Document Nnj6E7pBZav0aXqkm8EkbZng

690 CHAPTER 42 1965 Guide And Data Book Example 4- Select a condenser for 60 tons of refrigeration with 240 gpm water available for condensing at an entering tem perature of 80'F. Solution: The -water flow rate is 4 rom per ton (240 +;60), thus fixing the temperature rise' at 7 F aeg (28 gal degjtcr (min) (too)'+ 4 nun per ton), and the leaving water temperature at 87 F. From Fig. it at a loading of 10 sqft per ton-ana a flow rate of.4 qua per ton, the leaving terminal difference is approximately 5 F deg, and the corresponding water flow rate is,2.94 gpm per tobe.^Theresultingconde.nsingtemperature would'fee-87 +6 + 1.5 required u Horizontal posed Shell-and-Tube Condenser for Halocarbon Refrigerants. . - Horizontal closedshell-and-tube.condensers for halocarbon refrigerants are built in .sizes from-about five .tons up to those required on-the largest initfAlltinnRv'In 'large however,greater operating economy is nhtainfrri by a multiple arniHftnwpr intfadltmn Halocarbon refrigerant condensers have been made with many types of material;Bpecifications, -including all prime sur face ferrous-tribes and all-prime.surface or finned,-nonfetrous tubes. - The most common tube used is the rioriunal f-in. OD cop per tube-with integral fins on theouttide, 49 perinch-and'*ft in. high. These tubesare^ither tilverljrazed into thin copper drtstefcl/tnbesheets, ^or unitedUnto heaviermonfenxms or >rte41 tube sheets. finned-tube condenser for Refrigerant 12 'is very similar in construction to, andfias the same;advantages and disadvan tages as, the .ammonia horizontal shell-and-tube condenser. Fig. 12isa"typicalset of rating curves developed for the pur pose of economical condenser selection. Evamplft 5 illustrates the use of this diagram. Example 6: Determine-thelength of water travel and pressure drop required for a horizontal Refrigerant 12 .condenser with a leaving terminal difference of 5 F deg, a loadingiof .12 sqit per too, and a water flow rate of 3 gpm per ton. Solution: Enter theilower right quadrant of Fig. 12 at a termi nal difference of 6 F deg, proceed vertically to the 12 soft per ton line, then horizontally to the 3 gpm per ton curve (lower left quadrant), reading 24 ft travel at the bottom of the lower left quadrant. Proceed vertically into the upper left quadrant to the 3 gpm per ton curve, then horizontally to the 12 eq ft per ton line in upper right quadrant. From th point read the pressure drop of 2.5 psi at the top of the upper right quadrant. Other Types of Water-Cooled 'Condensers A .theU*md~U-tub6iCondenserls similar to a shell-and-tube condenser,iexcept that the cooling water is circulated through a bundle.of U-tubes terminating in.a single tube sheet. A sheUrand-coil condenser s one wherein the cooling-water is circulated through one ortnore continuous ,or assembled coils contained withinthe shell. .A jdaubfcpipe or tube-inrlube condenser consists of one.or ' more assemblies of two tubes, one within the other, in which the refrigerant vaporj& condensed either in,the anaular.-space or in the inner tube. ` Condensers 691 An atmospheric condenser consists of a number of horizontal tubes arranged one above the other. The refrigerant is con densed within the tubes, and the cooling water is distributed along the top tube, flowing by gravity over the tubes directly below. Sufficient water must be circulated to obtain uniform and complete wetting of the tubes. The principles of calculation and rating given for shell-andtube condensers apply equally for the other condensers. Noncondensable Gases Noncondensable gases circulate with the refrigerant, but unlike the refrigerant, do not liquefy in the condenser. They may be one or a mixture of two or more of the following: nitrogen, hydrogen, oxygen, chlorine, oil vapors, water vapors, etc. Noncondensable gases come (a) from outside the system by being drawn into the crankcase of the compressor along with- the oil and through leaks in piping, stuffing boxes, etc., when the system is being operated under a vacuum, and (b) from jflgjHft the system by improper evacuation of a new system, before charging, by decomposition of the lubricant under the heat of compression, and (c) by miscellaneous chemical reac tions within the system. When noncondensable gases are present, they collect on the frigh side of the system, resulting in a condensing pressure above that corresponding to the temperature at which the' refrigerant is actually condensing. The excess pressure is caused in part by the partial'pressure effect of the gases and in part by the effect of these gases in forming a resistance film over some of the condensing surface, thus lowering the heat transfer coefficient. Due to tiie continuous flow of vapor and gases into the con denser, the noncondensable gases will tend to accumulate in the coldest and least agitated part of the condenser, or in the receiver. The purge connection for the elimination of these, noncondensable gases should, therefore, be made to thatpart of the high-pressure side of the system. The presence of noncondensable gases in the condenser not only causes higher head pressures, but also increases brake horsepower, reduces capacity, and promotes oxidation of the oil, especially at the valves, due to the presence of oxygen at this point of high discharge temperatures. This has been con firmed by operating experience with all refrigerating systems. For steam and air, the film resistance coefficient between steam and a metal wall is up to six times as great with 4.5 percent air as it is with no air present.* According to Dalton's law of partial pressures, the presence of these gases, raises the condensing pressure P by .the amount of the partial pressure, P' The increase of the work of compression is proportional to the ratio P + P'/P and is greater with higher partial pressures and lower condensing pressures. A test for noncondensable gases is to shut down the re frigeration system while allowing the condenser water to flow for a sufficient length of time so that the refrigerant is at the same temperature as the water. If the condenser pressure is higher than the pressure corresponding to the refrigerant tem perature, there are noncondensable gases present. Experi mental data have shown that, with these gases present in the condenser under-the above conditions, with a 10 F deg differ ence between refrigerant temperature and.the'temperature corresponding to the condensing pressure, there wiU.be as much as 20 to 25 F deg increase in condensing temperature during operation. - Noncondensable gases can be eliminated to a great degree by purging. This may be accomplished by shutting down_the system as mentioned above, and purging by blowoff. directly OF NON-CONDENSABLE CASES 1. Amaotua, 45 F 2. Ammonia, 70 F 3. bfrfgwonl 12, 45 F 4. Refrigerant 12, 70 F Fig. 13 .... Loss of Refrigerant During Purging a! Various Gas Temperatures and Pressures into the atmosphere or by using devices for this purpose which will operate whUe the system is in operation. Purging connec tions should be made where noncondensable gases will ac cumulate in the system. Purging devices include a refrigerated surface within a' mntAinr into which is introduced a mixture of refrigerant gas and -noncondensable gases. The low temperature surface condenses a large amount of the refrigerant, and the remaining mixture is allowed to escape to the atmosphere. Fig. 13 shows the refrigerant loss when purging ammonia and Refrigerant 12 at various temperatures and pressures, and indicates the desirability of purging devices. Even though Refrigerant-12 losses are considerably greater than ammonia, it is economical to purge these systemsas thoroughlyas ammonia systems. This can- be determined by balancing the savings in compressor power requirements and increased wear on machinery against the loss of refrigerant.- Construction Codes Where state or municipal codes do'not take-precedence; rfogjgn pressures, materials, tedding, tests, and ' relief devices whnnM be in accordance with the ASMS Unfired Pressure Vessel-Code, Section 8, and'ASHRAE Standard 16-63 which is identical with the American Standard Safety Code for Mo chanieal Refrigeration, B9.1-1964. PART II: AIR-COOLED CONDENSERS The heat transfer process in an air-cooled condenser can be subdivided into three main phases:. (1) desuperheating, (2) condensing, and (3) subcooling. Fig. 14 shows the changes of state of Refrigerant 12 pasting through the condenser coil and the corresponding temperature change of the cooling air in contact with the outside of the condenser.' Depending on the refrigerant, the relative limits of desuperheating, condensing, and subcooling zones will vary slightly,' but Flg.-14 is typical for most of the commonly used refrigerants. .With Refrigerant 1.2, the desuperheating; zone. occupies - only 5.2'percent of the condenser coil, due to the large tem-