PAYING THE PIPER REFRIGERATION PART II BY DAVE DEMMA

Transcription

1 PART II BY DAVE DEMMA PAYING THE PIPER In Part 1 of Paying The Piper in the May/June issue of HPAC Magazine an explanation of the impetus behind R22 to HFC conversions was provided. A review of some of the considerations and steps involved in preparing for a conversion was started. In this article we review the remaining steps and considerations. As was stated in May/June, If you have not had the opportunity to complete an R22 to HFC conversion yet, do not worry. You will. As a first step, technicians should contact the equipment manufacturer before doing conversions. They should FIGURE 1 also ensure that the refrigerant is ASHRAE recognized, is sold in legal containers for transport and that its use is permitted under applicable codes. HEAT TRANSFER CAPACITY The condenser and evaporator allow heat to be transferred between the refrigerant and some other medium. This medium is typically air. Heat transfer capacity is based on the size and length of coil tubing, fin spacing, air flow, and TD (temperature difference between the entering air and refrigerant temperature). The type of refrigerant flowing through an evaporator or condenser does not significantly affect the heat transfer capacity of either. However, for a given heat transfer rating of an evaporator (or condenser), the required refrigerant mass flow to achieve the rated capacity will vary from refrigerant to refrigerant. Most coil manufacturers list a nominal capacity for each evaporator model regardless of refrigerant. Condenser capacities are shown as refrigerant specific. For a given condenser model, you can expect the R404A capacity to be approximately 98 per cent of the R22 capacity. TEV CAPACITY Thermostatic expansion valve (TEV) capacity is determined by the valve s pin and port dimensions, valve stroke, properties of the refrigerant in use, and system conditions. To be consistent, all TEV manufacturers rate their valves using the Air-Conditioning and Refrigeration Institute (ARI) Rating Conditions. The three system conditions that determine TEV capacity are: 1. Evaporator saturation temperature 2. Liquid refrigerant temperature 3. Pressure drop across the TEV port. This value will be the difference between liquid pressure at the TEV inlet less the sum of the pressure drop CONTINUED ON PAGE 52 Graphics Courtesy Sporlan Valve Division of Parker Hannifin 50 HPAC 80 years SEPTEMBER/OCTOBER 2006

2 St. Lawrence Chemical Inc. Exclusive distributor of Genetron refrigerants in Canada Ontario and Western Canada, Tel: Fax: Quebec and the Maritime Provinces, Tel: Fax: Circle #34

3 CONTINUED FROM PAGE 50 across the refrigerant distributor/tubes (if used), the pressure drop in the evaporator and the pressure at the evaporator outlet. For high-pressure refrigerants such as R22, R507 and R404A, the ARI Rating Condition is at a 100F liquid refrigerant temperature, 100 psi P across the TEV port, and 40F evaporator temperature. The capacity of the TEV at this condition is called the nominal tonnage of the TEV. Once a TEV has been selected, it can then be assumed that its pin/port dimensions, and stroke, will not change. However, if any of the three system conditions change, or if the refrigerant properties change, it will result in a TEV capacity change. Due to the difference in properties between R22 and R404A/R507, it is almost certain that the TEV will require replacement. Here is why: for a given application, the refrigerant mass flow requirement will be greater for R404A/R507 than for R22. The larger mass flow requirement will require a larger TEV port to deliver the necessary refrigerant mass flow. However, there may be some extenuating factors that would allow the existing R22 port to be sized correctly for R404A/R507. Because these HFCs operate at higher pressure than R22 at a given temperature, then the available pressure drop will be greater. This will have the affect of increasing the TEV capacity. Additionally, there is a greater benefit from reduced liquid temperatures with R404A/R507 as compared to R22. Particularly, in applications where a liquid subcooler is utilized in the system, the reduction in liquid temperature will yield greater TEV capacity increases with R404A/R507 as compared to R22. If the original R22 TEV was oversized, it is possible that it may have sufficient capacity when used with R404A. If the system is designed and operating properly, there should be no refrigerant vapour present at the TEV inlet. The large pressure drop which the refrigerant experiences across the TEV port results in a portion of the refrigerant flashing, yielding a refrigerant mixture of liquid and vapour at the TEV outlet. The biggest challenge for a multi-circuit evaporator utilizing one TEV is that each evaporator circuit receives an equal amount of the refrigerant liquid/vapour mixture. Taking the refrigerant liquid/vapour mixture and converting it into a homogeneous mixture (a mist of liquid/vapour) is the function of a properly-sized refrigerant distributor nozzle. It is nozzle pressure drop that accomplishes this task. For a high-pressure refrigerant such as R22 or R404A/R507, the distributor tubes and nozzle are considered 100 per cent loaded at a 35 psi P (10 psi P across the tubes and 25 psi P across the nozzle). PRESSURE DROP PROBLEMS Most supermarket display cases use refrigerant distributors that have fixed (non-changeable) nozzles. Changing refrigerant distributor nozzles in these applications will not be an option. Given the increased mass flow for R404A/R507 as compared to R22 mass flow, the correctly-sized distributor nozzle for R22 will be undersized for the larger R404A/R507 mass flow. This results in greater pressure drop across the nozzle. There are two possible problems with this. The first is that pressure drop is noisy. A large pressure drop across the nozzle in a refrigerant distributor can result in noise that is unbearable for supermarket customers. The second is that pressure drop across the TEV is one of the factors which determine its capacity. The larger the nozzle pressure drop is, the less pressure drop there is available for the TEV port. Reducing the available pressure drop across the TEV port (from excessive nozzle pressure drop) will cause a reduction in TEV capacity, possibly resulting in an undersized TEV. (Note: Most walk-in box evaporators use refrigerant distributors with replaceable nozzles. These should be replaced with the correctly-sized nozzle for the new refrigerant.) As an example, using one manufacturer s evaporator in a supermarket application at a 25F evaporator temperature, with a 10F TD (difference between the temperature of air entering the evaporator and the refrigerant saturation temperature in the evaporator), the evaporator is rated at 27,000 Btuh. Before selecting the TEV and refrigerant distributor nozzle, the system conditions must be determined. The evaporator temperature is fixed at 25F, and maintained by an evaporator pressure regulator. The design condensing temperature is 110F, although to reduce energy consumption, most supermarket refrigeration systems are set to operate at lower condensing temperatures in the spring/winter/fall months. We will use 70F as the minimum design condensing temperature. Condensing temperature determines the condensing pressure, from which the P across the TEV port can be determined. Because the P will vary substantially between the 110F condensing and 70F condensing condition, the TEV selection must be checked at both conditions. Liquid refrigerant temperature is the final condition: We will look at applications with a mechanical subcooler (60F liquid temperature), and without a mechanical subcooler (nominal 10F of subcooling from the condenser). Without the mechanical subcooler the liquid temperature will vary with the condensing temperature: 100F at the 110F condensing temperature, and 60F at the 70F condensing temperature. SELECTION PROCESS Table 1 displays all of the pertinent information required to select the TEV and distributor nozzles. Line A shows the TEV/Distributor nozzle selections for the original R22 application at the 110F condensing condition(column 3) CONTINUED ON PAGE HPAC 80 years SEPTEMBER/OCTOBER 2006

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5 CONTINUED FROM PAGE 52 FIGURE 2 SELECT THE CORRECT THERMOSTATIC CHARGE and without the mechanical subcooler (column 4). For the 27,000 Btuh evaporator (column 1) at the following conditions: 25F evaporator (column 2), and 100F liquid refrigerant temperature (column 5), the proper nozzle size is a #2 (column 6). Once the nozzle is selected, the TEV can be sized. Using the 35 psi P across the distributor nozzle and tubes (column 7), and assuming a five psi P between the condenser and the TEV inlet, the P across the TEV can now be calculated (137 psi P - column 9). The P, along with the evaporator temperature and liquid refrigerant temperature will determine the TEV capacity. For this model evaporator rated at 27,000 Btuh (column 1), a nominal three ton TEV (column 10) is selected, and would be operating at 61 per cent of its rated capacity (column 11). Your first reaction might be that this TEV is oversized, almost by a factor of two. Well, hold that thought for a moment. The selection must also be checked at the 70F condensing condi- tion (Line B). The lower liquid temperature (column 5) yields a larger distributor nozzle capacity, which results in a lower nozzle P for this condition (column 7). The lower condensing temperature also results in a reduction in the available P across the TEV port as well (column 9-45 psi P). The lower liquid refrigerant temperature will increase TEV capacity, while the lower P across the port will decrease TEV capacity. The net result from the 70F condensing temperature is the same nominal three ton TEV operating at 85 per cent of its rated capacity. That TEV that appeared a bit oversized at 110F condensing is sized much closer to the evaporator load at 70F condensing. THE TEV ELEMENT An understanding of the TEV element function and a brief review of the operational forces at work in the TEV are in order before proceeding in the analysis of the TEV selection. It must be understood that the element has nothing to do with the TEV capacity. As the sensing bulb fluid temperature changes, so to will its pressure. The sensing bulb pressure is exerted on the top of the element diaphragm, TABLE 1 TEV SELECTIONS FOR R404A/R507 REFRIGERANT CONVERSION LINE Typical Btu/h Evap. Temp. Cond. Liquid Liquid R-22 Pressure Pressure Pressure Case ( F) Temp. Sub-Cooler Refrig. Temp. Nozzle Size Drop Across Drop R-22 Drop Across Capacity ( F) ( F) R-22 Distributor TEV Port Distributor Tubes & (psi) & Nozzle (psi) Nozzle Using R-404A/R507 (psi) (1) (2) (3) (4) (5) (6) (7) (8) (9) TEV SELECTION R-22 TEV TEV % Replace Replace R-404A TEV % of Nominal of Rated R-22 R-22 R-507 Rated Capacity Capacity Element TEV With TEV Capacity With R-404A Nominal R-404A R-507 Capacity R507 TEV Element (10) (11) (12) (13) (14) (15) A 27, N B 27, N C 27, N X 2 96 D 27, N X E 27, N X 3 68 F 27, N X 3 80 G 27, Y H 27, Y I 27, Y X 2 63 J 27, Y X K 27, Y X 3 45 L 27, Y X HPAC 80 years SEPTEMBER/OCTOBER 2006

6 which then facilitates the pin carrier moving closer/farther from the port, closing/opening the TEV. That bulb pressure should be determined by the temperature range of the application and the type of refrigerant used in the application. Using the aforementioned evaporator at a 25F refrigerant saturation temperature with R22 would produce a 49 psi evaporator pressure, while R404A would produce a 62 psi evaporator pressure. The sum of the evaporator pressure and adjustment spring pressure comprises the total TEV closing force. This force acts on the underneath side of the TEV element diaphragm (see Figure 1). For R22 the combined closing force would be 59 psi, while the R404A combined closing force would be 72 psi. During normal TEV operation, the opening force (bulb pressure) must equal the closing force (evaporator pressure + adjustment spring pressure). It becomes rather elementary to illustrate the need for a different element charge when converting from R22 to R404. The resulting 13 psi increase in closing force requires a new element charge which is capable of providing an additional 13 psi at a 25F bulb temperature. Figure 2 shows the available thermostatic element charges for the various refrigerants. While it appears that there are a multitude of new thermostatic charges to operate with the new HFC refrigerants, a closer looks proves this wrong. Note the far right hand column labeled Actual Thermostatic Charges. In particular, note the following low temperature charges: LZ (R402A), SZ (R404A), RZ (R502), and PZ (R507). Each has its own peculiar nomenclature that denotes the refrigerant (first letter) and temperature range (second letter). The Actual Thermostatic Charge column shows them all to be an RZ charge (the original R502 low temperature charge). The pressure-temperature characteristics for each of these R502 replacements are close enough to the original such as to allow them to perform admirably with the original R502 bulb charge. DETERMINE CAPACITY The next step in the conversion process is to determine if the original R22 TEV, after having its thermostatic element Moving away from R-22 just got easier. Introducing DuPont ISCEON MO59 and ISCEON MO79. These non-ozone depleting refrigerants provide you with easy-to-use, cost-effective retrofit options to help your customers avoid costly equipment replacement or interruptions while reducing the use of ozone depleting substances. With over 75 years as a leader in the refrigerants industry, we re committed to providing you with reliable, proven and safe alternatives for CFC and HCFC systems. To find the right products, call your local DuPont Refrigerants distributor. DuPont Refrigerants. The Science of Cool. Copyright 2006 E.I. du Pont de Nemours and Company. All rights reserved. The DuPont Oval Logo, DuPont, The miracles of science, The Science of Cool and ISCEON are registered trademarks or trademarks of E.I. du Pont de Nemours and Company or its affiliates. Circle #36 replaced with the appropriate R404A thermostatic element, will have sufficient capacity for evaporator Btuh requirement. The R22 nominal three ton TEV becomes a nominal two ton CONTINUED ON PAGE 56 SEPTEMBER/OCTOBER years HPAC 55

7 CONTINUED FROM PAGE 55 TEV with R404A (column 14). Again, this difference in capacity is due to the properties of R404A. The evaporator and liquid temperatures will remain the same; however the P across the TEV port will be different. Because the original R22 distributor nozzle will be used, its P for R404A must be determined. Using this value, plus the operating conditions of the system, the new P across the TEV port can be determined (column 9). From these parameters, the TEV capacity can be determined. The nominal two ton R404A TEV will operate at 96 per cent of capacity at the 110F condensing condition, and 112 per cent of capacity at the 70F condensing condition. Clearly, an element replacement for the existing R22 TEV will not work. The R22 TEV will need to be replaced with an R404A nominal three ton TEV. This brings the per cent of rated capacity back to a respectable 68 per cent (110F condensing condition) and 80 per cent (70F condensing condition). For some applications a thermostatic element replacement may be acceptable. However, without working through the TEV selection procedure for each evaporator, it would only be a guess as to where this would happen. A marginally undersized TEV will operate with higher than desired superheat at the evaporator outlet. While lowering the evaporator pressure below the design condition will usually compensate for this and provide adequate discharge air temperature, it comes at a high price in terms of efficiency. It is imperative that the TEVs be sized correctly so that the entire evaporator heat transfer surface can be utilized, thereby allowing the evaporator pressure to operate at its highest possible setting for peak efficiency. In a refrigerant conversion this requires looking at the TEV selection for each evaporator. Product selection programs are available to determine TEV/distributor capacities at various conditions. Dave Demma holds a degree in refrigeration engineering and worked as a journeyman refrigeration technician before moving into the manufacturing sector where he regularly trains contractor and engineering groups. He can be reached at , ext RATE THE ARTICLE! Will this information be useful? Please circle the appropriate number on the Reader Postcard. Thank you. VERY USEFUL USEFUL NOT USEFUL Circle #37 56 HPAC 80 years SEPTEMBER/OCTOBER 2006