REQUIRED CONDITIONS FOR OPTIONAL OPERATION OF AN AIR-CONDITIONING SYSTEM

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1 Buetin of the Transivania University of Braşov Series I: Engineering Sciences Vo. 4 (53) No REQUIRED CONDITIONS FOR OPTIONAL OPERATION OF AN AIR-CONDITIONING SYSTEM G. NĂSTASE 1 I. BOIAN 1 Abstract: The paper discusses two aspects of refrigeration systems based on mechanica compression used for air conditioning: first the subcooing in reation with the refrigerant charge and iquid-ine temperature, and second the evaporator-eaving air temperature as a function of the fan-speed and of the entering temperatures (dry-bub and wet-bub). Both conditions are of importance for a proper operation of air conditioning systems and the anaytica interpretation is given in a simpe form of reationships. Key words: refrigeration, subcooing, proper airfow. 1. Introduction In vapor compression refrigeration systems subcooing is used so that when the refrigerant reaches the thermostatic vave, it is totay in its iquid form, thus aowing the vave to work propery and avoiding excessive and unnecessary misuse of power, mafunction and deterioration of severa components in the system. The subcooing of a refrigeration system is important to know because it gives an indication if the amount of refrigerant fowing into the evaporator is appropriate for the oad. If the subcooing is too high, then not enough refrigerant is being fed resuting in poor refrigeration and excess energy use. If the subcooing is too ow, then too much refrigerant is being fed possiby resuting in iquid getting back to the compressor and causing compressor damage. On the other hand, the air fow rate provided by the fan for the evaporator must be fitted to reach the desired temperature and humidity inside the air conditioned space. If the evaporator fanspeed is to ow the temperature of the eaving air is smaer than desired temperature and if the evaporator fan speed is to high the temperature of the eaving air is higher than desired temperature, both situations causing probems for the humidity and comfort too. 2. Study Objectives The anaytica correation of the iquidine temperature with the iquid-pressure at service vave is the aim of this paper, having the subcooing as a parameter. Vaues measured in practice [4] have been used for this purpose and the resuting equation represents a usefu too for the refrigerant charging process. The comfort to be achieved for the indoor space can be correated with the temperatures measured in front of the evaporator coi (dry and wet), and after this 1 Buiding Faciities Dept., Transivania University of Braşov.

172 Buetin of the Transivania University of Braşov Series I Vo. 4 (53) No. 1-2011 one (dry).

q s 0 h1 h5 > q0 = h1 h3 = [kj/kg]. (1) 3.

2 172 Buetin of the Transivania University of Braşov Series I Vo. 4 (53) No one (dry). The comfort condition can be reached by adjusting the fan speed in reation with temperature, using the equation obtained in this paper. Both equations resuted from practice for two refrigerants, i.e. R-410A and R-22 [5]. q s 0 h1 h5 > q0 = h1 h3 = [kj/kg]. (1) 3. Required Liquid-Line Temperature Figure 1 shows a schematic of a singestage vapour-compression system with parameters measured for an optima operation of the subcooer: The temperatures of the iquid: c - eaving the condenser T ; - eaving the subcooer T s ; The pressure of the iquid - eaving the subcooer p s. Fig. 2. Singe-stage vapour-compression cyce in p-h diagram for R- 410A refrigerant The subcooing expressed as a temperature differentia Ts = Tc Ts can be correated with the iquid pressure at the service vave p and with the iquid-ine temperature T c for R-410A refrigerant as shown in Tabe 1. Tabe 1 Required iquid-ine temperature [ o C] Fig. 1. Liquid-ine temperatures before/after the subcooer, and dry/wet-bub temperatures of the air fowing over the evaporator 3.1. Subcooing Additiona subcooing provides additiona capacity whie the power input to the compressor is not affected [3] as can be seen from Figure 2: From the tabe above it can be noticed that for a required subcooing (0 to 14 o C) and for a measured iquid-pressure at the service vave (1.5 to 3.9 MPa) a required iquid-ine temperature wi resut.

3 Năstase, G., et a.: Required Conditions for Optiona Operation of an Air-Conditioning System 173 These vaues presented in Figure 3 in a graphica form suggest a possibe anaytica correation: the required iquidine temperature c T as a function of the measured iquid pressure at the service vave p, and having as a parameter the required subcooing T i (temperatures in o C and pressure in MPa): 2 c = s T p p ( T ). (2) The same equation (2) appies for R-22 refrigerant but the pressure range is ower for R-22 compared with R-410A refrigerant. Considering a zero-vaue for the subcooing parameter the required iquidine temperature as a function of the iquid pressure at service vave is presented in Figure 5 for both refrigerants R-22 and R- 410A. Fig. 3. The required iquid-ine temperature correated with the iquid pressure at service vave and having the subcooing as a parameter for R-410A A simiar correation can be noticed for R-22 refrigerant, as shown in Figure 4. Fig. 5. The same correation exists for both refrigerants R-410A and R-22, except the iquid pressure range (having for R-410A) As a genera rue the required subcooing for R-410A refrigerant is 5±1.5 o C Adding or removing refrigerant Fig. 4. The required iquid-ine temperature correated with the iquid pressure at service vave and having the subcooing as a parameter for R-22 The ow-side foat, automatic expansion vave, and thermostatic expansion vave system are not sensitive to the amount of refrigerant in the system but the high-side foat systems and capiary tube systems are very sensitive to the amount of refrigerant charge. For the ast ones a correct refrigerant charge is very important [1].

4 174 Buetin of the Transivania University of Braşov Series I Vo. 4 (53) No When the right amount of refrigerant is not indicated by the manufacturer recommendation severa practica methods may be used to determine whether the refrigeration system has enough refrigerant. A sure way to determine if the system has sufficient refrigerant is to put a sight gass in the iquid ine. The appearance of vapor bubbes in the gass is a sign that the system is short of refrigerant. It shoud be charged unti the vapor bubbes disappear. A proper charge is best indicated by the frost on the evaporator. When frost starts to come down the suction ine, a itte of the refrigerant has to be purged out for a correct charge of the system. Besides these empirica methods a more precise method to determine the correct charge of the refrigerant resuts from the correation ahead presented. Adding or removing refrigerant in/from the system resut when comparing the existing temperature on the iquid-ine with the cacuated vaue by means of the equation (2): - If the measured iquid-ine temperature T, is 1.5 o C higher than its cacuated vaue c m c c T, (resuted from the equation (2) for the measured iquid-pressure at service vave p, and for the required subcooing T s ) than refrigerant must be added to the refrigeration system in order to ower this temperature. - On the contrary, if the measured iquid-ine temperature T c, m is 1.5 o C ower than its cacuated vaue T c, c (resuted from the equation (2) for the measured iquidpressure at service vave p, and for the required subcooing T s ) than refrigerant must be removed from the refrigeration system in order to raise this temperature. The required subcooing T s as the difference between the iquid-ine temperature after the condenser T c, and before the service vave T s, is determined by the iquid pressure p and by the refrigerant charge. 4. Proper Evaporator-Coi Leaving Air Dry-Bub Temperature In evaporator air-cooed refrigeration systems heat from the conditioned spaces or from products is carried to the evaporator by air circuation. When air circuation is inadequate, heat is not carried from the products or conditioned spaces to the evaporator at a sufficient rate to aow the evaporator to perform at peak efficiency or capacity. It is important aso that the circuation of air is eveny distributed in a parts of the cooed space and over the coi. Poor distribution of the air circuating can resut in uneven temperatures and dead spots in the air conditioned space. Uneven distribution of air over the coi surface causes some parts of the surface to function ess efficienty than other and owers evaporator capacity and efficiency. When air veocity is ow, the air passing over the coi stays onger in contact with the coi surface. More heat is removed and is cooed with a greater range. Thus, the temperature differentia increases, the refrigerant temperature decreases, resuting in a oss of capacity and efficiency because the rate of heat transfer is owered. As air veocity increases, a greater quantity of air is brought into contact with the evaporator coi. Consequenty, the temperature differentia decreases, the refrigerant temperature increases, resuting in a gain of capacity and efficiency because the rate of heat transfer is increased. Therefore air voume change across the coi wi increase or decrease the refrigerant temperature [2] Indoor entering air temperature If a proper evaporator-coi eaving air drybub temperature is desired then the fan speed must be adjusted in accordance with the indoor entering air dry-bub temperature and its reative humidity (or indoor entering wet-bub temperature). Resuted from

5 Năstase, G., et a.: Required Conditions for Optiona Operation of an Air-Conditioning System 175 practice the vaues presented in Figure 6 can be correated in an anaytica form. Referring to the evaporator in Figure 1 the dry-bub temperature of the indoor air entering the evaporator (indoor entering air dry-bub temperature) DBT i, and its Fig. 6. The proper evaporator-coi eaving air dry-bub temperature DBT o as a function of the indoor entering air wet-bub temperature WBT i (indoor entering air dry-bub temperature DBT i as a parameter) corresponding wet bub-temperature (indoor entering air wet-bub temperature) WBT i wi resut in the evaporator-coi eaving air dry-bub temperature DBT o for a given fan-speed. The proper evaporatorcoi eaving air dry-bub temperature DBT o can be cacuated as a function of the drybub, and wet bub temperatures of air entering the evaporator DBT i and WBT i (in degrees Cesius): 2 o, c = i i i + DBT 0.5 DBT + (0.049 DBT WBT ). (3) The proper airfow can be evauated for both refrigerants using Equation (3) Adjusting the fan-speed If the measured eaving air dry-bub temperature DBT o,m is 1.5 o C (or more) ower than the above cacuated temperature DBT o,c, the fan-speed of the evaporator shoud be increased. If the measured eaving air dry-bub temperature DBT o,m is 1.5 o C (or more) higher than the proper eaving temperature DBT o,c, it is an indication of ow capacity and the evaporator fan-speed shoud be decreased. In practice this evauation is caed Target Evaporator Exit Temperature or shorty TEET. To have a desired dry-bub temperature of the air eaving the evaporator coi the fan-speed must be adjusted in correation with the temperature of the air (dry-bub and wet-bub) fowing over the evaporator. 4. Concusions The appropriate charge of refrigerant in a vapour compression system used for air

6 176 Buetin of the Transivania University of Braşov Series I Vo. 4 (53) No conditioning is one of the conditions for its correct operation. The amount of refrigerant to be added or removed in/from the system is indicated by the measured temperature at the exit of the condenser and by the required subcooing. The anaytica correation worked out in this paper and resuted from the practice is a too for this purpose. Adjusting the speed of the fan fowing the air over the evaporator with more accuracy resuted from the equation deducted in this paper based on vaues from practice is the second condition for an improved operation of the air conditioning system. References 1. Athouse, A.D., Turnquist, C.H., Bracciano, A.F.: Modern Refrigeration and Air Conditioning. The Goodheart- Wicox Company, Inc. Pubishers, Hoder, R.D.: Troubeshooting an Air Conditioning System. Avaiabe at: Knowedge_Troubeshooting_AC_Syst em.pdf. Accessed: Radermacher, R., Yang, B., Hwang, Y.: Integrating Aternative and Conventiona Cooing Technoogies. In: ASHRAE Journa, American Society of Heating, Refrigeration and Air Conditioning Engineers - ASHRAE, Inc., Atanta, USA, 2007, p *** Emerson Cimate Technoogies, Inc. form No. 2006CC-151 issued 10/ *** Goodman Manufacturing Company, form No. 2006CC-36-R22 Issued 12/

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