Effects of Non-Uniform Refrigerant and Air Flow Distributions on

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ICR0040




  Effects of Non-Uniform Refrigerant and Air Flow Distributions on Finned-
                       Tube Evaporator Performance

                       Jong Min Choi, W. Vance Payne and Piotr A. Domanski

                          National Institute of Standards and Technology
                                 Building Environment Division
                               MS 8631, Building 226, Room B114
                               Gaithersburg, Maryland USA 20899
              Phone: 301-975-6663 Fax: 301-975-8973 Email: vance.payne@nist.gov

                                                   ABSTRACT
An experimental investigation was implemented to determine the capacity degradation due to non-uniform
refrigerant and air flow distributions, and to assess the potential to recover the lost capacity via controlling
refrigerant distribution between individual refrigerant circuits. The tests were performed on a three-circuit, three-
depth-row, finned-tube evaporator. Refrigerant inlet quality, exit saturation temperature, and exit superheats for the
individual circuits were controlled.

The study showed that capacity degradation due to refrigerant maldistribution can be as much as 30 %, even when
the overall evaporator superheat is kept at the target 5.6 C. Experimental data indicate that part of this capacity
degradation was caused by the internal heat transfer within the evaporator assembly. For the coil and air
maldistributions studied, the maximum capacity degradation was found to be 8.7 %. A 4.0% capacity recovery was
obtained by controlling refrigerant distribution to obtain the target 5.6 C at each circuit exit.


                                               INTRODUCTION
Most evaporators use an inlet expansion valve with a flow distributor to control the overall superheat at the
evaporator exit manifold. In the evaporator, the refrigerant flows through parallel refrigerant circuits, which are
designed to optimize between the benefit of improved refrigerant heat transfer and the penalty of refrigerant pressure
drop. The coil performs optimally when the superheat at individual circuit exits matches the desired overall
superheat in the exit manifold.

Evaporator air velocities may vary due to the geometry of the heat exchanger installation, nearness of the blower,
blockage of the air filter, and other factors. Non-uniform air flow can cause some circuits to have excessive
superheat while others may remain two-phase at the evaporator exit. In such situations, some circuits inefficiently
use coil area when transferring heat with superheated vapor instead of two-phase refrigerant.

Liang et al. (2001) conducted a numerical study of an evaporator with various refrigerant circuits operating with
uniform air flow. The researchers found that using a complex, optimized refrigerant circuit arrangement with
refrigerant circuits properly branched may reduce the needed heat transfer area by about 5 % for the same capacity.
Kirby et al. (1998) experimentally investigated the performance of a 5275 W window air conditioner under wet and
dry coil conditions with non-uniform air flow over the evaporator. The velocity variation over the evaporator varied
by a factor of 3, but upon correcting the non-uniformity of air flow, the investigators saw only a minor improvement
in performance. Chwalowski et al. (1989) performed a simulation and experimental study of an evaporator
operating with five different air velocities cause by different installations. In the extreme case, they reported a
capacity difference of 30 %. Lee et al. (1997) executed a simulation study of an R22 and R407C evaporator with
non-uniform air and refrigerant distributions. The study showed that the level of capacity degradation is affected by
the refrigerant circuitry design and air velocity profile relative to that circuit. For the cases studied, they reported
that the capacity of the evaporator showed a greater sensitivity to air maldistribution than to refrigerant
maldistribution.


                           International Congress of Refrigeration 2003, Washington, D.C.
                                                          1
                                                                                                                                    ICR0040


The goal of this study was to investigate the potential capacity improvements due to smart refrigerant distributors
capable of controlling refrigerant distribution within each circuit. The experimental setup used a three-refrigerant-
circuit evaporator with a system of valves, which allow controlling individual refrigerant superheats at the desired
level. The effects of non-uniform refrigerant and air distributions were studied. A more detailed report of the this
investigation is reported in Payne and Domanski (2003).

                      1. EXPERIMENTAL SETUP AND TEST PROCEDURE
1.1 Experimental Setup
Figure 1 shows a schematic of the experimental setup. The test rig consists of three major flow loops: (1) a
refrigerant flow loop containing an evaporator, (2) a water flow loop used for the condenser, and (3) an air flow loop
for the evaporator. The design of the rig allowed easy control of operating parameters such as condensing pressure
and subcooling at the inlet of the expansion valve (evaporator inlet enthalpy), evaporating pressure and superheat at
the exit of the evaporator.

An open-type reciprocating compressor with a variable speed motor was used to adjust the refrigerant mass flow
rate. Condensing pressure and subcooling were set by changing the supplied water flow rate and temperature from
the portable chiller to the condensing heat exchanger and subcooler, respectively. The control of condensing
pressure and subcooling allows evaporator inlet quality to be controlled to within  1 %.

Figure 2 presents the schematic of the R22 finned-tube evaporator used in this study. The evaporator has 54 smooth
copper tubes placed in three depth rows and three parallel circuits. The tubes have louvered aluminum fins. Inlets of
each circuit are connected to an individual manual expansion valve to adjust each circuit's exit superheat. A
pressure-regulating valve controls the evaporator exit pressure. A Coriolis-type mass flow meter measures
refrigerant flow rate in the liquid line between the subcooler and the expansion valves.

                                                                                                               Chiller



                                                   Water-cooled
                                                    subcooler               Water-cooled shell
                              Mass flow meter                               and tube condenser
                                                                                                                   Variable speed
                                                                                                                    compressor
                                    Filter drier



                                                           PT                 M                 PT
                                                           PT                                   PT
                               Turbine flow                       Needle valve M
                                  meter                    PT                                   PT
                                                                              M                           Pressure
                                                                                                          regulator



                           Blower


                                                          P          Flow                   Test
                                                                 straightener            evaporator    Dew point
                                                    Differential                 Dew point
                                                     pressure                     sensor                sensor
                                                    transducer      Thermocouple          Thermocouple
                                                                         grid                 grid



                                               Figure 1: Experimental setup schematic



The test evaporator was installed in a multi-nozzle air flow chamber, which was constructed according to
ANSI/AMCA 210 (1985). The exhaust fan installed at the exit of the air flow chamber controls air flow rate. All
measurements and data reduction were in accordance with ASHRAE Standard 37 (1998). The maximum difference
between the air and refrigerant side capacity was less than 5 %.




                          International Congress of Refrigeration 2003, Washington, D.C.
                                                         2
                                                                                                                                                ICR0040




                                    Coil Height
                                                  Coil height




                                                                Air Velocity


                     Figure 2: Evaporator circuiting with an example of non-uniform air velocity profile

1.2 Test Conditions and Procedure

The study included wet and dry-coil tests at 26.7 C dry-bulb temperature. The dew-point temperature for wet-coil
tests was 15.8 oC. R22 state at the inlet to the expansion valves was controlled to maintain an enthalpy equivalent to
a 48.9 C saturation temperature with a subcooling of 8.3 C, which resulted in an evaporator inlet quality of 25 %.
The evaporator exit was maintained at a pressure corresponding to saturation temperature of 7.2 oC. Indoor dry-bulb
and dew-point were stabilized for at least one hour before test data were taken.

Table 1 presents the tests performed. Superheat conditions in the individual circuits were set by adjusting R22 mass
flow through each circuit using three manual expansion valves. For tests with non-uniform air distribution, metal
mesh plates were attached to the upper half of the coil to alter the velocity profile. Two kinds of non-uniform air
flow tests were performed: 1) the volumetric air flow rate was held constant by increasing fan speed as the mesh
blockage was added; 2) the volumetric air flow decreased as the mesh blockage was added.

A hot wire anemometer was used to measure the air flow rate by traversing the coil at a minimum of 25 equally
spaced points at the face of the coil. This measurement agreed with the chamber air flow within 2 %.

                                                                               Table 1: Test matrix
                                                                                           Superheat at each circuit (oC)
      Parameter                           Air flow                                                                                       Test Name
                                                                                    Top       Middle         Bottom       Overall
  Imposed non-                                                                      5.6          5.6           5.6          5.6            Case A
uniform refrigerant            Uniform and constant                                 16.7        16.7          16.7         16.7            Case B
                                                                                     (1)
    distribution                                                                                16.7          16.7           5.6           Case C
                                                                                     (2)         (2)           (2)           (2)
  Imposed non-                                                                                                                             Case D
                                         Constant
 uniform air flow                                                                   5.6          5.6          5.6            5.6           Case E
                                                                                     (2)         (2)           (2)           (2)
  distribution by            Reduced due to the mesh                                                                                       Case F
  mesh blockage                    blockage                                         5.6          5.6          5.6            5.6           Case G
(1)   Expansion valve connected to top circuit was adjusted to set the overall superheat at 5.6 oC.            (2) Superheat was not adjusted; the expansion
      valves remained unchanged from the superheat set at 5.6 oC with uniform air flow.




                                 International Congress of Refrigeration 2003, Washington, D.C.
                                                                3
                                                                                                                                                              ICR0040


                                                                 2. RESULTS AND DISCUSSION
2.1 Maldistributed Refrigerant Tests with Uniform Air Flow
Refrigerant may be improperly distributed between different circuits because of bends or other blockages in the
distributor tubes, or because of non-optimal design. Figure 3 shows capacity variations at different imposed
superheats scenarios for each circuit for dry and wet coil tests. Case A and Case B represent uniform superheat for
all individual circuits of 5.6 oC and 16.7 C, respectively. For Case C, the middle and bottom circuit superheats are
16.7 oC, while the top circuit is controlled to set an overall superheat of 5.6 oC. This required overfeeding of the top
circuit, which resulted in its flooding.

                                                                                                             1.2
                  7500                                                                                                                                               3
                                                                                                                                                            1700 m /h
                                                                      Wet Coil                               1.1
                  7000                                                Dry Coil                                                    3
                                                                                                                            1300 m /h
                                                                                                              1




                                                                                         Relative Capacity
                  6500
                                                                                                             0.9
   Capacity (W)




                  6000
                                                                                                             0.8

                  5500
                                                                                                             0.7

                  5000
                                                                                                             0.6
                                                                                                                                                          Coil-E
                                                                                                                                                          Coil-EC
                  4500                                                                                       0.5
                                                3
                                                                                                                   Case A         Case B         Case A             Case B
                         Air flow rate : 1300 (m /h)
                  4000                                                                                                                  Test Number
                               Case A                   Case B       Case C
                                                    Superheat case                     Figure 4: Capacity for a regular coil and a coil with
Figure 3: Capacity for different superheat control                                     separated depth rows for different superheat control
scenarios for dry- and wet-coil tests at uniform air                                   scenarios referenced to the baseline capacity
flow
The tests show a rapid decrease in capacity when superheat increased in individual circuits. For Case B, the
capacity drops by 32 % as the individual and overall superheats increased to 16.7 C. It is interesting to note that a
similar capacity degradation, 30 %, was measured for Case C, even though the overall superheat was held at 5.6 C,
and only two out of three circuits had superheated refrigerant. It appears that the existence of one flooded and two
superheated circuits (and resulting significant temperature differences existing within the coil assembly) in Case C
promoted internal heat transfer within the coil, which affected coil performance detrimentally.

To examine the effects of internal heat conduction via fins between different tube depth rows, a special coil was
obtained from the manufacturer of the original coil. The original coil (Coil-E) and the special coil (Coil-EC) were
identical except that the fins in Coil-EC were not continuous between tube depth rows but were cut to produce three
separate tube banks. This arrangement prevented heat transfer via fins between tubes in different depth rows.

Figure 4 shows wet-coil tests results for Coil-E and Coil-EC for two air volumetric flow rates. For Case A, where
all individual superheats were at 5.6 C and no significant temperature differences existed in the coil assembles, the
capacity measured for both coils are very similar. (The slightly higher capacity measured for Coil-EC may be due to
the additional tripping of the air boundary layer by the additional cuts in Coil-EC fins separating the tube depth
rows.) For Case B, Coil-EC capacities were greater than Coil-E capacities by 10.2 % and 23.0 %, for 1300 m3/h and
1700 m3/h volumetric flow rates, respectively. In these tests, the superheat at all three circuits was controlled at
16.7 C. The cuts made in Coil-EC fins separating different depth rows appeared to be the only plausible
explanation for the Coil-EC higher capacity.

With the cross-counter flow of the refrigerant with respect to the air realized in the tested evaporator, superheated
refrigerant existed in the most upstream tubes. This provided the opportunity for heat transfer between highly
superheated refrigerant in the first-depth- row tubes with the two-phase refrigerant in the second depth row in Coil-
E. This heat transfer was parasitic in nature because it did not directly aid in cooling the air.


                                                    International Congress of Refrigeration 2003, Washington, D.C.
                                                                                   4
                                                                                                                                                                                                                          ICR0040


2.2 Non-Uniform Air Flow Tests
Evaporator face velocity may vary due to air filter blockage, blower arrangement, heat exchanger geometry, etc. In
this study, the combined effects of non-uniform air flow and evaporator superheat were examined by blocking the
upper portion of the test evaporator. Baseline tests were first performed with 5.6 C superheat for all circuits and
uniform air velocity. Then, the blockage was applied and no expansion valve adjustment was made (case D or case
F). Finally, the expansion valves were adjusted to yield 5.6 C superheat on all circuits (case E or case G). Two
series of tests, with a constant air flow and with the air flow decreased by the blockage were performed.

2.2.1. Constant Air Flow Rate: Tests with a constant air flow correspond to an installation scenario where the
installer adjusted the fan speed to obtain the specified flow rate. Figure 5 shows capacity for different air velocity
ratios, where velocity ratio is defined in this study as the ratio of the averaged air velocities between the top and
bottom halves of the coil. Figure 6 presents capacity for different air velocity ratios referenced to the baseline
capacity. For the velocity ratio of 1:1.26 and no superheat adjustment, the capacity decreased less than 1 %, and this
capacity loss was recovered when superheat was corrected. For greater differences in air distributions, the capacity
penalty was as much as 6 % (for the 2.59 velocity ratio), and when the superheat for each circuit was adjusted to
5.6 oC, the capacity recovered to within 2 % of the baseline value.
                        7200                                                                                                                                        1.04
                                                                                                               3
                                                                                    Air flow rate : 1300 (m /h)                                                     1.03                    Superheat case
                                                                                                                                                                                            O
                        6900                                                                                                                                        1.02                      : superheat depends on blockage



                                                                                                                            Relative capacity to uniform air flow
                                                                                                                                                                                                                              o
                                                                                                                                                                    1.01                    * : All superheats adjusted to 5.6 C
                        6600                                                                                                                                        1.00                                                          3
                                                                                                                                                                                                          Air flow rate : 1300 (m /h)
                                                                                                                                                                    0.99
         Capacity (W)




                                                                                                                                                                    0.98
                        6300
                        2400                Total        Latent Superheat case                                                                                      0.97
                                                                  Depend on blockage                                                                                0.96
                                                                                 o
                        2100                                      Adjusted at 5.6 C                                                                                 0.95
                                                                                                                                                                    0.94

                        1800                                                                                                                                        0.93
                                                                                                                                                                    0.92

                        1500                                                                                                                                        0.91
                                  0.8     1.0     1.2         1.4     1.6     1.8    2.0    2.2    2.4   2.6       2.8                                              0.90
                                                                                                                                                                              O              O               O               O
                                                                      Velocity ratio                                                                                       1.26   1.26*   1.62   1.62*   1.76     1.76*   2.59   2.59*
                                                                                                                                                                                                 Velocity ratio
Figure 5: Capacity for different air velocity ratios at
constant volumetric air flow rate
                                                                                                                         Figure 6: Capacity for different air velocity ratios
                                                                                                                         referenced to the baseline capacity at constant
                  14
                                  Air flow rate : 1300 (m /h)
                                                                      3                                                  volumetric air flow rate
                  12              Superheat depend on blockage

                  10                                                                                                     Figure 7 shows overall superheat and individual
                                                                                                                         circuit superheats that were caused by different air
                        8
                                                                                                                         velocity ratios via different mesh obstructions. Since
 Superheat ( C)




                                                                                      Superheat Circuit                  the volumetric flow rate of air was the same for all
 o




                        6                                                                       Top
                                                                                                                         tests, the blockage of the upper half of the coil was
                                                                                                Mid
                        4                                                                       Bottom                   simply displacing the air flow from the upper to the
                                                                                                Overall                  lower part of the coil. Consequently, increasing the
                        2                                                                                                blockage on the upper half of the coil resulted in
                        0
                                                                                                                         reduced superheat and flooding at the top circuit,
                                                                                                                         while the superheats at the middle and bottom
                    -2                                                                                                   circuits increased substantially. In the case of the
                            0.8     1.0     1.2         1.4         1.6     1.8     2.0    2.2    2.4    2.6       2.8   1:1.20 velocity profile, the difference in superheat
                                                                    Velocity ratio                                       between top circuit and bottom circuit increased by
Figure 7: Individual circuit and overall superheats for                                                                  10.4 oC. For the 1:2.6 velocity ratio, superheat
different air velocity ratios at constant volumetric air flow                                                            difference increased to 12.2 oC. The resulting overall
rate                                                                                                                     superheat decreased also. For the largest blockage


                                                                      International Congress of Refrigeration 2003, Washington, D.C.
                                                                                                     5
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