Document NGQrrnaxD3JzYB0XQdxKGMBob

Heating Ventilating Air Conditioning Guide 1939 Table 4. Capacity Balances for Maximum and Minimum Load Conditions Conditions Required at peak load conditions.................... Required at minimum load conditions........... Peak load equipment balance........ ................... Same equipment balanced at minimum load Same equipment balanced at maximum load conditions with 40 per cent by-pass.______ Same equipment balanced at minimum load conditions with 38,800 Btu per hour reheat Capacity in Tons Total 10.90 6.62 10.90 Sensible 7.90 3.36 7.90 Latent 3.00 3.26 3.00 9.85 6.58 3.26 8.38 5.05 3.33 6.62 3.36 3.26 Ratio 1.38 1.98 1.38 1.50 1.66 usual coils for comfort installations, this will not occur unless the evapo- rating temperature at the coil outlet is about 20 to 25 F. The exact value depends on the design of coil' and the amount of loading. Although it is not customary to choose coil and condensing units to balance at low tem peratures at peak loads, there is danger of this occurring when the load decreases. This is further aggravated if a by-bass is used so that less air is passed through the coil at light loads. It may be even worse if.the control is arranged for decrease of inside temperature with fall of that outside. Freezing can be avoided'by making the full load balance a high evaporating temperature and checking the balance at the minimum load condition. REFERENCES A.S.H.V.E. Research Report No. 997--Heat Transfer from Direct and Extended Surfaces with Forced Air Circulation, by G. L. Tuve and C. A. McKeeman (A.S.H.V.E. Transactions, Vol. 40, 1934, p. 427). A.S.H.V.E. Research Report No. 1029--Performance of Fin-Tube Units for Air Heating, Cooling and Dehumidifying, by G. L. Tuve (A.S.H.V.E. Transactions, Vol. 42, 1936, p. 99). ' A.S.H.V.E. Research Paper--Performance of Fin-Tube for Air Cooling and Dehu midifying, by G. L. Tuve and C. A. McKeeman (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, June 1937, p. 379). A.S.H.V.E. Research Paper--Performance of Surface Coil Dehumidifiers for Comfort Air Conditioning, by G. L. Tuve and L. J. Seigel (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, September, 1938, p. 605). Heat Transmission in Cooling Air with Extended Surfaces, by W. L. Knaus (Refriger ating Engineering, January, 1935, p. 23, February, 1935, p. 82). Rational Development and Rating of Extended Air Cooling Surface, by H. B. Pownall (Refrigerating Engineering, October 1935, p. 211); Dehumidification of Air with Coils, by William Goodman (Refrigerating Engineering, October, 1936, p. 225). Graphical Method of Determining Finned Cojj Capacities Described, by E. P. Wells (Heating, Piping and Air Conditioning, December, 1936, p. 665). Extended Surface Cooling Units, by R. H. Swart.(Refrigerating Engineering, February, 1938, p. 107). . PROBLEMS IN PRACTICE 1 Why are extended surface coils used in preference to plain tube coils and where is the advantage greatest and least? Extended surface is used to increase the flow of heat between an air stream and the external surface of a tube, where the resistance through the tube wall and from tube wall Chapter 24. Heat Transfer Surface Coils .. flowing in the tube are proportionately low. Where the latter resistances are t0 meniu ^ cornpare(j to the external resistance, as with dry air flowing over tube con- 1'"IvfX Water at high velocity, it is economical to use large ratios of extended to tube 441 Where the external resistance is already low, as where large quantities of water surtac . ^ Qver the surface, lower ratios are used, sometimes even plain tube coils. -- ,, ,,i,at one feature of construction does the performance of extended ^foce coils largely depend? An intimate and permanent bond between tube and fin. 3 What is the maximum number of fins per inch and why? p. ht fins per inch is the usual practical maximum. A closer fin spacing causes trouble with dirt accumulation and by holding up water between the fins when on dehumidifying duty. 4 What is the purpose of the liquid distributor in volatile refrigerant coils? The distributor is used to apportion the incoming liquid and gas mixture as evenly as oossible between the individual refrigerant circuits tnrough the coil, at the same time making it practical to arrange these circuits for a low refrigerant pressure drop. 5 9 What.is the usual practice as to the relative direction of flow of air over the tubes and the liquid or vapor in the tubes? With water and brine, counter-flow is used wherever possible. Volatile refrigerant coils are generally arranged for parallel-flow although cross-flow and combination arrange ments are also used. Steam heating coils are. usually arranged for cross-flow. 6 What factors influence the transfer, of heat from the air to fluid in the tubes? The magnitude of the propelling force or temperature difference. The design and surface arrangement of the coil. The velocity and character of the air stream. The velocity and character of the fluid in the tubes. ' 7 What effect has air velocity on the heat transfer? It has been found that the relation between total heat transfer coefficient and mass air velocity can be represented as a straight line on logarithmic paper. The slope varies with the coil design, number of rows deep and the fluid in the tubes. 81 Why do practical coil applications sometimes fall short of rated capacity? To attain rated capacity it is necessary that the incoming air be uniformly distributed over the coil face. Sometimes this is not attained due to poor layout of apparatus and air connections. 9 What are the three general conditions of operation to which cooling coils are subjected in practice? The coil may be entirely dry, part dry and part wet, or wet throughout, the conditions being based upon whether or not any, part or all of the coil surface is below the dew-point temperature of the air passing over it. 101 What is the load-ratio line for a cooling and dehumidifying coil? It is a straight line drawn on a nondogarithmic psychrometric chart through points representing the entering and leaving air conditions. The slope of this line gives directly the relationship between the sensible and latent cooling corresponding to the selected entering and leaving air conditions. 11 Upon what does the selection of a heating or cooling coil depend? An economic analysis of each case should be made together with the relative importance of space, air resistance and quantity, and available or desirable temperatures of heating and cooling media. For a specific design of coil there are a number of possible arrange ments for the same duty.