As described above, water is used as the transfer fluid in the single-panel LAMEE and the supply air state is regulated by the heater and humidifiers before entering the exchanger. The exchanger is tested at three different test conditions: air heating and humidifying (H&H), air cooling and humidifying (C&H), and air cooling and dehumidifying (C&D). The steady-state performance of the small-scale LAMEE is
31
evaluated at these conditions. Experiments at each condition are conducted at three different values for the number of heat transfer units (NTU) and constant Cr*. The Cr* was equal to 7 and NTU varied from 2.5, 3.5 and 4.5. The normal air and solution inlet conditions for each test are presented in Table 2-2; however, they are slightly different for each test case due to repeatability of the experiment at different NTUs. The measured air and solution inlet and outlet conditions for each experiment are reported in the results section. Also, the air and solution outlet conditions are measured at the steady-state operating condition to calculate the steady-state effectiveness of the single-panel LAMEE. The steady-state condition is determined when the temperature and relative humidity changes by time (d
d T t and d( ) d RH
t ) are smaller than their uncertainties in the
experiment, which are 0.1°C and 1%RH, respectively.
Afterwards, the numerical sensible and latent steady-state effectivenesses of the single-panel LAMEE at different supply air conditions are plotted on the psychrometric chart and verified with three experimental tests. The solution inlet temperature, NTU and
Cr* are set at 22˚C, 3.5 and 7 to generate those results. The designed air inlet and test
conditions for each test are shown in Table 2-2.
Table 2-2 Normal air and solution inlet test conditions
Test Case Air Inlet Solution Inlet
,°C
T W,g/kg T,°C
H&H 22 8.5 (52%RH) 30
C&H 35 7.1 (21%RH) 22
C&D 35 19.4 (55%RH) 16
Contour map, C&H#1 28 9.2 (39%RH) 22
Contour map, C&H#2 35 7.1 (21%RH) 22
32 2.5.1 Test Facility Mass and Energy Balances
It is necessary to do mass and energy balances for the SPEET facility before beginning the experiments to prove that the system is working at steady-state and generating accurate experimental data within acceptable uncertainty ranges. For this purpose, the conservation of mass and energy in the system is used to check the system bias error. If the mass and energy balance are not satisfied in the system at steady-state, it means that an unaccounted bias error exists (Simonson et al., 1999). Dry air and solution mass balances are checked in the energy exchanger to make sure that there is no air leakage in the system. To establish the dry air and solution mass balance the following equations should be satisfied (Simonson et al., 1999):
, , air
air air in air out m
m m m Un
(2.1)
, , sol
sol sol in sol out m
m m m Un
(2.2) where, is the mass flow rate and subscripts and refer to the air and solution (water) in the system, respectively. Also, subscripts and refer to the inlet and outlet of the air or solution, respectively.
air
m
Un and
sol
m
Un are the calculated uncertainty for measured air and solution mass flow rate differences in the system between the inlet and outlet, respectively. After establishing the dry air and solution mass balances, water vapor and energy in the system are required to be balanced to ensure valid experimental results. The water vapor and energy balances equations are provided as follow (Simonson et al., 1999):
mW
mW
air in,
mW
air out, msol in, msol out, UnmW (2.3)
mH
mH
air in,
mH
air out,
mH
sol in,
mH
sol out, UnmH33
where and are humidity ratio and specific enthalpy, respectively. Also,
mW
Un and UnmH are the calculated uncertainties for the water vapor and energy transfer in the system based on the measured air and solution properties in Eqs. (3) and (4), respectively. In Eq. (2.3), the water vapor transfer in the solution side, which is pure water in this experiment, is the water mass flow rate difference between the solution inlet and outlet. Additionally, the energy balance of the system can be represented in terms of percentage by using the energy exchange inequality (EEI) in the system. The energy exchange inequality in the LAMEE is the summation of the total energy transferred to/from the air and solution in the exchanger divided by the total input energy to the system and expressed as a percentage in Eq. (2.5).
, , , ,
, ,
100 air in air out sol in sol out
air in sol in mH mH mH mH EEI mH mH (2.5)
To verify the mass and energy balance, the small-scale LAMEE test facility is tested at a specific operating condition, where NTU and Cr* are set at 3.5 and 7, respectively. The steady-state air and solution inlet and outlet conditions and flow rates for this experimental test are presented in Table 2-3. To do the dry air mass balance, a mass flow controller with the capacity of 100 L/min and 2% of full-scale accuracy is used to supply the air into the system and measure the supply inlet air flow rate. A mass flow meter with the same capacity and 1% of full-scale accuracy is installed at the end of the air pipeline to measure the outlet air flow rate. Also, during the experiment the air side pressure in the system was monitored by using a barometer on the exchanger inlet to prevent any possible damage to the test setup due to over pressurizing the system because of using the mass flow meter at the outlet. According to the mass and energy balances
34
results in Table 2-3, the small-scale LAMEE test setup is balanced and is operating at a steady-state condition. Also, the water vapor and energy balances of the system during the experiments are checked based on the steady-state experimental results for the three designed test cases at NTU and Cr* equal to 3.5 and 7, respectively, and the results are presented in Table 2-4. The dry air and solution flow rates are assumed to be the same at the exchanger air and solution inlets and outlets for each test case. The results show that, the energy and mass are conserved within experimental uncertainties during the experiments.
Table 2-3 Air and solution properties measured at the inlet and outlet of the small-scale LAMEE to establish the mass and energy balance, and mass and energy balance results
Parameter Unit Value Uncertainty
Tair,in °C 23.1 0.2 Wair,in g/kg 1.82 0.15 Qair,in L/min 28 2 Tair,out °C 16.3 0.2 Wair,out g/kg 7.74 0.38 Qair,out L/min 25.9 1 Tsol,in °C 15.8 0.1 Qsol,in mL/min 55.3 2 Tsol,out °C 13.1 0.2 Qsol,out mL/min 54.1 1.1 air m kg/h 0.122 0.159 sol m kg/h 0.079 0.172
mW
kg/h 0.068 0.172
mH
W 3.4 3.7 EEI % 4.7 -35
Table 2-4 Water vapor and energy balance for small-scale LAMEE at different test operating conditions at NTU = 3.5 and Cr* = 7
Test case Water vapor balance Energy balance EEI
mW
, kg/h UnmW, kg/h
mH
, W UnmH, W %H&H 0.029 0.097 3.7 3.8 2.8
C&H 0.019 0.096 0.3 2.8 0.5
C&D 0.015 0.095 4.2 4.4 3.9