Page 396 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH11A 5/30/2001 4:50 PM Page 353

Refrigeration Systems 353

h (of mixed vapors corresponding to point 1B) 1. Establish total refrigeration tonnage for each
evaporator temperature level. When possible, combine
w e h 1c w 1 h 1 these into as few different levels as possible. Do not
, Btu/lb (11-14)
specify a lower temperature than needed to accomplish
w e w 1
the process refrigeration requirements. Allow a
where w e vapor from economizer, lb/min minimum of 5°F differential between the lowest
w 1 vapor from first-stage of compression, lb/min required process temperature and the evaporating
h 1 enthalpy of first-stage compression vapor, Btu/lb
refrigerant. The larger this t, the smaller can be the
h e enthalpy of vapor from economizer, Btu/lb
surface area in the evaporator. The lower the
evaporating temperature for any given refrigerant, the
Assuming a constant specific heat of the vapor, the tem-
higher the required horsepower for the compressor.
perature of the mixture is given by
The compromise suggested must be resolved by
comparative cost studies and judgment.
w e t e w 1 t 1A
t m (11-15) 2. Establish a heat balance for the refrigerant throughout
w e w 1
the entire system, using thermodynamic property tables
or diagrams for the particular refrigerant. 1, 2, 20
where t e is the saturated temperature of the economizer vapor
3. Allowing for pressure drop through piping, equipment,
(from saturation curve) and t 1A is the temperature of the first
and control valves, establish the expected operating
stage compressor discharge.
temperatures and pressures.
4. Prepare inquiry specifications for compressors and heat
System Design and Selection
exchange equipment following the forms suggested.
Basically the system design consists of the selection of
component parts to combine and operate in the most eco- Figure 11-50 illustrates the type of comparisons of perfor-
nomical manner for the specified conditions. Unfortu- mance that may be made to better interpret a given set of
nately, the specific conditions are not only for the design parameters.
evaporator where the refrigerant is actually used but include
all or part of the following. These conditions are identified Example 11-7. 300-Ton Ammonia Refrigeration System
whether the system is a separate component selection or a
package furnished assembled by a manufacturer. A process system requires the condensation of a vapor
stream at 15°F. The refrigeration load in three parallel evap-
1. Evaporator: temperature and refrigerant orators will be equally distributed and totals 3,600,000
2. Compressor: centrifugal, screw or reciprocating; elec- Btu/hr, including a 10% factor of safety and 5% system heat
tric motor, steam turbine, or other driver loss. Design a mechanical (not absorption) system using
3. Condenser: horizontal or vertical, temperature of cool- ammonia as the refrigerant. Ammonia was selected because
ing water, water quantity limit (1) the temperature level is good and (2) ammonia is com-
4. Receiver: system refrigerant volume for shut-down patible with the process-side fluid in case of a leak. The con-
refrigerant storage denser cooling water is at 90°F for three months during the
5. Operation: refrigeration tonnage load changes. summer and must be used to ensure continuous operation.
Refer to Figure 11-51A for a diagram of the system. The
Figures 11-49A—D are convenient to summarize specifica- selected conditions are also presented as a summary of
tions to a manufacturer. They are also used as a condensed expected operations, Figure 11-51B.
summary of a designed system. To allow operations at one-half load and flexibility in case
For final design horsepower and equipment selection, the of mechanical trouble, use two reciprocating compressors
usual practice is to submit the refrigeration load and utility capable of handling 150 tons of refrigeration each.
conditions/requirements to a reputable refrigerant system
designer/manufacturer and obtain a warranted system with Pressures Selected
equipment and instrumentation design and specifications
including the important materials of construction. Always 1. Compressor discharge: 214.2 psig (228.9 psia) at a con-
request detailed operating instructions/controls and utility densing temperature for ammonia of 105°F (see Refer-
quantity requirements. ence 1). This allows a 105°F 90°F 15°F t at the
cold end of the condenser. This is reasonable.
Design

A system is designed as follows: (Text continues on page 358)