Pipe work and system layout

There are several factors that

influence the design of piping:

Overall type of refrigeration system

Pressure loss in general

Pressure loss in vertical piping

Oil drainage and return

Circulation rates

Controls

Refrigerant

Many decisions and compromises are made

when designing and dimensioning the pipe

system for a cold store. This information pack

describes some important issues that

influence the energy bill.

In general terms at same load larger pipe

diameters have a lower pressure drop but

higher installation cost (incl. insulation). For

some piping the service dictates the

dimensioning.

Overall type of

distribution system

There are three overall common types of

distributions systems:

- Pump circulated systems: A low

pressure liquid distribution systems

which normally requires a refrigerant

pump to pump the cold refrigerant

from the central refrigeration unit to

the evaporators. This type is typically

utilizing R717 (NH3) as refrigerant

and used in large capacity systems.

- High pressure liquid distribution

systems, where the liquid is led to

the evaporators directly from the

condenser or high pressure receiver.

These are normally direct expansion

systems (DX) and are most

commonly used in smaller cold

stores not utilizing R717.

- Secondary refrigerant systems

where the distribution system is

designed for a media normally based

on water. These systems are

described in a different information

pack.

Generally pump circulated systems are used

when the capacity is high and the pipe work is

large and where many different evaporators

are served by a central refrigeration unit.

Further a separate pipe system for defrosting

can be present based on

Hot gas at discharge pressure

Warm liquid, typically in secondary

systems

Designing pipe

systems for cold stores is a trade between investment and running cost.

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INFORMATION PACK

The diagrams show the increase in

energy consumption versus

pressure loss and temperature

change. Further description in the

right column.

0,0%

0,5%

1,0%

1,5%

2,0%

2,5%

3,0%

3,5%

4,0%

-50 -40 -30 -20 -10 0

[kW

h/k

Pa]

Evaporation temperature [° C]

Extra energy consumption caused by pressure loss in suction line

R717

R134a

Motor efficiency : 0,9Isentropic efficiency: 0,7Condensing temperature: 35° CConstant cooling load

0,0%

0,5%

1,0%

1,5%

2,0%

2,5%

3,0%

3,5%

4,0%

-50 -40 -30 -20 -10 0

[kW

h/k

]

Evaporation temperature [° C]

Extra energy consumption caused by decreased evaporation temperature

R717

R134aMotor efficiency : 0,9Isentropic efficiency: 0,7Condensing temperature: 35° CConstant cooling load

0

1.000

2.000

3.000

4.000

5.000

6.000

7.000

8.000

9.000

-50 -40 -30 -20 -10 0

[kW

h/k

/Ye

ar]

Evaporation temperature [° C]

Extra energy consumption caused by decreased evaporation temperature

R717

R134a

Motor efficiency : 0,9Isentropic efficiency: 0,7Condensing temperature: 35° CConstant cooling load of 100 kWOperation period: 5000 h/Year

0,0%

0,5%

1,0%

1,5%

2,0%

2,5%

3,0%

3,5%

4,0%

10 15 20 25 30 35 40 45

[kW

h/k

Pa]

Condensing temperature [° C]

Energy consumption caused by pressure loss on the high pressure side

R717

R134a

Motor efficiency : 0,9Isentropic efficiency: 0,7Evaporation temperature: -20° C

0,0%

0,5%

1,0%

1,5%

2,0%

2,5%

3,0%

3,5%

4,0%

10 15 20 25 30 35 40 45

[kW

h/k

]

Condensing temperature [° C]

Energy consumption caused by increased condensing temperature

R717

R134aMotor efficiency : 0,9Isentropic efficiency: 0,7Evaporation temperature: -20° C

Pressure loss in

general

Transporting a primary or secondary

refrigerant from the refrigeration unit to the

evaporator and back to the unit involves a

pressure loss which can be calculated as:

id

L)(c 0.5 p 2

From the equation it is clear that the pressure

loss is depending on the flow velocity squared,

and proportional to the density, friction losses

and pipe lengths.

The pressure loss has direct and indirect

influence on the energy consumption.

Direct energy consumption.

If the primary or secondary refrigerant is

pumped through the pipes to the evaporator

and back the pump itself require energy which

is often neglected when calculating the energy

consumption of the entire refrigeration system

and COP. The required pump power is

calculated as:

pump

totpump

pqP

The direct pump power can easily make up 10

% of the total energy consumption of the entire

refrigeration system in commercial type

systems.

Indirect energy consumption.

The piping pressure drop courses an indirect

energy consumption in two ways:

Heating by the pump

Lowering the suction pressure of the

compressor

The direct energy consumption of the pump

ends up as heat in the refrigerant which the

refrigeration unit must cool.The shaft power

for open type pumps) and total electrical

consumption for semi hermetic pumps

Depending on the efficiency and the operation

conditions of the refrigeration unit the direct

pump power influence the power consumption

of the refrigeration unit in the following relation:

COP

pumpPrefP

Pressure loss suction side

The refrigeration unit is controlled so that it

can maintain a preset temperature in the cold

store. In practice this is done by controlling the

evaporation pressure in the evaporator(s) or

by activating additional evaporators. For small

variations in working pressures the power

consumption of the refrigeration unit is

approximately proportional with the pressure

ratio between the discharge pressure and the

suction pressure:

suctp

dischpref

ref

ref

refq

refP

So if a certain pressure is required in the

evaporator in order to maintain a preset

temperature in the cold room the suction

pressure at the compressure have to be lower

as a consequence of the pressure loss in the

pipe system:

esuctionlinpevapp suctp

In the world of refrigeration there is a tendency

to express pressures as the corresponding

saturated temperature of the refrigerant. This

is also the case for pressure drops and as a

rough rule of thumb 1°C pressure drop

on the suction or discharge side

results in 2-4% higher power

consumption for the compressor.

An example is shown in the left column for two

systems for 100kW cooling load and the same

service conditions utilizing R717 and R134a

respectively.

Pressure loss suction side

The first two diagrams show the percentage

increase in energy consumption for the

refrigeration systems at 35°C condensing

pressure only caused by the suction line

pressure loss at different evaporating

temperatures.

Both diagrams show the percentage change,

but the actual increase in kWh is higher at low

temperatures. This is illustrated in the third

diagram.

Similar curves can be made for other relevant

refrigerants.

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Pressure loss discharge side

The hot gas leaving the compressor is led to

a desuperheater (for heat recovery) or

directly to the condenser. The pressure loss

on this side of the compressor also causes

some extra energy. The fourth and fifth

diagram shows the relation similar to the

diagram above.

Summary

Generally the largest impacts on energy

consumption are the actual evaporation and

condensing temperatures where the focus

should be. Secondly the pressure loss on the

low pressure side is more important than the

pressure loss on the high temperature side.

Comparing the two diagrams it is clear that

the pressure loss (in Pa) on the suction side

is more important than the discharge

pressure loss.

Pressure loss

determined by the

service of the piping

In some parts of a refrigeration installation the

service of the piping is dictating the sizing

and thereby the pressure drop:

Vertical piping for two-phase

refrigerant flow (riser)

Vapor lines (suction lines) for

transportation of lubricant oil back to

the compressor

Often the refrigerant distribution system

(pipes) is placed outside on the roof of the

cold store or along the ceiling in order to

optimize the logistics in cold store and in

large systems to make easy access the

valves. In pump circulated systems more

liquid refrigerant is fed to the evaporator that

evaporated (circulation rate > 1) and more or

less liquid has to flow along the vapor against

gravity in a vertical pipe, a riser. The liquid

refrigerant (or oil) is to be pulled by the vapor

flow: If the pipe diameter is small the vapor

velocity is high and the liquid is drawn up, but

the frictional pressure drop is high. If the flow

velocity is too low (too large pipe diameter) oil

and liquid refrigerant will not be carried

along. In that case a liquid column with

vapor bubbles will build up creating a

static pressure loss. In other words the

riser has to be designed for the specific

cooling load (vapor amount) in order to

minimize the pressure drop. This also

implies that for a given load a specific

pipe size is optimal. This again means

that the design has to be carefully

examined by experienced personnel

when operating evaporators at part load.

In the case it is possible to install double

risers, but the design is out of the scope

of this info pack.

The impact by the riser pressure drop is

the same as already explained: In order

to maintain the right evaporator

performance and cold store temperature

the refrigeration compressor will operate

at a lower suction pressure which cost

extra energy.

An example: For too large riser the

potential static pressure is depending on

the refrigerant density. For R717 the

static pressure loss of pure liquid is 6,4

KPa/m which cost approximately 6400

kWh/year (@ -30° C, 5000 h/year, P_e =

100 kW).

Oil management

Depending on the compressor type and

the efficiency of the oil separation more

or less lubricating oil which will as

already mentioned be carried into the

piping systems.

The oil can be soluble or non-soluble and

the management of the oil in the system

has to be handled by the design of the

pipe system.

Soluble oils are used in DX type systems

in which the oil dissolved in the liquid

refrigerant has to be dragged back to the

compressor by the gas flow and

inclination of piping.

For refrigerant systems based on

ammonia (R717) as refrigerant non

soluble oil is used to lubricate the

compressor.

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Mixture of liquid and

gas flow in a riser

Valve station in

R717 system

ICE-E INFO PACK

ICE-E INFO PACK The oil management in these systems is

based on the fact that the oil is heavier than

the ammonia. The systems must therefore be

equipped with oil tapping valves spread

around the entire system at the points where

the oil concentrates.

If the pipe system is not properly designed or

serviced on a regular basis oil presence will

eventually increase the pressure drop and

reduce the evaporator performance, which

the refrigeration unit will compensate for by

decreasing the suction pressure

correspondingly.

Insulation

Piping in refrigeration systems are often

insulated in order to

Minimize parasitic heat entering the

system

Eliminate moisture condensation on

the outside and ice build up

Eliminate condensation of

refrigerant inside vapor lines

Concerning the parasitic heat the choice of

insulation thickness is like the pipe size a

trade-off between installation cost (thickness)

and running cost (extra power consumption

for the compressor and condensator).

Recommendations can be found in [1]

Outside the cold store itself the refrigeration

piping is often colder than the dew point of

the air which has to be taken into account

when choosing the type of insulation. Some

type has a very low permeability for water

vapor and others need to have a vapor

retarder at the outside in order to avoid water

vapor to penetrate and condensate inside the

insulation.

In both cases it is evident to keep the

insulation / vapor retarder undamaged in

order to keep the moisture out.

On subzero piping the moisture will build up

as ice inside the insulation which will further

damage the insulation. At warmer pipes

the moisture will vet the insulation.

In both cases the efficiency of the

insulation will be lowered resulting in

higher parasitic heat.

Corrosion

Water (and oxygen) has to be present in

order to corrode a metal surface. Even

though work in being done to keep the

insulation/vapor retarder intact it is nearly

impossible to keep moisture out of the

insulation and thereby the pipe surface.

Due to this carbon steel piping is normally

coted before insulation. More information

is to be found in [1].

References

[1] AHSRAE HANDBOOK, Refrigeration, 2010

ISBN 978-1-933742-82-3

Nomenclature

p = pressure difference [Pa]

ptot = total pressure difference [Pa]

density [kg/m3]

c = flow velocity [m/s]

friction factor [-]

L = pipe length [m]

di = internal pipe diameter [m]

sum of friction coefficients for bends,

contractions, expansions etc. [-]

Ppump = pump effect [W]

q = liquid flow [kg/s]

pump efficiency of pump [-]

Pref = compressor effect [W]

COP = Coefficient Of Performance [-]

ref = pressure ratio compressor

pdisch = outlet pressure compressor [Pa a]

psuct = suction pressure compressor [Pa a]

The work associated with this information pack has been carried out in accordance with the highest academic standards and reasonable endeavours have been made to achieve the degree of reliability and accuracy appropriate to work of this kind. However, the ICE-E project does not have control over the use to which the results of this work may be put by the Company and the Company will therefore be deemed

to have satisfied itself in every respect as to the suitability and fitness of the work for any par ticular purpose or application. In no circumstances will the ICE-E project, its servants or agents accept liability however caused arising from any error or inaccuracy in any operation, advice or report arising from this work, nor from any resulting damage, loss, expenses or claim. © ICE-E 2012

For more information, please contact: Lars Reinholdt (lre@teknologisk.dk)