Aug 16, 2017 - terms of heat transfer effectiveness, and core type. ... propagate can lead to system failure and/or damage. The existing operating ...... recovery systems, systems energy analysis, Energy and Buildings 42 (2010) 1833â1840.
Accepted Manuscript Experimental study of air-to-air heat exchangers for use in arctic housing Colin Beattie, Paul Fazio, Radu Zmeureanu, Jiwu Rao PII: DOI: Reference:
S1359-4311(17)32685-6 https://doi.org/10.1016/j.applthermaleng.2017.10.112 ATE 11310
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
21 April 2017 16 August 2017 19 October 2017
Please cite this article as: C. Beattie, P. Fazio, R. Zmeureanu, J. Rao, Experimental study of air-to-air heat exchangers for use in arctic housing, Applied Thermal Engineering (2017), doi: https://doi.org/10.1016/j.applthermaleng. 2017.10.112
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Experimental study of air-to-air heat exchangers for use in Arctic housing Colin Beattie, Paul Fazio†, Radu Zmeureanu, Jiwu Rao Centre for Zero Energy Building Studies, Department of Building, Civil and Environmental Engineering, Concordia University, Montréal, Québec, Canada
Abstract During the operation of heat recovery systems in Arctic housing, the heat transfer effectiveness of air-to-air heat exchangers diminishes due to the frost formation, and as a consequence the potential for energy savings due to the heat recovery is lost. If the frost formation is not detected, it might damage the HEE core. There is no standard technique for the detection of required starting time of the defrost operation. Manufacturers of such HEE units use a pre-set duration of normal operation that is followed by defrost period. This paper presents the experimental results under laboratory-controlled environment of an airto-air heat/energy exchanger (HEE), tested with four different cores, to assess the performance under Arctic climate conditions. The frost formation is not directly measured, but indirectly through the evaluation of the effect of frost formation and management on the exhaust air flow rate, and the sensible and latent heat transfer effectiveness. Correlation-based models are developed that predict the change in exhaust air mass flow rate in terms of heat transfer effectiveness, and core type. The practical usage of those correlation for the ongoing commissioning of existing residential ventilation systems stays with the estimation of the starting of core defrosting. Thus the use of factory-set defrost schedule can be replaced by 1
a scheduled that is adapted to the operating conditions of the cores. The paper also proposes the use of a heat recovery system with two HEE units, and evaluates the heat transfer effectiveness and supply air flow rate of that system during the operation.
KEYWORDS: air-to-air heat/energy exchangers; frost management; experiment; laboratory; ventilation; Arctic housing. Nomenclature Name Floor area Specific heat of air
Symbol
Units m2 J kg−1 K−1
Heat of vaporization of water
J kg−1
Mass flow rate
kg s−1
Minimum mass flow rate
kg s−1
Pressure
Pa
Saturation pressure
Pa
Temperature
°C
Volumetric flow rate
m3 s−1
Minimum volumetric flow rate for ventilation
L s− 1
Sensible heat transfer effectiveness
–
Latent heat transfer effectiveness
–
Humidity ratio
kgwater kgdry air−1
1. Introduction Inuit Nunangat is the ancestral homeland of the Inuit people of Canada with a population of 52,115 (in 2011) of which 83% identified as being Inuit [1]. The largest city in the region, Iqaluit, is located on Baffin Island in the Canadian territory of Nunavut and is one of the fastest growing communities in Canada with an 8.3% increase in population between 2006 and 2011 [2]. The need for more and better housing is not only linked to the increase in population. Inade2
quate temperature and humidity control, along with high air leakage in the building envelopes in northern housing has been found to increase the occurrence of moisture-related problems such as mould-growth, mildew and frost [3]. In the past, designers relied on air leakage through the building envelope as the primary source of outside air. However, the airtight housing construction has become common practice in Canada to reduce space heating energy consumption. The importance of indoor air quality in northern housing was underlined in [4], which concluded that the insufficient ventilation and overcrowding within the homes maybe the reason for the higher rates of infections. A follow-up study [5] in the same region found that the installation of the air-to-air heat exchangers reduced indoor air pollutants such as carbon dioxide, and lowered the relative humidity of indoor air. There was also a reduction in reported respiratory infection symptoms in Inuit children. Although several studies have been carried out in the past on the frost management of air-toair heat recovery equipment, there is still a need for additional work to quantify the performance of such units in very cold climate such as of Northern Canada. Issues that require additional work are presented below. Air-to-air heat/energy exchangers (HEEs) are utilized extensively across Canada to reduce the energy required for heating the ventilation air in airtight housing. Limitations, however, still exist for northern Arctic regions. Due to the extremely cold inlet supply air temperatures in these regions, the moisture in the exhaust air condenses and results in frost formation within the heat exchanger core. This phenomenon can lead to reduction in system efficiency and if allowed to propagate can lead to system failure and/or damage. The existing operating procedure for an air-to-air heat exchanger in Arctic climates typically requires the pre-heating of supply inlet air before entering the heat/energy exchanger in order to
3
reduce the potential for frost formation. This can be costly, as Northern Canada has some of the highest end-use energy costs nationally because of the import of fuel from the south [6]. Conventional defrosting practices, while adequate in more temperate locations in Canada, are not optimal for managing the accumulation of frost in an exchanger in those regions. During the operation of such a heat recovery system, the heat transfer effectiveness diminishes due to the frost formation, and as a consequence the potential for energy savings due to the heat recovery is lost. The iced formed inside the core could block the channels and reduce the airflow rate removed from the house, which might lead to the deterioration of indoor air quality. Moreover, if the frost formation is not detected, it might damage the HEE core. There is no standard technique for the detection of frost formation and the starting of defrost operation. Manufacturers of such HEE units use a pre-set duration of normal operation that is followed by defrost period. In most residential systems only one air-to-air HEE unit is used. When the exhaust air is recirculated through the core as a retroactive frost management, the outdoor air is not supplied into the house, which affects negatively the indoor air quality. The paper is organized as follows. The literature review of the traditional and alternative frost management methods of air-to-air heat/energy exchangers (HEEs) are discussed in section 2. The experimental details of one HEE with different cores are presented in section 3 as well as the proposed system with two HEEs in parallel. The results are presented in sections 4, 5 and 6. Main conclusions are listed in section 7.
2. Literature Review In the air-to-air heat recovery units discussed in this paper, there is no direct contact between the outdoor air stream and exhaust air stream; they are separated by sheets of different
4
materials that allow either for heat transfer between the air streams, or for heat and moisture transfer. The heat is transferred from the warm and humid exhausted air from the house, to the cold and dry outdoor air that is supplied to the house. Air-to-air heat exchangers and air-to-air energy exchangers are sometimes referred to as HRVs (heat recovery ventilators) or ERVs (energy/enthalpy recover ventilators). Both types are used in residential and commercial buildings for the pre-heating and/or pre-cooling of supply air. More recently Zeng et al. [7] presented a detailed review on the air-to-air heat exchanger technologies for building applications Exchanger cores that transfer only sensible heat are typically constructed of corrugated plastic sheets made of polycarbonate. Although metal (aluminum or stainless steel) fixed-plate cores are available on the market, they are more expensive. Vapour-permeable cores are typically recommended for hot and humid climates where they have been proven to provide considerable energy savings [8,9]. One of the most common types of vapor-permeable core materials is polymerized paper. Membrane-based cores (MERV) are becoming more popular as research advances in the vapour transfer. One commonly used membrane core consist of porous desiccantloaded polymer substrate that is coated on one surface with a thin layer of water-permeable polymer [10]. The primary operational concern of heat/energy exchangers in cold climates is the formation of frost in the exchanger core. At cold inlet supply air temperatures, moisture in the warm and relatively humid exhaust air condenses and freezes on the internal surfaces of the heat exchanger core. In sensible-only heat exchanger cores, frosting is observed when inlet supply air temperature decrease below -5°C at typical indoor air relative humidity levels [11]. Due to the transfer of water-vapour from the exhaust airstream to the supply airstream, vapour-permeable cores begin to frost at inlet supply air temperatures below -10°C. Frost formation in the ex-
5
changer core reduces the cross-sectional area, and as a consequence it reduces the warm exhaust air flow rate. This reduces the amount of energy available to be recovered by the supply airstream [12,13]. Frosting in air-to-air energy exchangers has been extensively reviewed in [14]. The authors covered many topics including frosting in different types of air-to-air heat/energy exchangers, research related to frost properties, and different frost management strategies. Methods for managing core frosting are divided into two categories: 1) the proactive approach of frost control, which uses vapour-permeable cores, and 2) the retroactive approach of defrost [15]. In the second category, pre-heating that is an effective frost control method used in arctic applications; however, it is the most detrimental to the overall system efficiency [15], and can be very expensive in arctic Canada where energy sources are limited. The recirculation of exhaust air within the heat/energy exchanger is the most commonly used and energy efficient retroactive defrost mechanism in residential applications [16]. When frost formation within the HEE reaches a critical level, the unit stops the supply airflow and allows only the recirculation of warm exhaust air to defrost the system. At very cold exterior temperatures (i.e. below -35°C) the recirculation method may not be sufficient to defrost the system. Moreover, the recirculation, while better than other retroactive frost control methods, still uses a significant amount of energy and is far from the ideal in Arctic climates [17]. Recent studies of cold weather heat/energy exchanger operation focused towards the use of two heat/energy exchangers for the management of core frosting. One study [18] proposed a design with two fixed-plate heat exchangers in series, with alternating airflow directions. The system was installed in a low-energy house in Greenland and monitored over a five-year period [19]. The sensible heat transfer effectiveness varied from 55% to 80%, with an annual average of 60%.
6
A multi-section, parallel configuration heat recovery system [20] was proposed for operation in cold climate. Within the active core, 90% of the exhaust air is allowed to exchange with 100% of the supply air. The remaining 10% of the warm exhaust air is diverted to the previously active core resulting in the defrosting of accumulated frost. After a pre-set period of time, the system switches between the cores repeating the cycle. The system was evaluated for long-term operation in a laboratory set-up under frosting conditions, and achieved a sensible heat recovery effectiveness of 88%. Another system [21], designed for sensible and latent heat recovery and moisture recovery, consists of a single air channel with a layer of heat storage material and a layer of moisture absorbing material. During the charging phase, the warm and humid exhaust air from the interior first travels through the water absorbent material resulting in a decrease in the moisture content of the air. Then the air passes through the thermal storage material where the warm exhaust air is cooled down. At a predetermined temperature drop across the system, the direction of air flow changes resulting in the discharge of both thermal and moisture storage. The system was capable of recovering up to 95% of energy and 70-90% of moisture from the exhaust air. Two alternating systems are required in order to provide continuous airflow. The system requires high electric power input to fans due to the pressure loss in the heat storage and moisture absorbing materials. The authors of this paper presented preliminary experimental results in [22] for the case with one sensible heat exchanger core, and one sensible and latent heat core that uses watervapor membrane. The scope of this paper is threefold: (i)
the evaluation of the effect of frost formation and management on the exhaust air flow rate, and the sensible and latent heat transfer effectiveness of an air-to-air heat/energy ex-
7
changer (HEE) with four different cores, which are produced by two Canadian manufacturers; (ii)
the development of a method for the ongoing commissioning of heat recovery units of residential ventilation systems. For this purpose, laboratory-controlled experiments were undertaken to generate data for the development of correlation-based models that can signal when the defrosting of exhaust channel should start. The reduction of exhaust airflow rate below the initial value is measured, and when it reaches the value corresponding to a set value of the reduction of heat transfer effectiveness, the defrost starts. The correlation-based models, which were developed for the four exchanger cores supplied by the manufacturers, should be implemented in the ongoing commissioning control program. The approached used in this study can be repeated in the laboratory for any other cores of different design and materials. It is beyond the scope of this study the development of first-principle models; and
(iii)
the assessment of the performance of a proposed heat recovery system with two HEE units with alternating defrost, which provides the minimum air ventilation rate, and thus avoids the risk of supplying less ventilation when only one unit is used under defrost operation.
3. Experimental details Fig. 1 identifies the notation of airflow inlets and outlets of one HEE, which is used in this paper, where: #1 = inlet supply air from outside; #2 = outlet supply air to the house; #3 = inlet exhaust air from the house; #4 = outlet exhaust air to outside.
8
#4
#1
#3
#2 Heat/Energy Exchanger Heat/Energy Exchanger Core
Fig. 1. Airflows identification of one HEE
3.1. Operation with one HEE The air-to-air HEE with a fixed-plate cross-flow exchanger [23,24], used in this study, was provided by a Canadian manufacturer. The exchanger is insulated from the ambient conditions with expanded polystyrene. Four different exchanger cores (HRV, ERV, MERV1 and MERV2) were provided by two manufacturers for the purpose of this study (Table 1); all are of cross-flow fixed-plate type with the same dimensions of 0.254 m x 0.254 m x 0.362 m.
Table 1. HEE core specifications
Core
Transfer between airstreams
Core material
Exchange surface area, A [m2]
HRV
Heat only
Polypropylene
9.7
ERV
Heat and moisture
Polymerized paper
9.5
MERV1
Heat and moisture
MERV2
Heat and moisture
Substrate with vapourpermeable coating
8.8 8.8
9
All the cores feature a sandwich-style structure (Fig. 2.a to 2.c). The HRV core consists of corrugated polypropylene sheets separated and supported by spacers of the same material. All vapor-permeable cores have a similar construction; heat and moisture permeable material is separated and supported by a corrugated metal sheet. The only difference between the MERV1 and MERV2 cores is the type of vapor-permeable coating applied to the membrane substrate.
Fig. 2.a. HRV core construction
Fig. 2.b. ERV core construction
10
Fig. 2.c. MERV1 and MERV2 cores construction
The defrosting of the exchanger core is accomplished through the recirculation of the exhaust air. When recirculation is triggered, the inlet supply air (#1) and outlet exhaust air (#4) ports are closed, and the inlet exhaust air (#3) is redirected to the outlet supply air (#2) port. The factory-set defrost schedule has different operational bands based on the inlet supply air temperature (Table 2). The energy exchanger core can operate without defrost until temperatures of 10°C, while the heat exchanger core requires defrost when inlet supply air temperatures is below -5°C. The defrost schedules are the same for the heat and energy exchanger cores when inlet supply temperatures are lower than -10°C. When the inlet supply temperature is -25°C, as an example, the HEE core operates normally for 25 min, followed by the defrost mode for seven minutes. Table 2. Factory-set defrost schedule [23,24] Defrost/operating time [min]
Inlet supply air temperature [°C]
Heat exchanger core
Energy exchanger core
Warmer than -5
No Defrost
No Defrost
-5 to -10
7/25
No Defrost
-10 to -27
7/25
7/25
Lower than -27
10/22
10/22 11
3.2. Proposed system operation with two HEEs in parallel The proposed system consists of two HEEs (#1 and #2), which operate in parallel. Fig. 3 shows the duration of the defrosting, normal operating and standby periods for the case of two exchangers in parallel, for one cycle, using the same factory-set schedules (Table 1). For instance, when the inlet supply air temperature drops below -27°C, the HEE #2 operates for 22 minutes, followed by 10 min in the defrost mode, and 12 min in standby. While the HEE #2 is in normal operating mode for 22 min, the HEE #1 is in the defrost mode for 10 min, followed by the standby mode for 12 min. Then the HEE #1 starts the normal operating mode of 22 min, while the HEE #2 enters the defrost mode followed by standby.
Tinlet,supply < -27 C Exchanger 1
10
Exchanger 2
12
22
22
10
12
-27 C < Tinlet,supply < -5 C Exchanger 1
7
Exchanger 2
18
25
25 0
7 20
18 40
60
mins Standby
Defrost
Operating
Fig. 3. Defrosting operational schedule with two HEEs working in parallel
3.3. Heat transfer effectiveness
12
The frost formation is not directly measured, but indirectly through the evaluation of the effect of frost formation and management on the exhaust air flow rate, and the sensible latent
and
heat transfer effectiveness. The mass of the core was also measured at the beginning and
end of each experiment, for each core and inlet supply air temperature, to evaluate the mass of ice. The effectiveness-NTU method can used for the assessment of the performance of air-to-air heat exchangers from flow rates, heat and mass transfer resistance, and heat transfer area [25-27]. However, the authors opted for the use of the effectiveness, in compliance with ASHRAE Standard 84-2013, Method of Testing Air-to-Air Heat/Energy Exchangers [28], which offers the adequate tool for the ongoing commission of residential ventilation system. The heat transfer effectiveness
or
of an air-to-air HEE is defined as the ratio between
the actual energy transferred and the potential maximum energy transferred between the airstreams [28]. In Eqs. (1) and (2) the denominators are calculated as the average values of the first five minutes of the evaluation period, when there was no frost.
[-]
(1)
[-]
(2)
where: = sensible heat transfer effectiveness (-) = latent heat transfer effectiveness (-) = inlet supply air temperature, °C = outlet supply air temperature, °C 13
= inlet exhaust air temperature, °C = inlet supply air humidity ratio, kg/kg = outlet supply air humidity ratio, kg/kg = inlet exhaust air humidity ratio, kg/kg = mass flow rate of dry air, kg/s = minimum mass flow rate of dry air between inlets #1 and #3, kg/s = heat of vaporization of water, J/kg = specific heat of dry air, J/(kg∙K).
3.4. Experimental setup The HEE unit was connected to the Environmental Chamber that is able to simulate the outdoor temperatures between -65°C and 50°C, and to the Room Chamber that controls the indoor thermal conditions, where the indoor air temperature was maintained at 21.7 ±0.3°C, and relative humidity at 35%, which correspond to measurements in Northern Canadian house [3]. Thus the effectiveness of those air-to-air heat exchangers was assessed for normal operating conditions in Northern houses. It was beyond the scope of this study to assess the performance at other indoor conditions. There are test-ducts (TD), installed at each inlet and outlet of the HEE unit, each one measuring the air temperature, humidity and pressure differential, from which the air flow rate was calculated [28]. The measurement recording interval was 30 s for all variables. Fig. 4 shows the experimental set-up for the case of two HEEs working in parallel. The ducts were wrapped with mineral wool insulation and sealed with vapor barrier in order to reduce the heat loss/gains from the ambient air in the laboratory and reduce the potential for condensation on the ducts.
14
The testing of the air-to-air heat/energy exchangers and cores complied with ASHRAE Standard 84-2013 [28] and CAN/CSA-C439-09 [29]. The air temperature measurements were conducted in accordance with ASHRAE Standard 41.1-2013 [30], using a T Copper and Constantan thermocouple traverse. Eight thermocouples were used to determine the average temperature across the cross-section of each test-duct. The measurement of moist air properties was conducted in accordance with ASHRAE Standard 41.6-1994 [31]. The relative humidity of the airstreams was determined using a collection of capacitive thin-film hygrometer sensors. The airflow measurement station measures the average pressure difference between the total pressure and static pressure, through a traverse Pitot-tube arrangement, in compliance with ASHRAE Standard 41.2-1987 [32] and ASHRAE Standard 41.3-1989 [33], from which the air mass flow-rate was calculated.
15
Fig. 4. Schematic diagram of the experimental setup of the proposed system with two HEEs in parallel
3.5. Measurement uncertainty and propagation of error The overall uncertainty of a measured variable consists of two components, the bias error and the random error [34]. The bias error is a constant offset that is normally given by manufacturers, or is due to the improper sensor installation. The random errors are the random differences from one measurement to another due to sensor noise and extraneous conditions. The random error for each measurement device was determined based on a sample of measurements.
16
The uncertainty of a variable that is calculated from measurements (e.g., mass flow rate, or sensible heat transfer effectiveness) was estimated by considering the error propagation through the data reduction equation. The overall uncertainty of measured and calculated variables is summarized in Table 3. Table 3. Overall uncertainty of measured and calculated variables Variable
Overall Uncertainty, Ux
Average air temperature, T Saturation pressure, Psat Relative humidity,
%
Humidity ratio, Volumetric flow rate, Mass flow rate, Sensible heat transfer effectiveness, Latent heat transfer effectiveness, Heat flow rate Exchanger core mass
±0.10 kW kg
3.6. Experimental conditions Six inlet air supply air conditions were selected for the experiments: -5°C, -10°C, -15°C, 20°C, -25°C and -35°C, which are representative for the outdoor air conditions experienced in Iqaluit, Nunavut [35]. Air temperatures above 0°C were not used because the scope of this project was limited to conditions where frost formation is a concern. The supply and exhaust streams of the experimental setup are equal to 38.9 L/s according to [36], for a house of 140 m2 with 4 people. All air ducts have the diameter of 152.4 mm (6 inches), which gives the average air inlet velocity of 2.13 m/s. 17
The experiments were separated into three phases. In the first phase, one HEE was operated without recirculation defrost, by disabling the control, for longer operation time than the factory-set schedules. This approach allowed for unrestricted frost formation in the cores. The duration of each test for all units was three hours for inlet air temperatures between -5°C and -20°C, except for the HRV core which was tested only for two hours only to prevent core damage caused by frosting. In the second phase, one HEE was operated with the recirculation defrost as controlled by the factory-set schedules (Table 1). All cores were tested at the inlet supply air temperatures of 25°C and -35°C for 3 hours. In the third phase, two HEEs units were operated in parallel with alternating recirculation defrost (Fig. 4). Three cores were selected, HRV, ERV and MERV1, and tested at the inlet supply air temperatures of -25°C and -35°C for 3 hours.
4. Results of phase 1 with one HEE unit without defrost This section presents the results for one HEE unit with ERV, MERV1, MERV2 and HRV cores, operating with the recirculation defrost disabled, and at different inlet supply air temperatures.
4.1. Reduction of exhaust air mass flow rate Due to the formation of frost on the exhaust air channel, the free cross-sectional area of the channel was reduced. As a consequence the mass air flow rate
was reduced as the fan capac-
ity was exceeded. Fig. 5 shows the reduction with time of the ratio between the instantaneous exhaust air mass flow rate
at any given time and the initial average air mass flow rate
.
18
The denominator was calculated during the first 10 minutes of the test, when there was no frost in the exhaust channel.
Fig. 5. Reduction of the ratio for a) ERV b) MERV1 c) MERV2 and d) HRV cores at different inlet supply air temperatures,
19
The exhaust air mass flow rate remains almost constant for all cores when greater than -10°C. The rate and magnitude of the decrease of
is equal or
varied between cores, with the
HRV core having the highest decrease when the inlet supply air temperature was lower than or equal to −25°C. ERV core experienced the smallest reduction in the mass flow rate at all tested inlet supply air temperatures, because the water vapor was transferred through the membrane, and experienced negligible frost. Thus the ERV core is a good alternative for the proactive frost management. For instance, when T1 = -25°C after two hours of continuous operation, the ERV core experienced a reduction of 4.4%, the MERV1 core had 21.7%, the MERV2 core had 23.5%, and the HRV core had 29.9%.
4.2. Increase of mass of cores The increase of mass of each core, as composed of the ice mass and the mass of water vapour that was absorbed by permeable material of cores such as ERV, was also measured (Table 4). The initial dry mass of the ERV, MERV1, MERV2 and HRV cores was 4.24 kg, 1.50 kg, 1.50 kg and 3.30 kg, respectively. The post experiment mass of all cores increased when the inlet supply air temperature decreases. Table 4. Increase in the mass of cores at different inlet supply air temperature Inlet supply air temperature [°C]
Increase in mass [kg] ERV
MERV1
MERV2
HRV
−5
n/a
n/a
n/a
0.28
−10
0.26
0.06
0.14
0.42
−15
0.32
0.14
0.24
0.56
20
−20
0.40
0.24
0.36
n/a
−25
0.50
*0.40
0.40
0.70
−35
0.46
0.24
0.42
0.44
* Measurement recorded after 3 hour experiment instead of 2 hours
The HRV core experienced the largest increase of frost mass compared to the other cores because there was no water vapour transferred to the supply airstream. In the case of
= −35°C
the test was aborted after 50 minutes due to concerns regarding the HRV core damage. The MERV1 and MERV2 cores experienced smaller accumulation of frost when compared to the HRV core. The results of the ERV core showed a higher increase in the core mass than in the two other vapour-permeable cores (MERV1 and MERV2). This is due to the material of the core which not only transfers but absorbs and retains a certain portion of the moisture in the exhaust airstream.
4.3. Sensible and latent heat transfer effectiveness The sensible heat transfer effectiveness of the test for
for all cores remained constant for the duration
= −10°C. Below this temperature, the core frost reduced the sensible heat trans-
fer between the exhaust and supply airstreams. The HRV core had a significant decrease in inlet supply air temperatures below −15°C. The decrease in
for
was witnessed for the membrane-
based cores only at inlet supply air temperatures below −20°C. The HRV core had the lowest latent heat transfer effectiveness
of about 0.05 for all tests
because the transfer of water vapour between the two airstreams is not permitted by the core materials. The MERV1 and MERV2 cores had
of about 0.55 and 0.43, respectively, for the dura-
21
tion of the tests when
was −10ºC and -15ºC. The latent heat transfer effectiveness reached 0.4
and 0.25, respectively, at lower inlet supply air temperature below −10°C. The membrane-type cores (MERV1 & MERV2) experienced a fast instantaneous transfer of water vapour between airstreams (Fig. 6). This was not the case for ERV core, where the
at
all tested temperatures required about 60 minutes to reach steady state. This delay was due to the absorption of water vapor by the core materials (polymerized paper). After reaching steady state, the latent heat transfer effectiveness of the core ERV was about 0.62, the highest compared to all other cores.
22
Fig. 6. Change with time of the adjusted latent heat transfer effectiveness, b) MERV1 cores at different inlet supply air temperatures, T1
for a) ERV, and
4.4. Correlation-based models From these experiments, some correlation correlation-based models are developed between the reduction of exhaust air flow rate, which is due to the frost formation, and the reduction of heat transfer effectiveness of the cores. The practical usage of those correlation for the ongoing commissioning of existing residential ventilation systems stays with the estimation of the starting of core defrosting. Thus the use of factory-set defrost schedule can be replaced by a scheduled that is adapted to the operating conditions of the cores. The coefficient of determination R2 has values between 0.91 and 0.99, which proves the strength of linear relationship between the reduction heat transfer effectiveness and the reduction of mass flow rate. The use of these correlations also avoids the use of any intrusive device for detecting the frost formation. The results for all cores when
= −10°C, and those from ERV core were ex-
23
cluded because there was almost no change of
and
for all tested inlet supply air tempera-
tures. The reduction of the exhaust air mass flow rate with respect to the value of initial conditions (
) versus the reduction of sensible heat transfer effectiveness with respect to the value of
initial conditions (
) is derived as follows, for the inlet supply air temperature between
−15°C and −35°C: = 1.43∙
– 0.43
for MERV1 and MERV2 cores, and 1 ≥ = 1.25∙ for HRV core, and 1.0 ˃
≥ 0.8 - 0.25
[4]
≥ 0.9 =
for HRV core, and 0.9 ˃
[3]
0.02
[5]
≥ 0.8.
As an example of use, if the maximum acceptable reduction of sensible heat transfer effectiveness is set to 10% of the initial value due to the frost formation, i.e.,
= 0.9, then the
core defrosting should start when the measured exhaust air mass flow rate reached 86% of the initial value for MRV1 and MRV2 cores, and 88% for HRV core.
5. Results of phase 2 with one HEE unit with defrost This section presents the results for one HEE unit with ERV, MERV1, MERV2 and HRV cores, operating with the core defrost enabled, and at the inlet supply air temperatures of −25°C and −35°C.
5.1 Reduction of time-averaged supply of outside air 24
The experiments with one HEE with defrost control proved that the use of factory-set defrost time is a conservative approach that adequately manages the formation of frost. As expected, the ERV core presented little to no change in the inlet exhaust air mass flow rate during the exchange-cycle for both tested inlet supply air temperatures. There was only a negligible decrease in the inlet exhaust air mass flow rates during the exchange-cycles for the MERV1, MERV2 and HRV cores when T1 = −35°C. For the ventilation of low-rise residential buildings [32], when the ventilation is intermittent, it is required that the time-averaged supply air volumetric flow rate over the period of three hours should meet or exceed the required ventilation rate (38.9 L/s in this study). However, the measurements revealed that was a reduction of the time-averaged supply of outdoor air as a result of the time-controlled defrost-cycles over the 3 hour period (Table 5). For instance at -25ºC the time-averaged supply air flow rate was only of 26.0 L/s compared with the required ventilation rate of 38.9 L/s.
Table 5. Duration of exchange and defrost cycles, and time-averaged supply air flow rate for the HRV core Cycles duration over 3 hour
Inlet supply air temperature [°C]
Exchange-cycle [min]
Defrost-cycle [min]
above −5
180
n/a
32.5
−25
144
36
26.0
−35
128.5
51.5
23.1
Time-averaged supply air flow rate [L/s]
If the HEE unit is selected based on the supply air flow rate when the inlet supply air temperatures are above -5°C (when defrosting is not required), the ventilation requirements might 25
not be met during times when a defrost-cycle is required. On the contrary, if the HEE unit is selected based on the flow rates when
= -35°C, it might result in over ventilation for periods
when defrost is not required.
5.2. Sensible and latent heat transfer effectiveness The average sensible and latent heat transfer effectiveness (Table 6) of each core were calculated for the last exchange-cycle of each three hour test, in order minimize the influence of the starting conditions. There was no significant difference between the results for each core, for the two inlet supply air temperatures used in this experimental phase, i.e., -25ºC and -35ºC. Due to the small thickness of heat transfer surface for all cores, the thermal resistance of the plates is negligible, and therefore the sensible heat transfer effectiveness should be almost the same for all exchangers. The HRV core has a larger heat transfer surface area than the other cores (Table 1). As a result, for this study the HRV core had higher
values of 0.83-0.84 com-
pared with 0.76-0.78 for the vapor-permeable cores (ERV, MERV1 and MERV2). The ERV core had the highest
values of 0.59-0.61 compared to the other cores. Since the HRV core is
impermeable to water vapor, its
value is less than the uncertainty of estimating
.
While there was a negligible difference in the sensible heat transfer effectiveness for the membrane-based cores (MERV1 and MERV2), there was a substantial difference between latent heat transfer effectiveness. The manufacturer did not disclose the material properties of the membrane cores. The results for the two membrane-based cores underline the importance of choosing the appropriate vapor-permeable coating for a given HVAC application. Table 6. Average sensible and latent heat transfer effectiveness of HEE with defrost controlled by the factory-set defrost schedule
26
Sensible heat transfer effectiveness [-]
Core
Latent heat transfer effectiveness [-]
T1 = −25°C
T1 = −35°C
T1 = −25°C
T1 = −35°C
ERV
0.78
0.77
0.61
0.59
MERV1
0.76
0.77
0.52
0.48
MERV2
0.76
0.76
0.46
0.43
HRV
0.83
0.84
0.0
0.0
6. Results of phase with two HEE units with alternating defrost Three cores, i.e., HRV, ERV and MERV1, were selected for the third experimental phase. The two HEE units, with HRV, ERV, or MERV1 cores, were operated for three hours with alternating recirculation defrost according to the schedule presented in Fig. 3 at the inlet supply air temperatures of −25°C and −35°C.
6.1. Air mass flow rates Fig. 6 shows the variation over two cycles of the inlet supply and inlet exhaust air mass flow rates of two HEE units with ERV cores at
= −25°C. During the first cycle, when the
HEE #1 was in exchange mode (EX), the inlet supply mass flow rate ( and the inlet exhaust (
) was about 0.055 kg/s
) was about 0.05 kg/s. During the same time, the HEE #2 was first in
defrost mode (DF) with the recirculation of inlet exhaust mass flow rate (
) of about 0.04
kg/s; there was no outside air through the inlet supply. There was no air circulated through the HEE#2 when it was in standby mode (SB).
27
The air mass flow rate of the exhaust air (
) increased for HEE#1 when the exchanger
was in the defrost cycle (Fig. 7). The air mass flow rate of the exhaust air
decreased for
HEE#2 when the exchanger was in the defrost cycle. The difference might be due to the configuration of the air ducts leading to and from each HEE unit. Since the ducting, to and from the two HEEs was not identical, it could explain the difference in the air mass flow rate of the exhaust air from the two HEE units. The average heat flow rate gained by the outdoor cold air stream in the HEE unit during the normal operation was calculated as 1.60 kW, while the average heat flow rate lost by the exhaust warm indoor air was 1.25 kW. Since the difference of heat flow rates is 0.35 kW exceed the uncertainty due to the propagation of errors of 0.1 kW (Table 3), one can conclude that the incoming cold air is also heated by the laboratory warm air through the shell of HEE unit.
28
Fig. 7. Inlet supply and exhaust air mass flow rates for two HEEs with alternating defrost: ERV core at = −25°C
In the case of two HEE units with alternating defrost, the heat transfer effectiveness is equal to that of one HEE (Table 6) since both operate with the defrost controller in operation. It is important to note that the heat transfer effectiveness presented in Table 6 was calculated for the last exchange-cycle of each three hour test, in order minimize the influence of the starting conditions. The main advantage of the operation with two HEE is the supply of a continuous air flow rate as a result of the alternating between the two HEE units. This is a better result compared to the case of one single HEE unit with defrost (Table 5) where there was a reduction in the time-average supply air volumetric flow rates below the minimum outdoor air rate. The increase in the amount of air supplied to a house was noticed when one HEE was in exchange mode and the other HEE was in defrost mode (Fig. 7). During those periods, the out29
side air supplied by one HEE was combined with the recirculated air used for defrost of the second HEE. This may prove to be useful when sizing HEE units for residential applications because the amount of air supplied to the home (time average volumetric flow rate) does not diminish when there is a defrost-cycle.
6.2. Sensible and latent heat transfer effectiveness The average sensible (Table 7) and latent (Table 8) heat transfer effectiveness were evaluated over the last six minutes of the third exchange-cycle. This time period was selected to minimize the effects of the initial test conditions, which are caused by the thermal mass and moisture absorption of the cores. The change of inlet supply air temperature from -25ºC to -35ºC had not change significantly the sensible heat transfer effectiveness for both HEE units with ERV and HRV cores, while there was a small decrease for the MERV1 core at
= −35°C when compared to
=
−25°C. The HRV core had slightly higher values compared to the other cores. The
values of the ERV and MERV1 cores (Table 8) were lower at colder inlet supply
air temperatures. Since HRV core is impermeable to water vapor, its
value was negligible.
Table 7. Sensible heat transfer effectiveness with alternating defrost Core
[-]
30
T1 = −25°C
T1 = −35°C
HEE #1
HEE #2
HEE #1
HEE #2
ERV
0.82
0.74
0.82
0.74
MERV1
0.82
0.72
0.75
0.69
HRV
0.83
0.78
0.86
0.78
Table 8. Latent heat transfer effectiveness with alternating defrost [-] T1 = −25°C
Core
T1 = −35°C
HEE #1
HEE #2
HEE #1
HEE #2
ERV
0.60
0.57
0.56
0.53
MERV1
0.56
0.49
0.41
0.40
HRV
0.04
0.02
0.05
0.01
The HEE#2 had smaller
and
values for all cores and inlet air temperatures compared
with the HEE#1. This difference was due to the air leakage between airstreams across the exchanger cores. The outlet supply volumetric air flow rate of HEE#1 increased by 8-12% compared with the inlet air flow rate, while the outlet exhaust air flow rate decreased by 1-8%. In the case of HEE#2 the outlet supply flow rate decreased by less than 4%, while the outlet exhaust air flow rate increased by 1-8%.
7. Conclusions The paper presented the experimental results under laboratory-controlled environment of an air-to-air heat/energy exchanger (HEE), tested with four different cores, to assess the performance under Arctic climate conditions, and the performance of proposed system with two HEEs
31
with alternating defrost. The analysis of experimental results led to the following main conclusions: 1. The reduction of exhaust mass flow rate due to the frost formation was influenced by the inlet supply air temperature and the core type. For instance, when T 1 =-25°C after two hours of continuous operation, the ERV core experienced a reduction of 4.4%, the MERV1 core had 21.7%, the MERV2 core had 23.5%, and the HRV core had 29.9%. 2. The vapor-permeable cores (ERV, MERV1 and MERV2) should be utilized instead of HRV core as proactive management of core frosting in Northern Canada and for any region where core frosting is of a concern. 3. The use of correlation-based models, which predict the change in heat transfer effectiveness in terms of the change in exhaust air mass flow rate, inlet supply air temperature and core type, can help to detect the frost formation in the cores. For instance, if the maximum acceptable reduction of sensible heat transfer effectiveness due to the frost formation is set to 10% of the initial value, i.e.,
= 0.9, then the core defrosting should start when the meas-
ured exhaust air mass flow rate reached 85% of the initial value for MRV1 and MRV2, and 88% for HRV. 4. When the operation of HEE unit is controlled by the timer, which reduces the risk of frost accumulation, all cores have almost the same sensible heat transfer effectiveness, which is independent of the cold inlet supply outdoor air temperatures of -25°C and -35°C. 5. The experiments with one HEE with defrost control proved that the factory-set defrost time is a conservative approach that adequately manages the formation of frost, however, there is a reduction of the time-averaged supply of outdoor due to defrost cycles. As an example, when
32
the HRV core is tested at -25°C, the time-averaged over three hours of supply outdoor air flow rate is reduced from 39 L/s to 26.0 L/s. 6. The proposed system of two HEE in parallel provided a continuous supply of outdoor air, which improves the indoor air conditions compared to the case of one single HEE with defrost, where the time-average supply outdoor air flow rate is below the recommended ventilation rate.
Acknowledgements The authors acknowledge the financial support from Natural Sciences and Engineering Research Council of Canada, through the Smart Net-Zero Energy Buildings Strategic Research Network, and Faculty of Engineering and Computer Science of Concordia. Additionally, the authors would like to thank Venmar Ventilation ULC and dPoint Technologies for providing technical support and experimental equipment.
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Highlights
Measurements of air-to-air energy exchangers under Arctic conditions are presented Reduction of exhaust mass flow rate due to frost formation in HEE is revealed Change in heat transfer effectiveness vs change in air flow rate is predicted Operation with one vs two HEEs with alternating recirculation defrost is compared
36
