Document 0rEQboVjqw4b8yjZ2BQ669ZO

706 CHAPTER 43 1965 Guide And Data Book Table 2 .... Minimum Design Working Pressures for Evaporators Refrigerant Number 11 12 22 114 717 Design Wotting Pressor* --prig 30 140 150 50 150 pressures are specified and design working pressures are implied by this code. Table 2 gives the minimum dpign working pressures generally nwH by manufacturers for evaporators considered to be pressure vessels. The pressure relief device must be set to relieve the pressure at not more trhn the design working pressure. Unfired pressure vessels are usually tested hydraulically at ISO percent of the de sign pressure and subsequently by air or gas pressure at 125 per- CeJ^ Herder**and Pressure Vessel Code, Unfired Pressure Vessels. Section VIII. This is the basic code which covers the method of calculation for strength, allowable stresses for ma terials used, factory tests, inspection, and application of the ASME symbol to the approved vessel. Most states have adopted the ASME code and, consequently, no pressure vessel may be installed in those states without the ASME stamp of approval. Insurance companies usually will not insure a refrigerant-containing vessel unless it has the ASMS approved stamp, where applicable. 5. Underwriters' Laboratories UX.-R07C Standard for Refriger ant Containing Components. This standard applies more particu larly to commercial rise equipment and has been adopted by a majority of states and municipalities as a means of protecting the safety and health of the consumer. It involves specific design criteria, use of materials, testing and initial approval by Under writers' Laboratories. Chemical Requirements Materialsfor Use with Refrigerant 717 (Ammonia). The most widely used matwriala in connection with ammonia are carbon steel and cast iron. Stainless steel, monel, cupro-nickel and similar alloys are satisfactory but more costly. Since copper . ftpfi high-copper alloys are attacked by ammonia when moisture is present, their use is avoided. Aluminum and alu minum alloys may be used with caution with ammonia. Materials for Use with Halocarbon Refrigerants. These re frigerants are used satisfactorily with all the common metals and alloys. Under certain conditions with moisture present they form acids which attack steel and even the non-ferrous metals. In properly cleaned and dehydrated systems this does not commonly occur. Materialsfor Use with Water. Relatively pure water is satis factory with both ferrous and non-ferrous metals. Brackish or sea water and some river waters are quite corrosive in contact with iron and steel and also with copper, aluminum, and many, of the alloys of these metals. Proper analysis of the water will indicate the proper materials to be used. . Materialsfar Use with Brines. Sodium and calcium brines are almost universally used with ferrous metals and with a' few non-ferrous alloys. They will attack copper and many of the copper-bearing alloys quite readily. Calcium brine is the more corrosive of the two. It is advisable, even with ferrous metals', to treat these brines periodically to hold the pH value near the neutral point. Ethylene glycol and propylene glycol are stable compounds and are less corrosive than brine. Performance Requirements Continuous or Steady-state Cooling. Where water;or brines ~ are used to remove heat, as in.product cooling or as in air conditioning, the water or brine cooling- temperature.range (difference between entering and leaving" liquid temperatures) can be established from practical considerations. ' Except in tank-end-agitator systems, the cooling ranges uyd iwnmnnly vary from 5 to 15 F deg. The water or brine circulation rate through the cooler will be: gpmpertr 24 ' Ai(sp gr)(e). . (1) where gpm per tr -- Sow rate of fluid, gpm per ton of refrigeration. 6t = <yiling range, Fahrenheit degrees. _ - gp gr _ specific gravity of fluid as compared to water. c a specific beat of fluid as compared to water. " The water or brine pump and the power requirements for its operation will depend upon both the circulation rate and the total pressure drop. Specifications for selection or design of water or brine cool ers should include: 1. Description of fluid cooled, if other than water. 2. Entering water or brine temperature. 3. Leaving water or brine temperature. 4. Circulation rate (gpm). 5. Permisible water or brine pressure drop. 6. Refrigerant used. 7. Saturated suction temperature of refrigerant corresponding to leaving refrigerant pressure. Batch Type Cooling. When stored products, or cooling fhrirto, are cooled from some initial temperature to some final temperature over a period of time, the selection or design of a liquid cooler for that application is complicated by the vary ing Irffld conditions imposed upon that cooler. - Since the time required to cool a batch over a certain tem perature range is inversely proportioned to the cooling ca pacity of the refrigeration system used, system capacities at various temperatures of liquid on should not be averaged. Tnqtpud the tiinft required will be proportional to the average of the reciprocals of the cooling capacities at the various liquid on temperatures. .... For example, assume that water in a storage tank is to be cooled from 80 F to 50 F. The calculated load is' 100 ton-hours (1,200,000 Btu) ad the system capacities with various water on temperatures are to be: Water On 80 F 70 F 60 F 50 F System Capacity-- Tons 100 80 60 40 Reciprocal 0.0100 0.0125 0.0167 0.0250 Sum of Reciprocals =0.0642 Average capacity - " 62.2 tr Approximate required cooling time --" 1-61 Hrs. A closer approximation would be to apply the trapezoidal rule to the reciprocals of the four tonnage rallies,' or Simpsons' Rule to three reciprocals.' FLUID FRICTION The flow of fluids is discussed in Chapter 6. Equation .31 of Chapter 6 is the bash equation for calculating the pressure drop of fluids flowing inside conduits: The friction factor / and Reynolds number relationship are shown in Fig.'S of Chapter 6. *' *,, , Allowance should be maria for pressure drop through .water Evaporators for Liquid Cooling 707 boxes,-inlet and outlet fittings, and return bends. Friction Ipftyg in cast water heads commonly are estimated as equiva lent to the loss in a tube length equivalent to 80 to 100 tube diameters for each inlet and outlet and each change in direc tion of flow plus inlet and outlet. The method used in predicting fluid pressure drop through conduits of constant cross-sectional area can not be used in transverse baffled heat exchangers because the Reynolds number is constantly changing. Some success has been re ported in solving this type of problem by the velocity head method but much work remains to be done before a procedure will be established that will successfully account for changes in tube outside diameter, tube pitch, tube layout, tube handle bypass, baffle leakage, and the baffle window shape or area.1-* Refrigerant Pressure Drop . The.pressure drop that can be tolerated in an evaporator depends upon the refrigerant used and its thermodynamic properties, the evaporator temperature, and what portion of the logarithmic mean temperature difference (LMTD) is ad versely affected by the change in saturation temperature due to'pressure drop within the evaporator. . Table 3 lists four of the commonly used refrigerants and their change in'saturation temperature, per psi change in pressure at.various ranges of evaporator temperature. Table 3 shows that a Refrigerant 22 evaporator can operate at a higher refrigerant pressure-drop than a Refrigerant 12 unit for the same F deg penalty in saturation temperature but a Refrigerant 22 evaporator operating at --30 F cannot toler ate the same pressure drop as it can at 40 F.. Refrigerant pressure drop for Refrigerant 12 and 22 erapo-; raters, when cooling water, are usually, held to a range of 2 to 6 pa and 3 to 9 psi respectively. Brine coolers employing Re frigerants 12 and 22 at typical conditions operate at !)> to. 4 psi and 2 to 6 psi respectively. The mathematical prediction of refrigerant pressure drop in evaporators is difficult due to the change of state as in boiling. Experience has shown that if the refrigerant pressure drop is established,on a given evaporator operating at a given weight rate of refrigerant and at given exit refrigerant conditions, then ,the pressure drop will vary almost directly as the spe cific volume of the exit gas for other evaporator temperatures on the same evaporator (providing the condensing tempera ture and temperature .of the -liquid refrigerant to the evapo rator are held reasonably constant). The volume of flash gas released by the expensing device in direct expansion evaporators has a decided effect on the n>r. frigerant pressure drop. Liquid subcooling, for instance, will lower the refrigerant pressure drop by decreasing the amount Of flftfih gftH HEAT TRANSFER . Heat flows from one medium to another by virtue of tem perature difference. The rate at which heat is transferred is a function of the temperature difference and the ability of the Table 3 .,.. Variation In Saturation Temperature for . 1 P*> Change in Saturation Pressure , Eraporahjr f*Bp, P . 30to 40Ff Oto 10 F -30 to -40 F .1! . 6.78 12.57 -- Bofrigstant 12 22 1.18 1.82 - 3.71 0.72 1.13 2.27 717 0.74 1.24 ` 2.86 fluids to give up or receive heat. Other parameters which must be considered are: (a) the effect of the refrigerant pressure' drop upon the overall terminal temperature .differences; (b) refrigerant distribution; (c) thickness of the tube wall and. fins, if such are present, and the thermal conductivity of the material of which the tube and fins are made;-(d) fouling deposits from the fluid being cooled; and (e) fouling deposits from the cooling medium. Except where wax deposits from oils develop, item e is negligible. In general, the heat transfer rate is given by: q - UrAfOTm (2) q ** heat transfer rate, Btu per hour. U, * overall heat transfer coefficient baaed on outside sur face, Btu per (hour) (square foot) (Fahrenheit degree). A, = outside surface area, square feet ATm TM logarithmic roe&n temperature difference, Fahrenheit v degrees. The overall heat transfer coefficient is given by: For water or brine inside of tubes ' U, ++()s*+(c)s* (3) For refrigerant inside of tubes 1 (F, (c)s* + T+X + (3a) where' K " heat transfer film coefficient for refrigerant, Btu per' (hour) (square foot) (Fahrenheit degree). * " thickness of tube wall, feet. k = thermal conductivity of tube material, Btu per (hour) (square foot) (Fahrenheit degree per foot). Sm " ratio of total outside surface to inside tube surface. A heat transfer film coefficient for water or brine side. - ---fouling resistance on water or brine side. The two film coefficients, A- for the refrigerant,' and A for the water or brine, largely determine the overall heat transfer coefficient UT. The resistance to heat flow through the tube ' wall (and fins, if any) x/k, and the water side fouling re sistance (1/A)(S*) (referred to the refrigerant side) are usually small as compared to the film resistances, 1/A of the refrigerant, and (l/hJ)Ss of the water or brine (referred, to tiie refrigerant side). Refrigerant Film Coefficient Factors affecting boiling film coefficients indude: ; - 1. Physical properties of the refrigerant. - -' 2. -Nature of the surface including its roughness. - 3.,The rapidity with which the vaporised refrigerant is re- moved from the' surfaces, te. how well can the surfaces be kept-' wetted? '' Boiling film coefficients in evaporators where the re frigerant surrounds plain or finned tubes are.influenced by: 1. Tube size (coefficient increases with a decrease in tube di ameter). 2. Tube sparing, arrangement, height of tube bundle. 3. .Tube roughness, fin arrangement. .. `4. Presence of oil. ` 5. Refrigerant distribution (distributing conduits, sprays, etc.).! 6. Presence of noocondensables. .* 7. Evaporator temperature. *^ ,8. Method of expansion, (valves, injectors, pumps, etc.). ` 9. Dryness of leaving vapor. ` .~