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	CO2 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

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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 CO2 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

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%.

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

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)

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

er (entrainment ratio):

er = msuction mmotive

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.

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0.5

<![if ! IE]> <![endif]>rao	0.4
<![if ! IE]> <![endif]>Entrainment	0.3
	0.2
	0.1
	0

0	2	4	6	8	10	12	14
			Suc on Superheat
		-10°C/26,5 bar(a)			-8°C/28 bar(a)
		-6°C/29,6 bar(a)			-4°C/31,3 bar(a)

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Application Guide | High pressure lift ejector and liquid ejector systems

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

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.

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.

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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.

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.

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.

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 index1)	130	130 + 100 = 230	130 + 100 + 100	130 + 100 +
			= 330	100 + 100 = 430
Starting capacity2)	46%	26%	18%	14%
Gap between	-	13%	9%	7%
compressor steps2)

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

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

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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 index1)	130	130 + 70 = 200	130 + 70 + 70 = 270	130 + 70 +
				70 + 70 = 340
Starting capacity2)	46%	30%	22%	17%
Gap between	-	0%	0%	0%
compressor steps2)

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

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

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 index1)	-	-	130 + 70 + 140	130 + 70 +
			= 340	140 + 140 = 480
Starting capacity2)	-	-	17%	12%
Gap between	-	-	0%	0%
compressor steps2)

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

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

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.

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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

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.

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 AK-

PC 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

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.

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Application Guide | High pressure lift ejector and liquid ejector systems

1.2Example of system load with a High Pressure lift ejector

	160.0								<![if ! IE]> <![endif]>Danfoss 80G449
		x = 17%
	80.0
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<![if ! IE]> <![endif]>Pressure	40.0
	20.0
	10.0
	150.0	200.0	250.0	300.0	350.0	400.0	450.0	500.0	550.0

Enthalpy kJ/kg

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.

	160.0								<![if ! IE]> <![endif]>Danfoss 80G450
			x = 55%
	80.0
<![if ! IE]> <![endif]>bar(a)	40.0
<![if ! IE]> <![endif]>Pressure
	20.0
	10.0
	150.0	200.0	250.0	300.0	350.0	400.0	450.0	500.0	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.

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Application Guide | High pressure lift ejector and liquid ejector systems

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

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 with an Adaptive Liquid Control (ALC) algorithm,

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.

2.1 Superheat control	Superheat across an evaporator, represented with a single tube:
including new Adaptive
Liquid Control (ALC)

•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 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.

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.

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.

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