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BEST PRACTICE IN HEAT NETWORK DESIGN Alexandra Ivanchuk, Applications Engineer – 14th June 2016, REHAU DH Workshop, Manchester
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20,000 employees worldwide – experts in polymer
manufacturing
Since 2000, involved in thousands of district heating
projects across Europe
Manufacturer of district heating pipes & fittings
Largest UK stock of district heating pipe
REHAU’s experience of district heating
2 June 2016 / Rowy 2642/ BT GB
REHAU are the only UK-
manufacturer of PE-Xa district
heating pipe. Production started
in May 2012. Est. 30% reduction
in CO2 from UK manufacturing.
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District heating pipe materials: steel (EN 253) or polymer (EN 15632)
Steel pipe with PU foam Polymer pipe with PU foam Polymer pipe with PEX foam
(bonded) (bonded) (non-bonded)
4 June 2016 / Rowy 2642/ BT GB
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Advantages:
- Very large diameter sizes available
- Capable of withstanding higher temperatures / pressure than polymers
- Strong material – resistant to impact damage
Disadvantages:
- Only straight lengths possible
- Joints required every 6-12m
- Expansion/contraction joints required
- High installation costs
- Corrosion problems (therefore warning systems are required)
- Specialist welding required
λ ≈ 0.022 W/mK
Steel district heating pipes
5 June 2016 / Rowy 2642/ BT GB
>75% of the failures in steel
district heating pipes were due
to on-site welding.
Source: German CHP & District
Heating / Cooling Association,
2013
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Advantages:
- More flexible compared to steel
- No water ingress if outer jacket punctured
- No thermal expansion (self-compensating)
- Long coil lengths possible (less joints)
Disadvantages:
- Steel has higher temperature / pressure limits
- Less flexible compared to non-bonded polymer
pipes
λ = 0.0216 W/mK
Polymer – PU insulation
(bonded)
7 June 2016 / Rowy 2642/ BT GB
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Advantages:
- Highly flexible
- Ideal for house connections / congested
installations
- Long coil lengths possible (less joints)
Disadvantages:
- Steel has higher temperature / pressure limits
- Lower insulation performance compared to PU
foam
λ = 0.043 W/mK
Polymer - PEX insulation
(non-bonded)
8 June 2016 / Rowy 2642/ BT GB
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Optimising heat network design – a balancing act
Total costs
Heat loss costs
Investment costs
Pump & pumping costs
Cos
ts
Pipe diameter
June 2016 / Rowy 2642/ BT GB
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4 key design areas to focus on
10 June 2016 / Rowy 2642/ BT GB
Correct heat loads
Diversity
Installation costs
Pipe sizing
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1. Calculating the correct heat load
11 June 2016 / Rowy 2642/ BT GB
Only roughly estimated or the old boiler load is
used
Pipes & plant will be oversized
leading to higher heat losses
Income from selling the heat
is less
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It is unlikely for every heat customer to use their peak
load at the same time. This is described as diversity.
The diversity factor is the ratio / percentage of the peak
load really used.
For a heat load of 1MW, a diversity factor of 0.7 means
you only need a 700 kW plant.
1000kW x 0.7 = 700 kW
2. The impact of diversity
12 June 2016 / Rowy 2642/ BT GB
Diversity factor in
the example on
the left
/ 13 June 2016 / Rowy 2642/ BT GB
Coldest days
Original – no diversity (1)
= 2,540 kW
Designed diversity of 0.85
= 2,160 kW
Feb 2012 April 2012
Real life measurement
of diversity from a
German DH network
with 80 connections
Actual = 1,600 kW => diversity 0.63 = 1600/2540
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Example diversity for heat network
14 June 2016 / Rowy 2642/ BT GB
= 10 houses = 5 houses = 1 house
Plant
0.9 0.9
0.9 0.9
0.9
0.70 0.75 0.79 0.83
1
1
0.96
0.96
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- Installing in soft-dig areas saves time & cost
- Polymer pipes reduce the installation costs due
to number of joints and time taken per connection
- Innovative use of tees / secondary spines
- Twin pipes instead of single pipes (where possible)
3. Reducing installation costs
15 June 2016 / Rowy 2642/ BT GB
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Trench routing optimisation
16 June 2016 / Rowy 2642/ BT GB
- Less big tees off header pipe
- More opportunity to use DUO
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2x UNO 25 = 10.9 W/m DUO 25 = 7.6 W/m
Heat loss reduction with DUO 30%
2x UNO 40 = 16.6 W/m DUO 40 = 11.6 W/m
Heat loss reduction with DUO 30%
2x UNO 63 = 19.5 W/m DUO 63 = 13.1 W/m
Heat loss reduction with DUO 33%
Data at 80/50°C using RAUTHERMEX pipe.
Benefits of using twin pipe
17 June 2016 / Rowy 2642/ BT GB
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4. Optimising the flow / return temperatures
Modern DH systems typically use a flow temperature of ca. 80°C.
Ensure return temperature is as low as possible (high ΔT)
- Reduces pipe size -> reduces capital costs
18 June 2016 / Rowy 2642/ BT GB
Flow / return temperatures (°C) Heat load Pressure loss (Pa/m) Pipe size required
82/71 1.1MW 161 160mm
80/60 1.1MW 183 125mm
80/50 1.1MW 161 110mm
70/40 1.1MW 166 110mm
60/20 1.1MW 280 90mm
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F/R temperatures (°C)
Heat load Pipe sizing OD
(mm)
Heat losses (vs 82/71°C)
PE-Xa lifespan
(24/7 operation)
82/71 1.1 MW 160 <25 years
80/60 1.1 MW 125 10% lower >25 years
80/50 1.1 MW 110 19% lower >25 years
70/40 1.1 MW 110 34% lower >50 years
60/20 1.1 MW 90 69% lower >50 years
19 June 2016 / Rowy 2642 / BT GB
Heat loss assumptions:
10°C soil temperature
1km distance
0.8m installation depth
1.0 W/m*K soil conductivity
CIBSE Code of Practice:
Best practice would aim to achieve return temperatures <55°C for existing buildings
and < 40°C for new buildings.
Optimising the flow / return temperatures
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Project details:
75 connections
20kW per house = 1,500kW
Total network length = 950m
Flow / return temps = 82/71°C
No diversity applied
All UNO pipes
List price = £313k
Heat network optimisation – original design
20 June 2016 / Rowy 2642/ BT GB
160mm (12m lengths)
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Project details:
75 connections
16kW per house = 1,200kW
Total network length = 950m
Flow / return temps = 82/71°C
No diversity applied
All UNO pipes
List price = £267k
Heat network optimisation – correct heat loads
21 June 2016 / Rowy 2642/ BT GB
125mm > 110mm 160mm > 140mm
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Project details:
75 connections
16kW per house = 1,200kW
Total network length = 950m
Flow / return temps = 82/71°C
Diversity applied of 0.8
All UNO pipes
List price = £218k
Heat network optimisation – using diversity
22 June 2016 / Rowy 2642/ BT GB
160mm > 140mm 125mm > 110mm 110mm > 90mm
75mm > 63mm
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Project details:
75 connections
16kW per house = 1,200kW
Total network length = 950m
Flow / return temps = 70/40°C
Diversity applied of 0.8
All UNO pipes
List price = £134k
Heat network optimisation – optimising flow / return temperatures
23 June 2016 / Rowy 2642/ BT GB
140mm > 90mm 110mm > 75mm 90mm > 63mm
House connections - 32mm > 25mm
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Project details:
75 connections
16kW per house = 1,200kW
Total network length = 950m
Flow / return temps = 70/40°C
Diversity applied of 0.8
Route optimised
All UNO pipes
List price = £128k
Heat network optimisation – route optimisation
24 June 2016 / Rowy 2642/ BT GB
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Project details:
75 connections
16kW per house = 1,200kW
Total network length = 950m
Flow / return temps = 70/40°C
Diversity applied of 0.8
Route optimised
Use DUO pipes if possible
List price = £123k
Heat network optimisation – use of DUO pipes
25 June 2016 / Rowy 2642/ BT GB
ALL DUO
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From these simple steps:
1) Heat load correction from 20kW to 16kW
2) Optimising the route and using DUOs
3) Changing flow temperature from 82/71°C to 70/40°C
4) Diversity factor of 0.8
Cost saving of 60% from the original design.
Heat losses reduced from 23.53kW to 8.98kW which is 63% heat loss reduction.
Heat network optimisation – conclusion
26 June 2016 / Rowy 2642/ BT GB
Can polymer be used for a large
scale heat network? REHAU
supplied 80km of RAUTHERMEX
to the bioenergy village of Lathen
in Germany of 11,000 residents.
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UK Case studies
28 June 2016 / Rowy 2642/ BT GB
- Gaunts Estate – AD plant – 1.6km network
- Soho luxury cottages – Biomass – 7km network
- Eleanor Social Housing – Gas CHP – 2km network
- Tattenhall Retirement Village – Gas – 2km network
- Dundee Social Housing – Gas CHP – 2.5km network
- Portmeirion Village – Biomass – 3km network
