Refrigeration System Piping

 Cryogenic and Refrigeration Systems Introduction

 
Cryogenic piping systems are those installations where the operating
temperature is below 20F. This limit is established on the basis of the
embitterment point of most carbon-steel materials. Many industrial
gases such as oxygen, nitrogen, and argon are stored and transported
in cryogenic containers and piping systems, since this is more efficient
compared to storage in gaseous form that requires high pressures and

therefore stronger vessels and pipes, which increases costs. Although
cryogenic vessels do not have to withstand higher pressures, the low
temperatures cause embitterment problems, resulting in larger expansion
and contraction of piping systems. These storage containers
and piping are subject to larger temperature differentials which cause
structural problems. Nevertheless, cryogenic piping and storage are
preferred for many industrial gases since they are more efficient and
more economical in the long run. 

Refrigeration piping systems are used with refrigeration equipment
to produce temperatures lower than normal for industrial and residential
use. A refrigerant fluid is used to create the low temperature
by absorbing heat from the surroundings and in the process it evaporates.
The evaporated vapor is compressed and condensed by using a
compressor in the system. The condensed liquid is then reduced in pressure
through an expansion valve after which it enters the evaporator to
start the cycle over again. Many volatile substances such as ammonia
are used as refrigerants to produce the lower temperatures required.Cryogenic and
Refrigeration Systems
 

Introduction

 
Cryogenic piping systems are those installations where the operating
temperature is below 20
F. This limit is established on the basis of the
embrittlement point of most carbon-steel materials. Many industrial
gases such as oxygen, nitrogen, and argon are stored and transported
in cryogenic containers and piping systems, since this is more efficient
compared to storage in gaseous form that requires high pressures and
therefore stronger vessels and pipes, which increases costs. Although
cryogenic vessels do not have to withstand higher pressures, the low
temperatures cause embrittlement problems, resulting in larger expansion
and contraction of piping systems. These storage containers
and piping are subject to larger temperature differentials which cause
structural problems. Nevertheless, cryogenic piping and storage are
preferred for many industrial gases since they are more efficient and
more economical in the long run.
Refrigeration piping systems are used with refrigeration equipment
to produce temperatures lower than normal for industrial and residential
use. A refrigerant fluid is used to create the low temperature
by absorbing heat from the surroundings and in the process it evaporates.
The evaporated vapor is compressed and condensed by using a
compressor in the system. The condensed liquid is then reduced in pressure
through an expansion valve after which it enters the evaporator to
start the cycle over again. Many volatile substances such as ammonia
are used as refrigerants to produce the lower temperatures required.

Several halogenated hydrocarbons are also used as refrigerants. Ethylene
glycol, propylene glycol, and brine are also used to produce lower
temperatures as secondary coolants. These fluids do not change from
the liquid to the vapor phase, however, as do other common refrigerants.

 

9.1 Codes and Standards
 

Cryogenic piping systems are designed and constructed in accordance
with the ASME B31.3 Process Piping Code. This code presents methods
to size pipe considering stresses due to internal pressure, weight of
pipe, weight of liquid, and thermal expansion and contraction of piping.
Piping material used for cryogenic piping systems must conform to
ASTM specifications which list material to be used based on operating
temperature and pressure.
Refrigeration piping is designed to the American Standard Safety
Code for Mechanical Refrigeration. This standard is sponsored by the
American Society of Heating, Refrigerating, and Air-Conditioning
Engineers (ASHRAE). Many state, city, and local codes also regulate
refrigeration piping, but most of these adopt the ASHRAE standards.
This code is also referred to as ANSI/ASHRAE 15. The American
National Standard Code for Pressure Piping, ASME B31.5, is also used
in structural design, construction, and testing of refrigeration piping.

9.2 Cryogenic Fluids and Refrigerants

Various cryogenic fluids such as helium and hydrogen are used in indus-
trial processes. Table 9.1 lists the properties of some common cryogenic
fluids.
Enthalpy and entropy versus pressure and temperature charts are
also used in conjunction with cryogenic piping calculations. One of the
properties used for cryogenic piping calculations is the density, which
is also the reciprocal of the specific volume. As an example, for nitrogen
at a temperature of 200 K and a pressure of 0.1 MPa the density
is 1.75 kg/m3. 

When a cryogenic liquid flows through a throttle
valve, flashing may occur. This flashing produces vapors resulting in
two-phase flow. Two-phase flow results in a larger pressure drop compared
to that of single-phase flow. Larger pressure drops require a larger
pipe size, and hence two-phase flow must be avoided. As far as possible,
cryogenic piping systems must be maintained in single-phase flow.
Refrigeration systems use secondary coolants and refrigerants. Brine
and glycol solutions such as ethylene glycol and propylene glycol are
secondary coolants. Refrigerants include ammonia and halogenated hydrocarbons.
Table 9.2 lists commonly used refrigerants in refrigeration
systems. 

 

 

 

9.3 Pressure Drop and Pipe Sizing

 
Pressure drop in cryogenic piping may be calculated based on single-phase (liquid or gas) or two-phase flow (liquid and gas) depending upon whether a single-phase or two-phase flow exists in the pipeline. Single phase liquid calculations are similar to that of water and oil piping systems. Single-phase gas calculation systems follow the methods usedwith flow of compressed gases in pipes. We will first address pressure drop in cryogenic piping systems for the liquid phase followed by that for the gas phase and finally that for two-phase flow. For more detailsof single-phase liquid or gas flow, please refer to Chaps. 6 and 7. 

9.3.1 Single-phase liquid flow

 
The density and viscosity of a liquid are important properties required to calculate the pressure drop in liquid flow through pipes. The density is the mass per unit volume of a liquid. For example, the density of water is 62.4 lb/ft3 at 60◦F. The density of liquid oxygen is 1134 kg/m at 54 K. Viscosity is a measure of a liquid’s resistance to flow. Consider a liquid flowing through a circular pipe. Each layer of liquid flowing through the pipe exerts a certain amount of frictional resistance to the adjacent layer. This is illustrated in Fig. 9.1, where a velocity gradient is shown to exist across the pipe diameter.

 

 

The velocity gradient is defined as the rate of change of liquid velocity along the pipe diameter. The proportionality constant µ in Eq. (9.1) is referred to as the absolute viscosity or dynamic viscosity. In SI units µ is expressed in poise [(dyne · s)/cm 2 or g/(cm · s)] or centipoise (cP). In U.S. Customary System (USCS) units absolute viscosity is expressed as (lb · s)/ft2or slug/(ft · s).For example, water has a viscosity of 1 cP at 60
F and liquid oxygen has a viscosity of 0.189 cP. Another term known as the kinematic viscosity of a liquid is defined as the absolute viscosity divided by the
density. It is generally represented by the symbol ν. Therefore,Kinematic viscosity ν =
absolute viscosity µ◦density ρ (9.2)In USCS units, kinematic viscosity is measured in ft
2/s. In SI units,
kinematic viscosity is expressed as m2/s, stokes (St), or centistokes (cSt).
One stoke equals 1 cm2/s. We will next discuss some important parameters relating to liquid flow and how they affect the pressure loss due to friction. Velocity of liquid in a pipe, the dimensionless parameter known as the Reynolds number, and the various flow regimes will be covered first. Next we willintroduce the Darcy equation and the Moody diagram for determining the friction factor. The analytical method of calculating the friction factor using the Colebrook-White equation will be discussed, and examples of pressure drop calculation and pipe sizing for single-phase liquid flow will be shown. 

Velocity.

The speed at which a liquid flows through a pipe, also referredto as velocity, is an important parameter in pressure drop calculations.The velocity of flow depends on the pipe diameter and flow rate. If the flow rate is constant through the pipeline (steady flow) and the pipe diameter is uniform, the velocity at every cross section along the pipe will be a constant value. However, there is a variation in velocity along the pipe cross section. The velocity at the pipe wall will be zero, increasing to a maximum at the centerline of the pipe. This is illustrated in Fig. 9.2.