Danfoss High pressure lift ejector and liquid ejector systems Application guide

Application Guide

High pressure lift ejector and
liquid ejector systems

Application Guide | High pressure lift ejector and liquid ejector systems

Contents 1. High Pressure lift ejector system - a general description ............................................................................3

1.1 System design with a High Pressure lift ejector (HP) ...................................................................................4

2. The Liquid Ejector (LE) system – a general description ................................................................................9

2.1 Superheat control including new Adaptive Liquid Control (ALC) ..........................................................9

2.2 CO Adaptive Liquid Management (CALM™) ...............................................................................................10

2.3 Suction accumulator design .............................................................................................................................. 11

2.4 Example of system load with Liquid ejector ................................................................................................ 12

3. System design with a High Pressure lift ejector (HP) and a Liquid Ejector (LE) ............................. 13

3.1 Example of system load with Combi (HP + LE) ejector ............................................................................ 14

4. Configuration of the AK-PC 782A .........................................................................................................................15

4.1 HP control ............................................................................................................................................................... 15

4.2 Receiver control......................................................................................................................................................17

4.3 CALM™ set-up in Pack Controller AK-PC 782A ............................................................................................18

4.4 I/O Configuration ................................................................................................................................................... 19

4.5 CALM™ set-up in System Manager AK-SM 8xx ...........................................................................................19

5. What is an ejector, and how does it work? ....................................................................................................... 22

6. Coolselector®2 Selecting components ..............................................................................................................25

6.1 HP ejector selection .............................................................................................................................................. 25

6.2 LE ejector selection ............................................................................................................................................... 26

6.3 Combi ejector selection (HP+LE) ..................................................................................................................... 27

7. Multi Ejector Solution™ ............................................................................................................................................. 28

2 | AB351943880096en-000101 © Danfoss | DCS (vt) | 2020.10

0.6

Entrainment rao

-6°C/29,6 bar(a) -4°C/31,3 bar(a)

s

80G445

Application Guide | High pressure lift ejector and liquid ejector systems

1. High Pressure lift ejector
system - a general
description

A parallel compression system is one of the
concepts that can be used in warmer climates
to enhance the efficiency of a transcritical CO
system. The system is using the same layout
as the booster system with an extra suction
group connected to the receiver. For parallel
compressors and their suction line we will use
the designations IT (intermediate) compressors
and IT suction group.

In cold periods, the system works as a booster
system, but in warmer periods the amount of
flash gas after high pressure expansion grows
and parallel compressors should take over while
the gas bypass valve should close. Depending on
the variable capacity IT compressor selection and
its minimum mass flow capacity, typical ambient
conditions for changeover should be between

The High-Pressure lift ejector (HP ejector) is an
add-on to enhance the parallel compression
system.

An ejector is a device that utilizes the expansion
work from the high-pressure expansion. This
expansion work is transformed into compression
work that compresses some of the MT evaporator
flow into higher pressure (receiver). Since the
expansion work is given by the conditions
(temperature and pressure out of the gas
cooler together with the receiver pressure),
the compression work is also given (assuming
constant efficiency). That means that the system
offers some flexibility.

We can adjust the pressure in the receiver in a
way such that the ejector produces a high lift
(pressure difference between receiver and MT
evaporator), but since the expansion work is
given, the flow cannot be high at the same time.
If the receiver pressure is then lowered and the
ejector thereby enabled only to produce a low
lift, the mass flow will be higher. However, it will
reach some limits, based on developed chocked
flow in the suction flow. When chocked limits are
reached, the suction flow can be higher, despite
the low requested pressure lift.

The suction mass flow is controlled by the
entrainment ratio (ratio between suction mass
flow and motive mass flow). The entrainment
ratio is a characteristic of the ejector and depends

15 °C and 20 °C. At this condition the gas mass
flow in the receiver that needs to be bypassed
increases to 25 – 30% of gas cooler mass flow and
should be taken by the gas bypass valve (GBV) or
preferably the IT suction group.

In very warm periods, close to ambient
conditions 40 °C, the mass flow ratio between
MT suction and IT suction will be MT 55% / IT
45%. Because of the higher suction pressure of
the parallel compressors, the system efficiency
will rise allowing to reduce installed MT/IT
compressors total swept volume comparing
to the standard booster system. Depending
on yearly climate conditions, the parallel
compression system will provide an annual
energy reduction compared to standard booster
system at a level between 5 – 9%.

on the ejector geometry and operational
conditions (pressure temperatures and densities
in the inlets and outlet of the ejector).

er (entrainment ratio):

m

suction

er =

m

motive

The optimal entrainment ratio is between
20 – 25% and the pressure lift between 5
– 14 bar. The pressure lift will be highest at
high ambient temperatures where we need
most of the ejector’s benefit. There is also a
dependency on suction flow super heat, as
laboratory experiments show that the highest
entrainment ratio is between 4 K and 12 K and
our recommendation is to take the flow from
the MT evaporators instead of common suction
line from MT evaporators and LT compressors
as the superheat can be higher depending on
LT/MT ratio. The high superheated gas in the
suction line will also result in reduced system
performance (COP).

Reduced COP can also occur if the LT mass flow
is relatively high in comparison to residual MT
evaporators’ flow, resulting in significantly high
MT compressors’ suction superheat and high
discharge temperatures at extreme ambient
conditions. To avoid this, an LT discharge
desuperheater can be applied or liquid injected
into the MT compressors’ suction line to maintain
acceptable superheat at those conditions.

Danfos

0.5

To the right is an example with different MT
evaporating temperatures (pressures) and ejector
suction superheat between 0 K and 15 K but
keeping constant:

• Temperature out of the gas cooler = 35 °C

• Pressure in the gas cooler = 90 bar(a)

• Pressure in the receiver = 38 bar(a)

0.4

0.3

0.2

0.1

0

0246810 12 14

Sucon Superheat

-10°C/26,5 bar(a) -8°C/28 bar(a)

© Danfoss | DCS (vt) | 2020.10 AB351943880096en-000101 | 3

Application Guide | High pressure lift ejector and liquid ejector systems

The high-pressure flow (ejector motive flow)
from the gas cooler enters the ejector which in
this system works like a high-pressure control
valve. The opening of the different ejectors is
controlled in order to maintain the high pressure
with the purpose of achieving optimum system
COP. The pressure/temperature curve and the gas
cooler outlet, contributing to optimum COP of
the system, is predefined and integrated in the
Danfoss pack controllers. In the ejector, energy
from the high-pressure side is used to entrain
gas from the suction side of the MT evaporators
and lifts it to the receiver. In the receiver, the gas
and liquid are separated and if the amount of
gas is higher than the minimum capacity of the
IT suction group, the controller should use IT
compressors instead of a gas bypass valve.

The benefit of the HP ejector is that it unloads
the MT compressors and enhances the IT
compressors by feeding gas that should be
compressed by the MT compressors. If we
consider the same design criteria for a variable
capacity IT compressor and its minimum mass
flow, the typical ambient conditions for engaging
the IT compressors will be lower than in a parallel
system without HP ejectors.

In well-designed IT suction group it will be
possible to start compressors at ambient
conditions from 15 – 20 °C, while comparing
to system with HP ejectors, and allowing the
receiver pressure fluctuation by using the IT
optimize function (one of the receiver control

options in AK-PC 782A pack controller), the
typical ambient conditions that can enable IT
compressors will be 5 – 7K lower. This control
strategy will contribute with more hours utilizing
the IT suction group during the year.

When using HP ejectors and the IT Optimize
function in regions with ambient conditions up
to 40 °C, the mass flow ratio between MT suction
and IT suction will be MT 35% / IT 65%. This will
give a significant reduction in annual energy
consumption compared to a standard booster
system, between 9 – 15%, depending on yearly
climate conditions and the ratio between LT and
MT load.

The energy consumption reduction is considered
without implemented Heat Recovery. When Heat
Recovery is applied in CO2 transcritical systems,
the pack controller will increase the pressure
in the gas cooler to above the critical point (74
bar(a)) in order to enable more heat rejection.
By doing this, the HP ejector will again have
a high potential to lift mass flow from the MT
evaporators to the receiver, like in warm ambient
conditions.

As for parallel compression, the oil management
of the parallel compressor enhanced with a high­pressure ejector needs special attention because
the mass flow ratio for the parallel compressor
is high. However, it is possible to build a system
with safe oil return if the oil carry-over is
managed.

1.1 System design with a
High Pressure lift

ejector (HP)

Systems with High Pressure lift ejectors (HP
ejectors) are in many ways identical to parallel
compression systems, but the ejector enhances
the operation of the parallel compressors. The
ejector takes mass flow from MT compressors
and delivers it through the receiver to the IT
compressors. Consequently, the MT compressors
can be reduced in size and since the parallel
compressors are running at a higher suction
pressure the total swept volume will be reduced.

Oil return to the MT compressors are just as
on the standard system with gas bypass. The
oil/refrigerant mixture will even be richer on
oil. As there is no oil returning to the parallel
compressors through the suction line, extra
attention needs to be paid to this part of the oil
management system.

4 | AB351943880096en-000101 © Danfoss | DCS (vt) | 2020.10

Application Guide | High pressure lift ejector and liquid ejector systems

Design of MT/IT suction groups

To extract the maximum performance of a
system with ejectors it is important to be careful
when sizing the MT and IT suction group. For
the ejector and the overall system to yield a high
performance the receiver pressure should follow
the variable reference that as realized by the “IT­optimize” reference mode for the IT compressor.
This however make it less trivial to size the
compressor groups (IT and MT) as it is important
to look at part load performance and not only
at the high design temperature and maximum
system load. Temperatures out of the gas cooler
between 15 – 20 °C and a part load operation
with a load of approximately 40 – 60% of the full
load condition (depending of the application)
should be considered. As the load on the IT
compressors are highly depended on the ejector
and the receiver pressure, they should be able to
cover a larger load span than a system without
ejectors, this makes it difficult to construct
an efficient system with only one parallel IT
compressor.

Parallel IT compressor capacity control

For MT and LT compressors suction groups, it
is important to have possibility to match the
actual compressor suction mass flow with the
system load. If there is no match, it will result
in suction pressure oscillations when cutting
in capacity steps (for increasing capacity) or
cutting out steps (for decreasing capacity). For
that reason, one or two Variable Speed Drives
(VSD) are used on leading compressors. When
considering the design of IT suction compressors,
having an as linear capacity control as possible
is even more important, because oscillations
in the receiver pressure control will affect both
the MT/LT evaporating pressure and the gas
cooler pressure, resulting in an unstable system
with compressors and fans running capacity
oscillations.

If cost is the driver, the performance at the
very warmest conditions can be sacrificed by
designing the compressors at part load condition
and then accept that the energy performance
at maximum load and maximum temperature
is not ideal. This is the same consideration as
with parallel compression systems. In case of
maximum load and maximum temperature,
the receiver pressure is allowed to increase and
therefore the entrainment ratio is decreasing,
leaving more gas to the MT compressors. In this
way the system can be cost optimized with only a
small penalty on energy.

The same design analogy can be applied to HP
ejectors by installing a High Pressure expansion
valve in parallel to it in order to cover the gas
cooler mass flow that cannot be taken by the HP
ejectors at very high temperature conditions. It
is possible to install more HP ejectors connected
in parallel, and in AK-PC 782A it is possible to
configure up to four ejector blocks in this way.

Higher receiver pressure oscillations can lead
to the IT compressors frequently reaching their
pump down limit. The pump down limit pressure
level will switch off IT compressors and start GBV
operation resulting in even more unstable system
operation. It is important to avoid long periods
with the MT and IT running large part of the
installed capacity in part load. At part load the
compressor is really inefficient. We have seen that
installing an ejector which removes load from
the MT and adds to the IT cause longer periods of
both MT and IT running part load causing overall
worse efficiency compared to not running with
the ejector. This is also an important argument
for splitting the capacity on more compressors.

Compressor suction group capacity control:

Example 1:

All compressors are same nominal size.
The first compressor has VSD control from 30 – 65 Hz. The rest are single step compressors.

Compressor index

Starting capacity

Gap between
compressor steps

1)

Compressor index represents capacity of each compressor as a relative number.

2)

Capacities represented as % of total suction line mass flow capacity.

1)

2)

2)

130 130 + 100 = 230

46% 26% 18% 14%

- 13% 9% 7%

130 + 100 + 100

= 330

130 + 100 +

100 + 100 = 430

© Danfoss | DCS (vt) | 2020.10 AB351943880096en-000101 | 5

Application Guide | High pressure lift ejector and liquid ejector systems

Example 2:

The first compressor has VSD control from 30 – 65 Hz. The rest are single step compressors, but 30%
smaller capacity compared to VSD nominal capacity at 50 Hz.

Compressor index

Starting capacity

Gap between
compressor steps

1)

Compressor index represents capacity of each compressor as a relative number.

2)

Capacities represented as % of total suction line mass flow capacity.

1)

2)

2)

130 130 + 70 = 200 130 + 70 + 70 = 270

46% 30% 22% 17%

- 0% 0% 0%

130 + 70 +

70 + 70 = 340

Example 3:

The first compressor has VSD control from 30 – 65 Hz. First single step compressor is 30% smaller
capacity compared to VSD nominal capacity at 50 Hz. Second and third double size of first single
step compressor. This configuration is applicable in systems with 1 VSD and 2 or more single step
compressors with Step control mode “Best fit”.

Compressor index

Starting capacity

Gap between
compressor steps

1)

Compressor index represents capacity of each compressor as a relative number.

2)

Capacities represented as % of total suction line mass flow capacity.

1)

2)

2)

- -

- - 17% 12%

- - 0% 0%

130 + 70 + 140

= 340

130 + 70 +

140 + 140 = 480

Conclusions:

• Example 1: More single step compressors will allow us to start the VSD compressor much earlier in
relation to total suction group capacity. Second, with more compressors there will be smaller gaps
between capacity control.

• Example 2: Using 30% smaller capacity single step compressors in comparison to nominal VSD
compressor, will result in linear capacity control (VSD control from 30 – 65 Hz). Drawback of such
configuration is less total capacity comparing to Example 1. because all single step compressors are
smaller.

• In Danfoss pack controllers it is possible to select a “Best-fit” option which makes it possible to select
different sizes of single step compressors (Example 3.). To achieve linear control and higher capacity,
it is important that the first single step compressor has 30% less capacity than a VSD nominal capacity
at 50 Hz. Other bigger size compressors should not be larger than double size of the first single step
compressor. Having this kind of regulation, it is possible to achieve a higher total capacity comparing
to systems in Example 1 and 2.

• Examples 2 and 3 presents optimal sizing compressors to achieve linear control and maximum
overall capacity. In reality, it is good to have certain capacities overlapping in the area between two
steps it does not immediately trigger stating a new compressor and lower the speed on the VSD.

• When evaluating IT suction group design, to avoid system oscillations, it is extremely important to
have linear capacity control. By doing this we can utilize IT compressors and HP ejectors in optimum
way through the year and provide energy savings.

6 | AB351943880096en-000101 © Danfoss | DCS (vt) | 2020.10

Application Guide | High pressure lift ejector and liquid ejector systems

Single parallel compressor system design

The ideal solution is to have - as a minimum - two
IT parallel compressors and two MT compressors
with linear capacity control, but very often this
is not possible in smaller systems due to cost
reasons. Therefore, it is often seen that there
is only one parallel compressor. If this parallel
compressor is selected for maximum load, it
would give the optimum performance at high
ambient temperatures, but it will give very
poor performance in the medium ambient
temperature, where there are many operating
hours. The turning point, when the IT compressor
should start and the gas bypass valve should be
closed, is the condition where the minimum gas
mass flow IT compressor capacity (at minimum
speed) should match the gas mass flow in the
receiver. The challenge of selecting the right
compressor is even bigger considering that
lower ambient conditions (lower pressure in the

Maximum utilization of parallel compressor
system with HP ejectors design

In general, when selecting the compressors, the
following scenarios need to be considered:

• In a well-designed system with IT compressors
and HP ejectors the change-over between
the gas bypass valve and the IT compressors’
operation happen at ambient temperatures
between 12 – 15 °C. So below that turning
point, the minimum mass flow that the IT
compressors can handle needs to be added to
the MT compressor mass flow capacity. This is
not a huge issue as the gas quality is low and
the MT compressors will have more capacity
because the pressure in the gas cooler will be
low too.

• Receiver pressure: The system can be tuned
by optimizing the receiver pressure. This
functionality is embedded in the Danfoss AK­PC 782A controller and is activated by selecting
Reference mode for the IT compressors
with the function “IT optimize”. At higher
temperatures out of the gas cooler (Sgc), the
receiver pressure is lifted gradually, depending

gas cooler), will result with higher compressor
capacity comparing to high extreme ambient
conditions.

As a result of the above considerations and
compromises, single parallel compressor systems
should be sized for the part load condition and
medium temperature conditions to achieve more
operation hours. With such configuration at full
load condition and high ambient temperature,
an IT compressor will run at full speed and the
suction pressure (receiver pressure) for the IT
compressor is allowed to rise to a determined
limit, causing the gas bypass valve to open
and release the surplus gas mass flow that
the IT compressor cannot handle to the MT
compressors. This will of course increase the load
on the MT compressors and should be taken into
account when designing the MT suction group.

on the MT suction and the gas cooler pressure.
If the receiver pressure is too low, savings will
be lower because the pressure is too close
to the MT suction pressure. A higher receiver
pressure also yields a smaller compressor due
to the higher suction pressure. Keep in mind
that a higher suction pressure also gives a
higher oil carry-over, so the compressor and
oil management need to be able to handle it.
There is a limitation on the receiver pressure
given by the manufacturer receiver design.

• Gas cooler mass flow: Due to mass flow
balances in systems with a HP ejector there
will be a higher mass flow through the gas
cooler than in parallel compressor systems
only. Depending how you control the pressure
in the receiver, the mass flow increase will
be different. If you run with the “IT optimize”
function at high ambient temperatures
increase will be up to 3%. If you run with
constant pressure in the receiver, the increase
at high ambient temperatures can be 6%
higher.

© Danfoss | DCS (vt) | 2020.10 AB351943880096en-000101 | 7

160.0

Pressure bar(a)

Enthalpy kJ/kg

80G449

s

80G450

160.0

Pressure bar(a)

Application Guide | High pressure lift ejector and liquid ejector systems

1.2 Example of system load
with a High Pressure lift
ejector

At above conditions (Po-MT/Prec/Pgc), the HP ejector is unable to make a necessary lift of 8.5 bar
and the amount of gas after the HP expansion is relatively small, so the system works as a standard
booster. To enable the ejector to provide a lift at specified Po-MT/Pgc conditions it will be necessary
to decrease the Prec pressure to 32 bar(a). This can be achieved using the receiver control mode “IT
optimize” and allowing lower minimum pressure reference in the receiver. There is also a feature for
keeping a minimum pressure difference between the receiver and the MT suction.

x = 17%

80.0

40.0

20.0

10.0

150.0 200.0 250.0 300.0350.0 400.0450.0 500.0550.0

Danfoss

x = 55%

80.0

40.0

20.0

10.0

150.0 200.0 250.0300.0 350.0400.0 450.0500.0 550.0

Enthalpy kJ/kg

In warm ambient conditions, the HP ejector has the ability to create a higher pressure lift. To optimize
system efficiency, we can gradually increase the receiver pressure and by doing this control the ejector
suction mass flow, keeping the entrainment ratio between 0.2 and 0.25. At above conditions, the
ejector suction mass flow is 24% of the motive mass flow (er = 0.24). Any gas after the HP expansion
and the ejector suction gas must be taken by the IT compressors.

Danfos

8 | AB351943880096en-000101 © Danfoss | DCS (vt) | 2020.10

Application Guide | High pressure lift ejector and liquid ejector systems

2. The Liquid Ejector (LE)
system – a general
description

2.1 Superheat control
including new Adaptive
Liquid Control (ALC)

The Liquid Ejector is designed for both standard
booster and parallel compression systems.
The working principle in the Liquid Ejector is
similar to the gas ejectors. The Liquid Ejector
is optimized to lift liquid from the suction
accumulator and returning it to the receiver. In
a Liquid Ejector system, the evaporators operate
with a very low superheat and a fraction of liquid
is returned to the suction accumulator, located
downstream of the evaporators. With the support
of an appropriate and intelligent case controller

the evaporator functions more efficiently at
a higher suction pressure. This enables the
suction pressure to be raised, thereby reducing
the energy consumption on the compressors.
Typically, the pressure in MT evaporators will
be increased, utilizing network Master Control
function Po optimization by 3.5 – 5 bar.

As the Liquid Ejector is powered by the work
which would have been otherwise lost, there is
no additional energy used to accomplish this.

with an Adaptive Liquid Control (ALC) algorithm,

Superheat across an evaporator, represented with a single tube:

• Zero superheat can be measured in all of the
evaporator where fluid exists (until Dry-Out
point).

• The ALC point (Adaptive Liquid Control point)
can be found at the Dry-Out point on the
borderline between the Annular Flow area
and the Mist Flow area. With ALC the highest
evaporator efficiency is realized but with
droplets of refrigerant leaving the evaporator,
which is not a safe situation compressor wise,
so a suction accumulator must be applied.

The key to achieving the performance benefits
enabled by the Liquid Ejector is the use of the
Adaptive Liquid Control (ALC) algorithm available
in the newest generation of Danfoss case
controllers. This algorithm makes full use of the
entire surface of the evaporator, maximizing its
efficiency and operating with the lowest possible
superheat (close to 0 K).

The amount of liquid is difficult to estimate,
but a good estimate form experience is approx.
10% from the MT evaporators that are in liquid
mode (by using the Danfoss evaporator/case
controllers with the ALC functionality), also only
those evaporators which are maximum loaded
are in liquid mode, resulting in a reduced amount
of returned liquid. In general, the experience
shows that approx. 30% of the evaporators are
running in liquid mode at a given time, so the
amount of liquid returned is then approximately
10% x 30% = 3% of the total MT evaporators’
mass flow returning to the suction accumulator.

• The MSS point (Minimum Stable Superheat)
can be found on the borderline between the
Mist Flow area and the Single Phase Flow. With
MSS control the highest evaporator efficiency
is achieved while all liquid is evaporated and
only superheated gas is leaving the evaporator,
which is a safe situation compressor wise.

• Dry SH control. Stable superheat can be
measured when the superheat is higher than
the MSS point.

This assumption is valid for systems with many
evaporators of similar size like in a supermarket
application (more than 10). For systems like
cold storage plants with only a few evaporators
where some of them can represent a high ratio
in total MT evaporator capacity, the amount of
liquid returned as a percentage of the total MT
evaporator mass flow can be significantly higher.

Fig. 1: Returned liquid estimation of returned liquid as a
function of evaporators quantity.

© Danfoss | DCS (vt) | 2020.10 AB351943880096en-000101 | 9