Page 364 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH11A 5/30/2001 4:50 PM Page 321

Refrigeration Systems 321

compression but require larger amounts of gas per ton of Methane 200 to 300°F
refrigeration. The reciprocating compressor operates on Ethane 75 to 175°F
the reverse condition; the lower molecular weight gases Ethylene 75 to 175°F
allow more gas to be pumped in a particular size cylinder. Propane 40 to 50°F
Adiabatic head, column (E), is a direct measure of the Propylene 40 to 50°F
number of stages of centrifugal compression. In actual rat-
ing, the polytropic head must be used. As a guide, 8,000— Methane is not used frequently in industrial plants for this
10,000 ft of head are developed per stage of centrifugal service, due to mechanical sealing and safety related prob-
compression, depending upon speed. lems. Due to the danger of air being drawn into hydrocar-
The lb/min/ton of refrigeration, column (G), is an indi- bon systems, a positive pressure should always be
cation of the latent heat of the refrigerant. The greater the maintained.
latent heat, the lower the flow rate per ton. The flow rate, Although these hydrocarbons have good refrigerant
cfm/ton of refrigeration, is an important guide, because properties for many applications, it is important to avoid
refrigerants with low cfm/ton are the best for reciprocating internal pressures in the systems that are below atmospheric
compressor application. For centrifugal compressor appli- pressure because of the danger of air in-leakage and possi-
cation, the low cfm/ton refrigerants are better for the large ble explosion of an air-hydrocarbon mixture. Mehra 8-11 pre-
tonnage requirements, and the high cfm/ton are better for sents useful charts for designing and comparing these
the small tonnage loads. hydrocarbon refrigerants. Methane is not included because
The approximate minimum tons for centrifugal applica- of its somewhat special handling requirements.
tions, column (H), is a rough guide based upon 2,000—3,000 Frequently, some plants use mixtures of some of the
cfm at inlet conditions being an efficient minimum capacity. hydrocarbon refrigerants because of local convenience. In
Some designs can be efficient at lower cfm values, depend- such cases it is important to develop the appropriate mix-
ing upon the particular manufacturer’s equipment. The ture’s physical property and enthalpy charts for design,
tons of refrigeration is actually a function of the evaporator because the properties of only one of the components can-
level and condenser temperature, and therefore, the cfm not define the mixture.
must actually be considered for each particular condition. To specify the system performance requirement, the fol-
Refrigerants 11 and 113 are probably not good for this appli- lowing must be defined: (1) lowest refrigerant temperature,
cation due to the very low suction pressure condition. taking into account the loss in heat transfer T (may be esti-
The approximate number of stages for a centrifugal com- mated at first) that can occur in the evaporator and (2) con-
pressor, column (I), is a function of the adiabatic (and actu- densing temperature of the refrigerant, again taking into
ally the polytropic) head and varies with the efficiency and account the heat transfer T based on the coolant circulat-
physical properties of the gas. ing to accomplish the refrigerant condensing. From these
The minimum recommended saturated suction tempera- initially established values, system pressures can be defined
ture on single-stage reciprocating applications, column ( J), or established from the thermodynamic charts.
is based on a compression ratio of about 9 to 1. The refrig- To design hydrocarbon refrigeration, it is necessary to
erants 114, 11 (soon to be replaced by R-123), and 113 are have available accurate Mollier diagrams, vapor pressure
not included due to the large cfm/ton. charts, etc. (see Figures 11-26 through 11-33 8-11, 15 ). By using
The minimum recommended saturated suction tempera- the convenient estimating charts and excellent presentation
ture on series multistage reciprocating-centrifugal applica- of Mehra 8-11, 30 (Figures 11-34 through 11-46) or some other
tions, column (K), represents an approximate reasonable equivalent convenience charts, the performances of various
limit on suction temperature. The temperatures shown cor- refrigeration systems can be examined and approximately
respond to suction pressures below atmospheric. optimized. These charts assume equal ratios of compression
per stage for centrifugal compressors with a polytropic effi-
Hydrocarbon Refrigerants ciency of 0.77. A pressure drop of 1.5 psi has been allowed at
the suction to the compressor, a 5 psi drop across the refrig-
The use of methane, ethane, ethylene, propylene, and erant condenser for ethylene and ethane, and a 10 psi drop
propane pure light hydrocarbons as refrigerants is quite for propylene and propane. 8-11 See Example 11-3 and 11-4
common, practical, and economical for many hydrocarbon and Figure 11-47.
processing plants. Examples include ethylene manufacture
from cracking some feedstock, ethylene or other hydrocar-
bon recycle purification plants, gas-treating plants, and
petroleum refineries.
Commonly used hydrocarbon refrigerants and their cool-
ing temperatures are: 30 (Text continues on page 328)