is the 
Fanning friction factor; Re is the Reynolds number; 
μ is the kinematic viscosity [m
2
s
–1
]; v
f
 is the air face 
velocity through the RHE, [ms
–1
].
1.4  Calculation of Life Cycle Costs
The LCC are assumed as the sum of costs of the RHE, 
electricity consumed in the supply and exhaust air 
fans, and the heat consumed in the air heating coil, 
displaced after the RHE in the supply air stream. The 
lifetime of the RHE is assumed to be equal to the 
typical lifetime of AHU, which is 10 years. 
The cost of the RHE is assumed as the sum of 
two parts: the cost of manufacturing and the cost of 
aluminium foil. The manufacturing cost was fixed 
at 345 EUR, because the same size of all RHEs 
investigated. The cost of aluminium foil is variable 
because of the different lengths and porosity of the 
RHE. The price of aluminium foil was assumed to 
be 8.62 EUR per kg. The volume of aluminium foil 
consumed is calculated under Eq. (17):
 VD L   02 51
2
..  (17)
The electrical power and energy consumed in the 
fans are calculated using the following equations:
 Pq p
el vf an e


10
3
 /,  (18)
 
EP Pt
P
el el supe le xo p
el



  
,,
/. 21 0 365 12 57 (19)
where q
v
 is the volumetric airflow; Δp
fan
 is the pressure 
difference of the fan, calculated by adding the 200 

Strojniški vestnik - Journal of Mechanical Engineering 67(2021)6, 302-310
306 So k o lnik as,	I.	–	Čiuprinsk as,	K.	–	Čiuprinskienė,	J.
Pa of the ventilation system aerodynamic resistance 
to the resistance of the RHE obtained from CFD 
calculations; η
e
 is the efficiency of the fan assembly, 
assumed to be constant and equal to 0.625; t
op
 is 
the annual time of the fan’s operation. It is assumed 
that the ventilation system is used 12 hours per each 
working day for 10 years.
The heat consumed in the air heat coil is calculated 
using the following equations and assumptions:
 Pq ct tt
hv ps as RHEf an
 

 max0 ;,
,
 (20)
 tt tt
sR HE oa RHEe ao a ,
,     (21)
where c
p
 is the specific heat of the air (1 kJ kg
–1
K
–1
); 
t
sa
 is the temperature of the supply air (20 °C); t
ea
 is 
the temperature of the exhaust air (20 °C); t
oa
 is the 
temperature of the outdoor air; t
s,RHE
 is the temperature 
of the air after the RHE; Δt
fan
 is the increase of the air 
temperature due to the supply air fan operation (1 °C);  
η
RHE
 is temperature efficiency of the RHE obtained 
from CFD calculations.
The heat energy calculations were performed 
for each hour of the ventilation system uptime. 
The outdoor air temperatures were obtained using 
RETscreen software [20].
As used for the purposes of the LCC calculations, 
the price of electricity was 0.099 EUR per kWh, and 
the price of the heat 0.0463 EUR per kWh.
1.5  Validation of the Models
In order to compare the experimental and calculation 
results, tests with three different condensation rotary 
heat exchangers were performed at a laboratory 
located at the factory of KOMFOVENT, a company 
that specializes in the production of ventilation 
equipment. Over the course of the testing procedure, 
the temperature efficiency and pressure loss of the 
RHE was measured at air speeds between 0.5 ms
–1 
and 
4 ms
–1
.
The length of every RHE tested was 200 mm 
while the diameter was 1,000 mm, the rotational 
speed was 12 revolutions per minute and the thickness 
of the foil was 0.065 mm. The characteristics of 
aluminium foil used were as follows: density of 
2,730 kgm
–3
, specific heat capacity of 900 Jkg
–1
K
–1
, 
thermal conductivity coefficient of 175 Wm
–1
K
–1
. 
The experiments were carried out under the following 
conditions: outdoor air temperature of 2 °C, relative 
humidity of 80 %, indoor air temperature of 22 °C, 
relative humidity of 45 %.
As a result, the methodology for calculating the 
temperature efficiency and pressure loss of a rotary 
heat exchanger was validated. The comparison of the 
results for three different cases is provided below; 
Table 1 shows the following parameters of the heat 
exchangers under comparison: the air velocity during 
the test, the wave height, and the wavelength of the 
foil.
As shown in Table 1, the calculation results are 
rather similar to the experimental results. In terms of 
temperature efficiency, the most significant relative 
difference compared to the experimental result was 
2.54 %. As for pressure loss, the maximum relative 
difference was even smaller, only 1.19 %.
Minor differences between experimental and 
calculation results most likely occur due to calculation 
errors for the wavelength and wave height of the 
rotary heat exchanger since these parameters vary 
across the entire area of the rotor.
In addition to these aspects, there might be other 
reasons that cause the aforementioned differences: 
the accuracy of the instruments used to measure 
temperature, the pressure and airflow rate and the 
excessive sparsity of the finite volume network.
However, a 2.54 % margin of calculation error 
for temperature efficiency and a 1.19 % margin 
of calculation error for pressure loss of a rotary 
heat exchanger constitutes a very good result. This 
confirms that appropriate methods were selected to 
determine the temperature efficiency and pressure loss 
of a rotary heat exchanger.
2  RESEARCH SCOPE
In the search for the optimal set of RHE parameters, 
establishing the scope of research is key since there 
Table 1.  Results of experiments and calculations
Parameters Experiment Calculations Absolute difference Relative difference
v,
[ms
-1
]
a,
[mm]
b,
[mm]
Temperature 
efficiency, [%]
Pressure 
loss, [Pa]
Temperature 
efficiency, [%]
Pressure 
loss, [Pa]
Temperature 
efficiency, [%]
Pressure 
loss, [Pa]
Temperature 
efficiency, [%]
Pressure 
loss, [%]
2.5 1.69 3.85 79.36 129.8 78.61 129.29 0.75 0.47 0.95 0.36
2.5 1.39 2.60 85.17 234.0 83.01 235.22 2.16 -1.22 2.54 -0.52
1.5 1.39 4.04 84.82 102.5 82.77 103.70 2.05 -1.22 2.42 -1.19

Strojniški vestnik - Journal of Mechanical Engineering 67(2021)6, 302-310
307 Minimization of the Lifecycle Cost of a Rotary Heat Exchanger Used in Building Ventilation Systems in Cold Climates
are many variables, and the number of possible 
combinations thus grows exponentially. The time 
for calculating temperature efficiency using the 
ANSYS FLUENT software was reduced as much 
as possible. The primary calculations took more 
than 24 hours; however, the time required for the 
calculations was reduced from 30 min to 40 min 
(computer specifications: i5-5200U CPU @ 2.2 GHz, 
16 GB RAM). This was achieved by shortening the 
airflow ducts from 3 m to 1 m, reducing the number 
of finite volumes from 105,867 to 8,087, increasing 
the time-step from 0.05 s to 0.25 s and expanding 
the conditions for the solution from Δt = 0.1 °C to 
Δt = 0.3 °C. The consistency between the calculation 
results and the experimental data shows (see Table 1) 
that these changes are acceptable.
The literature review has shown that once the 
rotational speed of the rotary heat exchanger reaches 
a certain value, there is virtually no effect on the 
temperature efficiency [2], [5] and [13]. The results of 
a large number of studies show that this value does 
not exceed 12 revolutions per minute. Therefore, in 
this study, taking into consideration the duration of 
calculations, the rotational speed of all rotary heat 
exchangers was pegged at 12 revolutions per minute.
During this study, in search for the optimal set 
of RHE parameters, air velocity was considered a 
constant, primarily due to the fact that in almost all 
the cases of preliminary calculations, the best results 
were achieved at the lowest air velocity. However, this 
imposes the largest dimensions of the AHU, which 
quickly becomes unacceptable from the practical point 
of view. The analysis of the heat exchangers tested 
at the KOMFOVENT laboratory showed that the 
highest temperature efficiency is achieved when the 
air velocity is about 1.5 ms
-1
. Therefore, in this study, 
the air velocity in all analysed rotary heat exchangers 
was the same: 1.5 ms
-1
.
Having established the rotational speed and 
air velocity of the RHE as immutable values, this 
research focuses on the length of the RHE, the length 
and width of the foil wave and foil thickness. A matrix 
of parameter values was made considering the time 
required for model calculation. These values are 
provided in Table 2.
Table 2.  Variable parameters of rotary heat exchangers
Parameter Values of the parameter
L, [mm] 200 300 400 - - -
s, [mm] 0.06 0.08 0.10 - - -
a, [mm] 1.4 1.5 1.6 1.7 1.8 -
b, [mm] 2.5 3.0 3.5 4.0 4.5 5.0
Two hundred and seventy different combinations 
of rotary heat exchanger parameters were simulated 
and compared in total. The heat exchangers were 
compared on the basis of LCC.
3  RESULTS AND DISCUSSION
The LCC of different RHE cases are provided below 
(Fig. 4). A heat exchanger with a length of 400 mm, 
foil thickness of 0.06 mm, wave height of 1.8 mm, 
wavelength of 5 mm, pressure loss of 116.5 Pa and 
Fig. 4.  The life-cycle costs of different rotary heat exchanger options (each vertical line represents a separate case simulated)

Strojniški vestnik - Journal of Mechanical Engineering 67(2021)6, 302-310
308 So k o lnik as,	I.	–	Čiuprinsk as,	K.	–	Čiuprinskienė,	J.
temperature efficiency of 88.2 % produces the lowest 
lifecycle costs. The lifecycle costs of this kind of 
heat exchanger are 3,596 EUR over 10 years, i.e., 
31 % lower than the heat exchanger with the highest 
lifecycle costs of 5,204 EUR over 10 years.
As seen in Fig. 5, there is a certain, visually quite 
obvious correlation (R
2
 = 0.50) between the LCC and 
temperature efficiency. It can be stated that the lowest 
LCC is typical of heat exchangers with a temperature 
efficiency between 85 % and 90 %. The figure also 
shows that both an excessively low and excessively 
high-temperature efficiency negatively affects the 
LCC. In this case, when the temperature efficiency is 
very high, the pressure loss is also significant, leading 
to increased LCC.
Fig. 6 shows the dependence of the LCC on 
pressure loss. A quite obvious correlation (R
2
 = 0.52) 
is evident here as well. In this case, to reduce the 
Fig. 5.  The dependence of the LCC on temperature efficiency (each dot represents a separate case simulated)
Fig. 6.  The dependence of the LCC on pressure loss (each dot represents a separate case simulated)

Strojniški vestnik - Journal of Mechanical Engineering 67(2021)6, 302-310
309 Minimization of the Lifecycle Cost of a Rotary Heat Exchanger Used in Building Ventilation Systems in Cold Climates
Obviously, the total foil surface area exposed to the air 
has a much greater influence. This parameter affects 
the RHE’s temperature efficiency and its pressure 
drop.
5  CONCLUSIONS
1. The literature review reveals a lack of specific 
recommendations for the constructional 
parameters of rotary heat exchangers. Most 
research projects focus on simulating heat and 
mass transfer processes and on improving the 
efficiency of these processes. There is also a lack 
of discussion about the criteria of rotary heat 
exchanger optimization. Most cases only refer to 
the efficiency of heat and moisture recovery as a 
criterion.
2. Good coherence of the calculation results and 
laboratory tests made at the KONFOVENT 
laboratory proves the suitability of the CFD model 
for the calculation of the temperature efficiency 
of rotary heat exchanger described. These tests 
also prove the suitability of the analytical model 
presented in [13] for the calculation of pressure 
drop of a rotary heat exchanger.
3. The lifecycle costs analysis of 270 different 
RHE variants calculated using the CFD and 
analytical models has shown that the best results 
are achieved when the length of the rotary heat 
exchanger is maximal (400 mm in the cases 
analysed), foil thickness is minimal (0.06 mm in 
the cases analysed), and pressure loss is between 
100 Pa and 180 Pa. The temperature efficiency of 
an RHE in cold climate conditions, in this case, is 
expected to be between 85 % and 90 %.
4. This research confirms that it does not take the 
best heat recovery efficiency to achieve the best 
energy efficiency of a rotary heat exchanger due 
to its pressure loss. The difference between the 
heat exchangers with the lowest LCC and the heat 
exchangers with the highest LCC can reach 31 
% in the same weather and economic conditions; 
this difference depends mainly on the geometrical 
characteristics of the rotary heat exchanger.
8  REFERENCES
[1] EIA (2012). Commercial Buildings Energy Consumption 
Survey: Energy Usage Summary., from https://www.eia.gov/
consumption/commercial/reports/2012/energyusage/, 
accessed on 2021-01-22.
[2] Al-Ghamdi, A.S. (2006). Analysis of Air-to-Air Rotary Energy 
Wheels, PhD Thesis, Ohio University, Ohio.
LCC, the pressure loss of the heat exchanger should 
be within 100 Pa and 200 Pa. This chart, much like 
the one above, shows that both an excessively low 
and excessively high-temperature efficiency leads 
to increased LCC. Pressure loss that is too low leads 
to lower efficiency, which prevents more significant 
energy savings, while high-pressure loss leads to 
increased electricity costs.
In order to determine the optimal parameters of 
rotary heat exchangers, all heat exchangers with the 
LCC below the considered limit of EUR 3,800 were 
analysed (i.e., a total of 22). The length of the first five 
heat exchangers with the lowest LCC is 400 mm. It 
should also be noted that the foil thickness of all these 
heat exchangers is 0.06 mm, which further tightens 
the limits of the pressure loss that should be taken into 
consideration when producing a heat exchanger that is 
the most appealing in terms of the LCC, i.e., 101 Pa to 
179 Pa. In this case, the temperature efficiency of heat 
exchangers is between 85 % and 90 %.
Table 3.  Optimal parameters of heat exchangers
The length 
of a RHE 
[mm]
The number of 
heat exchangers of 
respective length [-]
b/a
[-]
s 
[mm]
a 
[mm]
b
[mm]
200 2 1.73 0.060 1.45 2.50
300 11 1.86 0.060 1.71 3.18
400 9 2.59 0.060 1.72 4.44
Table 3 lists the average parameters of heat 
exchangers. It becomes evident that the most typical 
lengths of heat exchangers with the lowest LCC are 
300 mm and 400 mm (11 and 9 units, respectively). 
Only two heat exchangers are 200 mm long. The 
table also shows that the foil of these heat exchangers 
is the thinnest out of those analysed in this study: 
0.06 mm. The average ratio of wavelength to wave 
height, in this case, is 1.73, 1.86, and 2.59 for the heat 
exchangers with the length of 200 mm, 300 mm, and 
400 mm, respectively. The average wave height of heat 
exchangers that are the most appealing in terms of the 
LCC is 1.45 mm (for 200 mm heat exchangers), 1.71 
mm (for 300 mm heat exchangers), and 1.72 mm (for 
400 mm heat exchangers). The average wavelength of 
heat exchangers is 2.50 mm for 200 mm, 3.18 mm for 
300 mm, and 4.44 mm for 400 mm.
Neither the wave height and wavelength nor 
the ratio of the two have been observed to have any 
impact. This can be explained by the narrow gauge 
of the air channels. While they are narrow enough 
to keep the physical proprieties of the air stream 
homogenous in the same cross-section, the shape of 
the air channel’s cross-section makes no difference. 

Strojniški vestnik - Journal of Mechanical Engineering 67(2021)6, 302-310
310 So k o lnik as,	I.	–	Čiuprinsk as,	K.	–	Čiuprinskienė,	J.
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