Design of Helium Refrigeration and Liquefaction Systems
Simplified Concepts & Practical View Points By VenkataRao Ganni, Ph. D.
March, 2004
Thomas Jefferson National Accelerator Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Preface
Dedicated to my guru Raymond Moore Jr. for promoting inquisitive minds to think about
‘What is an optimum system and how can we provide it?’
• What is an optimum system? Does it result in the:
• Minimum operating cost • Minimum capital cost • Minimum maintenance cost • Maximum system capacity • Maximum availability of the system • Before we can make an attempt to answer these, we
“Need to understand the fundamentals”
Some of these are new concepts have never been formally published, although I shared with many of my colleagues over the years.
Thomas Jefferson National Accelerator Page 2 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Table of Contents
1Introduction 2 Carnot Helium Refrigeration and Liquefaction Systems 3 Idealized Helium Systems and Carnot Step 4 The Theory behind the Cycle Designs 5 System Optimization 6 The Facts and Myths of LN2 Pre-cooling 7 Typical Helium Cryogenic System and its Basic Components 8 Helium Refrigeration Systems for Below 4.2 K Operations 9 System Capacity Specification and Margin Allocation 10 Design Verification and Acceptance Testing 11 Some of the lessons Learned over the Years 12 Optimal Operation of the Existing Helium Refrigeration Systems 13 Some Areas of Interest for Future Developments 14 Conclusions
Thomas Jefferson National Accelerator Page 3 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy 1. Introduction
The helium refrigeration and liquefaction systems are an extension of the traditional household refrigeration systems.
Let us start with the question, what is a refrigeration system?
The refrigeration system is one that transfers heat energy from low temperature to high temperature.
Normally, the term refrigeration is used for absorbing heat energy at a constant temperature.
Thomas Jefferson National Accelerator Page 4 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Introduction (cont.)
The input power required for these systems mainly depends upon:
• The temperature from heat energy extracted (load temperature)
• The temperature to heat energy is rejected (ambient temperature)
• The process used (e.g.) — Vapor compression process — Hampson process — Claude process
• The efficiency and the effectiveness of the components used (e.g.) — Compressor — Expander — Heat exchanger
• How well the real components match the process
• How the system is controlled
Thomas Jefferson National Accelerator Page 5 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Introduction (cont.)
System Performance & Efficiency
¾ For typical industrial processes approximately 1kW of electrical energy is required to produce 3 kW of cooling. ¾ Why is any input energy required to transfer heat energy from a cold to a hot temperature reservoir? ¾ Using an electrical analogy, a thermal transformer that permits the heat energy transfer from cold temperature to hot temperature, with no input work does not exit. ¾ This is quite unlike an ideal electrical transformer, which will permit the transfer between voltage and current with no additional input power. ¾ This ‘transmission’ (or transfer) limitation of heat energy between temperatures implies that there is a ‘quality’ of heat energy.
Thomas Jefferson National Accelerator Page 6 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Introduction (cont.)
• The source and sink temperatures sets this limit on the conversion of thermal to mechanical energy. • It is around 40 % for typical industrial processes and approximately 3kW of thermal energy is required to produce 1kW of electrical energy. • In contrast, for ideal systems the conversion from mechanical to electrical energy (or visa-versa) can be 100%. • This thermodynamic limitation is expressed by the 2nd Law of Thermodynamics and embodies the concept that the thermal energy has a ‘quality’ (or ‘availability’) that varies with the temperature from which it is being supplied and received. • For our topic of refrigeration, the input energy required is due to the loss in ‘availability’ (or decrease in ‘quality’) of the thermal energy as it is transferred from a low temperature (load) to a high temperature (environment).
Thomas Jefferson National Accelerator Page 7 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Introduction (cont.)
• For refrigeration systems the coefficient of performance (COP), which is the amount of cooling obtained per unit of input work, is used as a measure of performance.
• A more useful term is the inverse of the COP or COPINV. This tells us how many watts of input power (i.e., the additional compressor input power with the expander output power utilized) are required to produce one watt of cooling power.
Thomas Jefferson National Accelerator Page 8 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Introduction (cont.)
• So, the Carnot work (WCARNOT) required for 4.4 kW of cooling is 1 kW.
• Note: This is not a violation of the first law of thermodynamics since a refrigerator is transferring energy from one temperature to another and not converting it. The input work is the energy equal to the difference in the ‘quality’ (‘availability’ or ‘exergy’) of the thermal energy between T1 and T2.
Thomas Jefferson National Accelerator Page 9 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Introduction (cont.)
• The Carnot cycle is an ‘ideal’ cycle in the sense that it does not have any ‘irreversibilities’ (i.e., ‘lost work’). • However, the term ‘idealized cycle’ will be relegated to a practical system that one can visualize using ideal components. • What differentiates the Carnot cycle is that it has the minimum COPINV (or the maximum COP) for the process of transferring heat energy between two thermal reservoirs. • It is this distinction that gives the Carnot cycle the recognized qualification for ‘efficency’ comparisons (e.g., termed ‘Carnot efficiency’ or ‘efficiency to Carnot’) of other cycles performing the same function.
Thomas Jefferson National Accelerator Page 10 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Introduction (cont.)
Thomas Jefferson National Accelerator Page 11 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Introduction (cont.)
For thermal systems, we use the Carnot cycle (i.e., a cycle that has the minimum COPINV with no irreversibilities) as a reference to compare to all other cycles.
The Carnot cycle is a reversible cycle and has the maximum efficiency thermally and from a fundamental process viewpoint.
To be clear, so when liquefiers are discussed, the term ‘Carnot cycle’ (as well as ‘Carnot work’ and ‘Carnot efficiency’) are not confined only to the diagram shown in Figure.
They are applicable to an ideal, reversible process that performs the specified process function.
Thomas Jefferson National Accelerator Page 12 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy 2. Carnot Helium Refrigeration and Liquefaction Systems
Clausius (In)equality
∆∆QQ LH= TTLH
Thomas Jefferson National Accelerator Page 13 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Carnot Helium Refrigeration and Liquefaction Systems (Cont.)
Carnot Refrigeration System: Minimum required input work for a given rate of thermal energy transfer between two thermal reservoirs.
The work input for the Carnot system expressed as:
WTcarnot =⋅0 ∆S−∆H • This is a very powerful equation. • The terms are as follows:
• TS0 ⋅∆ is the heat rejected to the environment or, the input power to an isothermal compressor • ∆H is the heat absorbed or the ideal refrigeration or, the ideal work output from an ideal expander • is the ideal net input work required Wcarnot which is the difference between (a) and (b) above
Thomas Jefferson National Accelerator Page 14 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Carnot Helium Refrigeration and Liquefaction Systems (Cont.)
• A refrigerator transfers heat energy from a low temperature reservoir to a higher temperature reservoir. Most helium refrigerators transfer heat energy from approximately 4.22K (or in some cases at sub-atmospheric pressures).
• A liquefier is different from a refrigerator since the objective is to cool a quantity (flow rate) of high (or ambient) temperature fluid to a specified low (or load) temperature, which then leaves the cycle (at a low temperature). What leaves the cycle is the liquefaction flow, and may be returned at a higher or ambient temperature. In comparison to the refrigerator, in a liquefier the temperature at which the heat energy is being transferred (removed) is constantly varying (decreasing as it is being cooled), although it is rejected at the same (high or ambient) temperature reservoir.
Thomas Jefferson National Accelerator Page 15 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Carnot Helium Refrigeration and Liquefaction Systems (Cont.)
Carnot Helium Refrigerator
WT⋅∆s−∆h(300)⋅−(4.833) 20.42 ⎡W ⎤ wT=⋅∆s−∆h carnot 0 carnot 0 COPINV == = 70 ⎢ ⎥ QhL ∆ 20.42 ⎣W⎦
⎛⎞T2 300 ⎡W ⎤ COPINV =−⎜⎟11= −=70⎢ ⎥ ⎝⎠TW1 4.224 ⎣ ⎦ Carnot Helium Liquefier
wTC = 0 ⋅∆s=(300)⋅(27.96) =8387 [W/(g/s)]
whX =∆ =1564 [W/(g/s)] (or 18.6% of wC )
wwCarnot =−C wX =6823 [W/(g/s)] (or 81.4% of wC )
Thomas Jefferson National Accelerator Page 16 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Carnot Helium Refrigeration and Liquefaction Systems (Cont.)
• Carnot work required for a given liquefaction load
10
9
8
7
6 )
/s 2. T0*∆s [kW/(g/s )] g 5 / (
kW 4
1. T 0*∆s-∆h [kW/(g/s )] 3
2 3. ∆h [kW/(g/s )]
1
0 1 10 100 1000 Te m pe r a t u r e [ K]
Thomas Jefferson National Accelerator Page 17 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Carnot Helium Refrigeration and Liquefaction Systems (Cont.)
Carnot work required for liquefaction load for a given temperature range
Temperature T0*∆s % ∆h % Range (K) [W/ (g/s)] [W/ (g/s)] 300 - 80 2058 24.5% 1143 73.0% 80 - 4.22 6329 75.5% 421 27.0% 300 - 4.22 8387 100.0% 1564 100.0%
Thomas Jefferson National Accelerator Page 18 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Carnot Helium Refrigeration and Liquefaction Systems (Cont.)
Carnot work for different fluids
Fluid Tsat,0 Liquefaction Refrigeration [K] ( W/(g/s) ) (W/W) Helium 4.22 6823 70 Hydrogen 20.28 12573 13.8 Neon 27.09 1336 10.1 Nitrogen 77.31 770 2.9 Argon 87.28 477 2.4 Oxygen 90.19 635 2.3 Methane 111.69 1092 1.7
Thomas Jefferson National Accelerator Page 19 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy 3. Idealized Helium Systems and Carnot Step
Carnot Step
The Carnot step is defined (by the author) as the process step required for accomplishing a given task with minimal energy expenditure. Or, in other words, accomplishing the given task with minimal exergy usage.
The three main parts to a helium system.
Thomas Jefferson National Accelerator Page 20 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Idealized Helium Systems and Carnot Step (Cont.)
• The load and distribution system Carnot Step: ¾ The process for load interface that has the least entropy increase (or exergy usage) is the ‘load Carnot step’
• The cold box Carnot Step: ¾ The cold box provides a process path analogous to walking up the stairs from a deep basement floor (4.2K) to the ground floor (300K). ¾ The size and arrangement of steps that requires a minimum expenditure of energy are the ‘Cold Box Carnot steps’
• The compressor system Carnot Step: ¾ Isothermal compression requires the minimum work and this is the ‘Compressor Carnot Step’
Thomas Jefferson National Accelerator Page 21 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Idealized Helium Systems and Carnot Step (Cont.)
Idealized Helium Refrigeration Systems:
12 24
11 22
2 1 10 20 9 18
t ] s l tm [J/g] n 8 16 ta o
o [a )
s = c T h f
= p 7 14 o p const l) re ( ra u 6 12 eg ess 5 10 nt Pr er Output Work (I 4 8 m Su upply S 3 6 Expand
TEMPERATURE (T) 2 4
1 2 f Q g 0 0 1 10 100 1000 Temperature [K] ENTROPY (S)
Thomas Jefferson National Accelerator Page 22 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Idealized Helium Systems and Carnot Step (Cont.)
QH,1 QH,2 QH,i QH,N
RHP1 RHP 2 RHPi RHPN
T1
QL,1
T 2
QL,2
T 3
T i
QL,i
T i+1
T N
QL,N
T N+1
mL
Thomas Jefferson National Accelerator Page 23 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Idealized Helium Systems and Carnot Step (Cont.)
P h P l C m L T1 mx,1
X 1 HX1
T mx,2 2
X 2 HX 2
T3 E (T) [K] R
U mx,i Ti T A X i HXi
TEMPER Ti+1
mx,N TN
X N HXN
TN+1 mL
ENTROPY (s) [J/g-K]
φ Trr==P const.
⎛⎞T1 N TTrT, ==⎜⎟()r ⎝⎠TN +1 AAnT() nT() N ==rT,,rT φ ⋅AAnP()rrn(T)
Thomas Jefferson National Accelerator Page 24 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Idealized Helium Systems and Carnot Step (Cont.)
• So, the Carnot step is the same for each expander stage (i.e., Tr is the same for each stage).
• As an example, for a 300K to 4.2K liquefier (e.g., T1 = 300K, TN+1 = 4.2K), with an expander pressure ratio of 16 (e.g., Pr = 16), the total temperature ratio is, Tr,T = 300 / 4.2 = 71 and the temperature ratio for each expander stage is, Tr = (16)0.4 = 3.03.
• So, the ideal number of expander stages is, N = ln(71) / ln(3.03) = 3.85 ≈ 4. As we will see later, more stages are actually required to compensate for component losses as well as process and fluid non- idealities.
Thomas Jefferson National Accelerator Page 25 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Idealized Helium Systems and Carnot Step (Cont.)
• Referring to Figure, for an idealized gas liquefier, each expander flow is the same and equal to the liquefaction flow, e.g.,
• With the expander flow the same for each Carnot step (or stage), the (ideal or isothermal) compressor work for each stage is also equal .
• However, the Carnot work for each stage is not the same. This is the case since the expander output work, which is recovered by the Carnot liquefier and used to reduce the compressor input power, is not the same for each Carnot step.
Thomas Jefferson National Accelerator Page 26 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy 4. The Theory Behind Cycle Design
Dewar Process
m2 m 1 = m 2 - mm
x m 2
(1-x)m 2
Q r(m - m ) mR L R L hfg
mL
QL
Thomas Jefferson National Accelerator Page 27 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy The Theory Behind Cycle Design (Cont.)
Dewar Process
Thomas Jefferson National Accelerator Page 28 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy The Theory Behind Cycle Design (Cont.)
COMPRESSOR m 2 1
2 1 W t c t s s n n HEAT EXCHANGER o o c c = = p 3 p Wx EXPANDER HX g f 3 TEMPERATURE (T) s = const 1 atm f g QL
ENTROPY (S)
QL
Thomas Jefferson National Accelerator Page 29 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy The Theory Behind Cycle Design (Cont.)
Possible High Efficiency Helium Liquefaction System
3.50 η = 1 3.25 X
) 3.00 X θ 2.75 η X = 0.9 o ( i
t 2.50 a
R 2.25 η X = 0.8 e r u 2.00
at η X = 0.7 r e 1.75 p η X = 0.6 m
e 1.50 η X = 0.5 T 1.25 1.00 1234567891011121314151617181920 Pressure Ratio (Π )
Thomas Jefferson National Accelerator Page 30 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy The Theory Behind Cycle Design (Cont.)
Thomas Jefferson National Accelerator Page 31 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy The Theory Behind Cycle Design (Cont.)
Thomas Jefferson National Accelerator Page 32 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy The Theory Behind Cycle Design (Cont.)
In summary, the Carnot step was explained. For a given number of expansion stages, these Carnot steps (i.e., the temperature ratio) are the same for both refrigerator and liquefier, and result in the minimum compressor flow (and therefore the minimum input power). This is indirectly saying that the ideal placement of the expanders with respect to temperature for both refrigerator and liquefier are the same. Due to practical limitations, the system will likely operate at slightly different temperatures between the two modes, and also slightly away from the Carnot step.
These limitations can be the fixed flow coefficients and efficiencies of the expanders and compressors, insufficient heat exchanger area or operating for a different optimal condition (e.g., at maximum system capacity rather than the minimum input power condition). The main difference between operating as a refrigerator vs. a liquefier in the configuration shown in Figure 4.3.2, is that the mass flow through each expander is approximately the same for a liquefier but not for the a refrigerator operating at the optimal minimum input power condition. In a refrigerator each Carnot step above the final cold expanders (i.e., above ~20K) is only required to handle the heat exchanger losses and heat leak. In contrast, in a liquefier all the expanders handle an approximately equal amount of compressor flow, and therefore an (approximately) equal amount of compressor isothermal compressor input power.
Thomas Jefferson National Accelerator Page 33 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy The Theory Behind Cycle Design (Cont.)
1
B A L A N CE 0.60 D ) D K E S
4.2 I G N 0.50 N U M W B I E T R H H RE O E F N (g/s @ E A A X T L P C A E ID N X EA O S C M I H L SYS P O FACTIO O N A S N N T G E A TEM T G E S QUID R E I S L L L IM IM I IT T E E D D 50 60 0 0 REFRIGERATION (W @ 4.2K) (80) 100 (150)
Thomas Jefferson National Accelerator Page 34 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy 5. System Optimization
What is an optimum system?
Does it result in the:
• Minimum operating cost • Minimum capital cost • Minimum maintenance cost • Maximum system capacity • Maximum availability of the system
Traditionally a design for maximum efficiency is referred as the optimum system design.
Thomas Jefferson National Accelerator Page 35 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy System Optimization (Cont.)
• The above five factors (or perhaps more) are rarely looked at as the optimization goals. As explained earlier, the demand on equipment varies substantially between operating as a refrigerator (i.e., heat exchanger dominance) and liquefier (i.e., expander dominance). • The challenge is to envision a cycle with these optimization goals, using real components, that is capable of operating close to the maximum efficiency, independent of the load; which may shift from a maximum to a minimum and from total refrigeration to total liquefaction mode or any partial combination.
Thomas Jefferson National Accelerator Page 36 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy System Optimization (Cont.)
• The trade-off relationship between the first two factors the minimum capital cost and minimum operating cost can be quantified to some extent by exergy analysis and the evaluated power cost.
• In the process industry, typically $1000 of capital investment is worthwhile if it reduces the electrical input power by 1 kW (@~$0.04/kW)
Thomas Jefferson National Accelerator Page 37 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy System Optimization (Cont.)
Pressure ratio constraints • Generally the compressor suction is maintained slightly above atmospheric pressure, the maximum discharge pressure sets the pressure ratio . • Many of the critical components used in the system designs • The pressure ratios selected for the cold box components must match both the type of the compressors and their operating characteristics • A higher pressure ratio exposes the components to higher stresses • The peak efficiency for the screw compressors is nominally in between the pressure ratios of 2.5 and 4.0 • Nominally the peak efficiency for turbo expanders is a pressure ratio between 2 and 5
Thomas Jefferson National Accelerator Page 38 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy System Optimization (Cont.)
The effect of higher mass flow through the cold box
• Increase the size of the heat exchangers required (and thus the size of the cold box). • Increase the heat exchanger thermal losses associated with the stream temperature difference. This effect is approximately proportional to the flow. • Increase the pressure drop. • Increase the capital cost of the system
Thomas Jefferson National Accelerator Page 39 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy System Optimization (Cont.)
• Balanced System Design {Ganni Helium Process Cycle(s) (patent pending)}
The new helium cryogenic refrigeration and liquefaction cycle has been developed to maintain high plant operational efficiencies at full and reduced plant capacities.
The following Figures illustrates the base variable pressure cycle design and the several cold end and warm end cycle configurations
Recycle flow pressures vary with % full load, to operate at optimal pressure ratio for the compressors and the expanders
Thomas Jefferson National Accelerator Page 40 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy System Optimization (Cont.)
Thomas Jefferson National Accelerator Page 41 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy System Optimization (Cont.)
HP MP1 LP LP HP MP1 LP LP
TC1 HX-C1 HX-C1
TC1 HX-C2 HX-C2 HX-C3 HX-C3 TC2 TC2 HX-C4 HX-C4 HX-C5 TC3 HX-C5 HX-C6 HX-C6 TC3 HX-C7 HX-C7 HX-C8 P TC4 HX-C9 LL P LL 1.8 atm SC2 HX-C7B /1.3 atm 1.8 atm SC2 HX-C9B /1.3 atm
HX-C8 D LOAD
N HX-C10 LOA O
TI TION C EFAC A LL F E
LL U LIQU K LIQ X 4.5 SC1 HX-C8B 5K
SC1 HX-C10B 4. 1.2 atm 1.2 atm COLD BOX COLD BO
4.5K REFRIGERATION, LIQUEFACTION 4.5K REFRIGERATION, LIQUEFACTION & SUBATM LOADS & SUBATM LOADS LOAD LOAD
Thomas Jefferson National Accelerator Page 42 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy System Optimization (Cont.)
HP MP1 LP LP
HX-C1 TC1 HX-C2
HX-C3 TC2 HX-C4
HX-C5 COLD COMPRESSORS TC3 HX-C6 HX-C7 P LL
1.8 atm SC2 HX-C7B /1.3 atm
HX-C8 LOAD
N O ACTI
LL X O 5K LIQUEF
SC1 HX-C8B B 1.2 atm 4. COLD
4.5K REFRIGERATION, LIQUEFACTION & SUBATM LOADS LOAD
Thomas Jefferson National Accelerator Page 43 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy System Optimization (Cont.)
HP MP2 MP1 LP HP MP2 MP1 LP
HX-W1 TW1 HX-W1 HX-W2 TW1 HX-W2 HX-W3
HX-W4 HX-W3 TW2 HX-W5
HP MP1 LP
HX-W1 HP MP2 MP1 LP
HX-W2
HX-W1 TW1 HX-W3 TW1 HX-W2 HX-W4
HX-W3 TW2 HX-W5
HX-W4 TW2 HX-W6
HX-W5 TW3 HX-W7
HX-W6 TW3 HX-W8
HX-W7 TW4 HX-W9
Thomas Jefferson National Accelerator Page 44 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy 6. The Facts and Myths of LN2 Pre-cooling
What Does LN2 Pre-Cooling Do?
It provides the refrigeration capacity required for:
¾ The liquefaction load (to cool the helium make-up gas) from 300K to 80K (cooling load of the beds to 80K).
¾ The heat exchanger (HX) losses associated with the cooling curves and heat leak from 300K to 80K.
Thomas Jefferson National Accelerator Page 45 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy The Facts and Myths of LN2 Pre-cooling (Cont.)
Advantages:
• Lower capital investment for a given cold box (refrigeration or liquefaction) capacity. For a given system, it increases the LHe production or the refrigeration capacity by a factor of 1.5 or more. • Smaller cold box and compressor size for a given capacity (i.e., a smaller building or les. building space). However, space is required for a LN2 dewar (normally outside). • Provides thermal anchor point of 80K for the adsorber beds. • Stable operation over a larger operating range and a larger turndown capability. • Fewer rotating parts and lower maintenance costs. • Able to keep the load temperature at 80K during partial maintenance of the cold box sub systems (i.e., turbines etc.). • Any impurities in the helium stream are frozen in the ‘warm’ HX and thereby protecting the lower temperature turbines from contamination and erosion damage. • Extremely useful to handle cool down loads. In general approximately ~ 80% of the cool down loads are from 300K to 80K.
Thomas Jefferson National Accelerator Page 46 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy The Facts and Myths of LN2 Pre-cooling (Cont.)
Disadvantages:
• Requires the coordination of deliveries of the LN2. • Different fluids in the system. Cross leaks are more detrimental. • LN2 requires additional oxygen deficiency hazard (ODH) monitoring. • Typically operating costs are greater.
Thomas Jefferson National Accelerator Page 47 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy The Facts and Myths of LN2 Pre-cooling (Cont.)
4.5 g/s He GN 1 g/s 6.5 g/s 2 1 g/s He 5 atm 1 atm 5 atm 1 atm He 300 K 290 K 300 K 300 K 300 K 295 K 300 K HX-1 HX-1 188 K 80 K 85.6 K 117 K 83.7 K 117 K =0.7 T1 153 K HX-B 2.3 atm HX-B =0.7 80 K 97.3 K 80 K 78 K 80 K =0.7 T2 LN 78 K TYPE - A TYPE - B TYPE - C
Thomas Jefferson National Accelerator Page 48 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy The Facts and Myths of LN2 Pre-cooling (Cont.)
LN2 Cost Break-Even Point Analysis Units TYPE-A TYPE-B TYPE-C Helium Liquefaction Flow g/s 1.0 1.0 1.0
LN2 Flow g/s 2.7 ⎯ ⎯ l/hr 12 ⎯ ⎯ Expander Efficiency(s) ⎯ 0.7 0.7 Compressor Recycle Flow g/s ⎯ 6.5 4.5 Comp. Isothermal Eff. ⎯ 0.5 0.5 Comp Power Input kW ⎯ 13 9 Given:
LN2 Cost $/liter 0.06 ⎯ ⎯ Electric Power $/kW-h 0.04 ⎯ ⎯ LCF ⎯ 1.38 2.00
Thomas Jefferson National Accelerator Page 49 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy The Facts and Myths of LN2 Pre-cooling (Cont.)
10 g/s He 10 g/s He
GN2 0.404 g/s GN2 0.404 g/s
290 K 300 K 290 K 290 K 300 K 290 K HX-B1 HX-1 HX-B HX-1 80 K 184 K 80 K 80 K 81.5 K 80 K HX-B2 81.5 K
LN2 HX-2 HX-2 1.105 g/s LN2 1.105 g/s TYPE - 1 TYPE - 2
Thomas Jefferson National Accelerator Page 50 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy 10 g/s He 10 g/s He GN GN2 2
290 K 300 K 290 K 290 K 300 K 290 K HX-1 HX-1 80 K 85.6 K 80 K 80 K 81.5 K 80 K HX-B
81.5 K HX-2
LN2 1.105 g/s HX-2
LN2 1.105 g/s TYPE - 4 TYPE - 3
Thomas Jefferson National Accelerator Page 51 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy The Facts and Myths of LN2 Pre-cooling (Cont.)
10 g/s He 10 g/s He GN GN2 0.22 g/s 2 277.6 300 K 277.6 290 K 300 K 290 K HX-1 HX-B1 HX-1 80 K 81.5 K 80 K 80 K 85.6 K 80 K HX-B2 HX-2
LN 2 81.5 K 2.67 g/s HX-2
LN 2 1.105 g/s TYPE - 3A TYPE - 5
Thomas Jefferson National Accelerator Page 52 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy The Facts and Myths of LN2 Pre-cooling (Cont.)
Comparison of HX Parameters
Duty ∆ Tm Cmin Cmax UA NTU ε
Type HX [W] [K] [W/K] [W/K] [W/K]
Type-1 HX-1 10888 4.48 49.83 51.85 0.961 2430 48.8 0.993 HX-B 458 4.48 2.10 2.18 0.961 102 48.8 0.993
Type-2 HX-1 10888 4.48 49.83 51.85 0.961 2430 48.8 0.993 HX-B1 243 40.14 1.16 2.10 0.552 6 5.2 0.955 HX-B2 215 24.18 2.10 ∞ 0.000 9 4.2 0.986
Type-3 HX-1 11347 4.48 51.93 54.03 0.961 2532 48.8 0.993
Type-4 HX-1 11134 7.59 51.93 53.02 0.979 1467 28.3 0.975 HX-B 213 3.11 51.93 ∞ 0.000 68 1.3 0.732
Type-5 HX-1 10889 7.59 50.79 51.85 0.979 1435 28.3 0.975 HX-B1 245 7.59 1.14 1.17 0.979 32 28.3 0.975 HX-B2 213 3.11 51.93 ∞ 0.000 68 1.3 0.732
Type-3A HX-1 11347 7.73 51.93 57.42 0.904 1468 28.3 0.993
Thomas Jefferson National Accelerator Page 53 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy 7. Typical Helium Cryogenic System and its Basic Components
CONTROLS
POWER He GAS STORAGE COOLING WATER
INSTRUMENT AIR
He DELIVERIES
LN2 DELIVERIES MAIN COMPRESSORS He PURIFIER
MAIN COLD BOX LN2 STORAGE (4.5 K) (DEWAR)
SUB ATM COLD BOX LHe STORAGE VACUUM (2 K) (DEWAR)
DISTRIBUTION
LOADS
Thomas Jefferson National Accelerator Page 54 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Typical Helium Cryogenic System and its Basic Components (Cont.)
Thomas Jefferson National Accelerator Page 55 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Typical Helium Cryogenic System and its Basic Components (Cont.)
Thomas Jefferson National Accelerator Page 56 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Typical Helium Cryogenic System and its Basic Components (Cont.)
Thomas Jefferson National Accelerator Page 57 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Typical Helium Cryogenic System and its Basic Components (Cont.)
Thomas Jefferson National Accelerator Page 58 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Typical Helium Cryogenic System and its Basic Components (Cont.)
Thomas Jefferson National Accelerator Page 59 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Typical Helium Cryogenic System and its Basic Components (Cont.)
Thomas Jefferson National Accelerator Page 60 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Typical Helium Cryogenic System and its Basic Components (Cont.)
Thomas Jefferson National Accelerator Page 61 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Typical Helium Cryogenic System and its Basic Components (Cont.)
Thomas Jefferson National Accelerator Page 62 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Typical Helium Cryogenic System and its Basic Components (Cont.)
Thomas Jefferson National Accelerator Page 63 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Typical Helium Cryogenic System and its Basic Components (Cont.)
Thomas Jefferson National Accelerator Page 64 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Typical Helium Cryogenic System and its Basic Components (Cont.)
Observed Turbine Efficiencies in Helium Service 0.85 0.80 0.75 0.70
cy 0.65
en 0.60 ci
i 0.55 f f 0.50 E 0.45 0.40 0.35 0.30 0 5 10 15 20 25 30 Turbine out put (kW)
Thomas Jefferson National Accelerator Page 65 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Typical Helium Cryogenic System and its Basic Components (Cont.)
Thomas Jefferson National Accelerator Page 66 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Typical Helium Cryogenic System and its Basic Components (Cont.)
Thomas Jefferson National Accelerator Page 67 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Typical Helium Cryogenic System and its Basic Components (Cont.)
Thomas Jefferson National Accelerator Page 68 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Typical Helium Cryogenic System and its Basic Components (Cont.)
Thomas Jefferson National Accelerator Page 69 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy 8. Sub Atm. Systems
1. Vacuum pumping on the helium bath 2. Sub atmospheric refrigeration system design 3. Cold compressors 4. Hybrid systems
Thomas Jefferson National Accelerator Page 70 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Sub Atm. Systems (Cont.)
VACUUM LHe SUPPLY PUMP PURIFICATION GAS RECOVERY (4.5K) SYSTEM
LOAD
Thomas Jefferson National Accelerator Page 71 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Sub Atm. Systems (Cont.)
VACUUM PUMP GAS RECOVERY 4.5K 300K PURIFICATION HELIUM SYSTEM REFRIGERATION SYSTEM HX
(4.5K)
LOAD
Thomas Jefferson National Accelerator Page 72 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Sub Atm. Systems (Cont.)
Thomas Jefferson National Accelerator Page 73 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Sub Atm. Systems (Cont.)
Thomas Jefferson National Accelerator Page 74 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Sub Atm. Systems (Cont.)
Thomas Jefferson National Accelerator Page 75 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Sub Atm. Systems (Cont.)
CC.t Eff.t_poly Eff.t_s=c Eff.t_t=c 0.8 0.7 0.6 0.5 t
ff. 0.4 E 0.3 0.2 0.1 0.0 25.00 30.00 35.00 40.00 45.00 50.00 55.00 Pr. Ratio.t
Thomas Jefferson National Accelerator Page 76 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Sub Atm. Systems (Cont.)
Thomas Jefferson National Accelerator Page 77 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy 9. System Capacity Specification and Margin Allocation
Specify the system requirements
Capacity or Carnot capacity and Carnot Efficiency
For at least the three following cases; They are:
¾ 100% Liquefaction ¾ 100% Refrigeration ¾ 50% Liquefaction and 50% Refrigeration
Thomas Jefferson National Accelerator Page 78 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy System Capacity Specification and Margin Allocation (Cont.)
Some of the factors to be considered for margin are:
• Uncertainty of the primary load estimates • Uncertainty of the heat leak into distribution system load estimates • Capacity for system and load control (~5%) • Uncertainty of instrumentation • System degradation with time due to contamination and allowance for valve leaks before requiring a system shut down and rebuild • Cool down and bringing the loads to steady state conditions • Critical reduced-capacity operation with some component failures (make sure each major component failure is analyzed) • In general the sum of all these factors adds up to 25 to 50% of the primary loads.
Thomas Jefferson National Accelerator Page 79 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy 10. Design Verification and Acceptance Testing
The required devices to test the equipment should be designed into the system components itself.
They will help to: verify the system performance from the vendor operate the system at partial loads using the test devises if required verify the system capacity from time to time; especially after major maintenance and for major component change out periods handle the load and system transients by providing the capacity modulation
Thomas Jefferson National Accelerator Page 80 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy 11. Some of the Lessons Learned over the Years
We can learn a great deal from operating systems and they can be good, bad or even ugly.
Analyze the data carefully before reaching any conclusions.
Make sure the decisions are based on the proven test data and operational experience. Many times the perceptions are very different from the realities.
For new projects, make sure the scope includes all the items required for the project.
Thomas Jefferson National Accelerator Page 81 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Some of the Lessons Learned (Cont.)
It helps a great deal if the project is well organized; the cost estimates are based on present data, and schedule planning and execution are performed by an experienced team.
Make sure at least the following areas of concern are addressed and the lessons learned in these areas are implemented in the new system designs.
Thomas Jefferson National Accelerator Page 82 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Some of the Lessons Learned (Cont.)
Compressors:
• Oil removal under sizing in general and in particular for minimum capacity or reduced pressure (capacity) operation • Improper selection of compressor frame sizes (either too large or too small) • Number of compression stages
Thomas Jefferson National Accelerator Page 83 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Some of the Lessons Learned (Cont.)
Cold Box
• Cycle selection and not addressing all aspects of operation by single point design • Undersized HX’s • Over estimated turbine efficiencies • Inadequate cool down and warm up taps • Nonfunctioning 80K and 20K beds (leaky isolation valves) • Oil contamination
Thomas Jefferson National Accelerator Page 84 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Some of the Lessons Learned (Cont.)
Sub atm Cold box design • Evaluation of cycle options and cold box design • Undersized HX’s • Over estimated compressor pressure ratios and efficiencies • Underestimated torque requirements or over estimated torque performance
Auxiliary Systems • Inadequate Purifier Size • Excessive time to regenerate the purifier beds
Thomas Jefferson National Accelerator Page 85 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Some of the Lessons Learned (Cont.)
Helium Storage • Dewar(s) size requirement estimates • Operable dewar specification including heaters • Warm storage sizing
Distribution System • High heat leak to transfer lines and distribution systems
Thomas Jefferson National Accelerator Page 86 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy 12. Optimal Operation of the Existing Helium Refrigeration Systems
Optimal operation addresses the following goals:
• Operation of the system at the design TS diagram. • Operation of the system at optimal operating conditions to meet the present loads. Again the same five questions need to be asked as explained in the chapter 5, System Optimization. • Modify the system to fit optimal operating conditions to meet present loads with current optimal goals.
Thomas Jefferson National Accelerator Page 87 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Optimal Operation of the Existing Helium Refrigeration Systems (Cont.)
End Station Refrigeration System (ESR) at JLab
Thomas Jefferson National Accelerator Page 88 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Optimal Operation of the Existing Helium Refrigeration Systems (Cont.)
Central Helium Liquefier (CHL) at JLab
Thomas Jefferson National Accelerator Page 89 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Optimal Operation of the Existing Helium Refrigeration Systems (Cont.)
Thomas Jefferson National Accelerator Page 90 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Optimal Operation of the Existing Helium Refrigeration Systems (Cont.)
Thomas Jefferson National Accelerator Page 91 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Optimal Operation of the Existing Helium Refrigeration Systems (Cont.)
Thomas Jefferson National Accelerator Page 92 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Optimal Operation of the Existing Helium Refrigeration Systems (Cont.)
Thomas Jefferson National Accelerator Page 93 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy 13. Some Areas of Interest for Future Developments
Variable system capacity operating modes with modulating pressures and with the appropriate gas management scheme should be implemented at other labs and helium system users in general. This already saved many MW of electric power for JLab, MSU, BNL and will be for SNS.
Software models for operating helium cryogenic systems (with the real components in the system) to help them operate close to the practically possible maximum efficiency for the actual operating load conditions
Thomas Jefferson National Accelerator Page 94 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Some Areas of Interest for Future Developments (Cont.)
Bayonet design:
More and more labs are using bayonet type connections between the systems similar to the ones developed originally at FNAL. It has been proven many a times that the load from these components far exceeds the load estimates. The majority of the heat load estimates are based on pure longitudinal conduction and neglect the convective loops in the annular gap.
Thomas Jefferson National Accelerator Page 95 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Some Areas of Interest for Future Developments (Cont.)
Industry has used tight clearances or end seals to minimize this effect. It is also questionable how well the heat intercepts on these bayonets are anchoring the temperature or in fact, contributing to the overall load increase by setting up the convective loops.
Mass flow meter: Develop a mass flow meter to measure the vapor flow from a load with very low pressure drop.
Thomas Jefferson National Accelerator Page 96 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Some Areas of Interest for Future Developments (Cont.)
Screw compressor:
Test a screw compressor to establish the effect of built-in volume ratio on the pressure ratio and the efficiency for helium systems. Also establish the other process parameters like operating temperature for minimum input power and minimum maintenance.
Thomas Jefferson National Accelerator Page 97 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Some Areas of Interest for Future Developments (Cont.)
Expanders:
Develop turbo expanders which can utilize commercially available magnetic bearing technology and operate with a large pressure ratio and high efficiency.
Develop multi-cylinder reciprocating expansion system, which can utilize the mass produced parts from the auto industry.
Thomas Jefferson National Accelerator Page 98 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Some Areas of Interest for Future Developments (Cont.)
High efficiency small helium refrigerator / liquefier:
Develop a high efficiency small helium refrigerator / liquefier for labs (can be used for small 4.5K targets etc.) and universities
Thomas Jefferson National Accelerator Page 99 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy 14. Conclusions
For new systems, develop the requirements very carefully while taking into consideration the load requirements including all of the anticipated transients and the practical strengths and weakness of the components chosen for the system.
Make sure the design goals are carried through the selection of all the components and into the detailed system design and are not skewed with perceived constraints.
If the end user does not understand the requirements or how to achieve them in the specification, it certainly can not be assumed that the vendor will provide it.
•
Thomas Jefferson National Accelerator Page 100 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Conclusions (Cont.)
Hopefully these and other information will help you to design and operate cryogenic systems at optimal conditions and minimize the input power (the wasted natural resources) and feel good about your positive contributions.
Hope you saw the
“Need for understanding the fundamentals”
Thomas Jefferson National Accelerator Page 101 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Conclusions (Cont.)
What is an optimum system? Does it result in the:
1) Minimum operating cost 2) Minimum capital cost 3) Minimum maintenance cost 4) Maximum system capacity 5) Maximum availability of the system
Or, A combination of some or all the above Or, Some other factors? What do you think?
Thomas Jefferson National Accelerator Page 102 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Conclusions (Cont.)
Thomas Jefferson National Accelerator Page 103 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Conclusions (Cont.)
My special thanks to the members of JLab cryo department and especially to Peter Knudsen for helping me in developing these notes.
Thank you all for showing the interest and hopefully you all sincerely participate in reducing the wasted natural resources.
Thomas Jefferson National Accelerator Page 104 Facility Operated by the Southeastern Universities Research Association for the U.S. Department of Energy