This page contains a Flash digital edition of a book.
BSEE BOILERS & HOT WATER


A well‐designed heat network scheme has the potential to deliver on efficiency and provide accurate metering for end users but designing these systems correctly and ensuring they deliver real savings is crucial. Pete Mills, Commercial Technical Operations Manager at Bosch Commercial and Industrial outlines some of the potential pitfalls when it comes to sizing a heat network.


he main draw of a heat network is its promise of heat provision at a lower cost and in an efficient way. However, the industry, from designers, contractors and operators cannot afford to be complacent, as a small number of failing schemes have demonstrated. Many district heating designers oversize heat network schemes and the plant heating source as part of a conservative approach; sometimes overcompensating to allow for future expansion and the connection of further networks. There is a lack of experience and guidance on design best practice for heat networks in the UK and as a result, designers tend to over-specify to allow extra capacity for unknown elements.


T


This may seem like a minor error at first, only affecting capital expenditure, but in reality, oversizing a network from the outset will have huge ramifications for the long-term efficiency, overall performance, and return on investment of a system.


An iterative approach is therefore necessary, refining elements of the design to ensure that realistic heating and DHW loads are used, which can mean spending more time modelling loads and accurately assessing heat losses.


Oversized and inappropriate boilers


A problem with the design of many smaller heat networks is a lack of renewable or low carbon energy heat sources within proposed schemes, as it is the efficiency gains from these sources that deliver reduced energy costs. Peak load boilers should only operate when the load cannot be met by the renewable or low carbon heat sources and should be selected based on good turndown ratios.


It is important to note that these heating systems spend a lot of their life operating at between 10% and 25% of their peak due to 24 hour operation with periods of low demand, so pumps and boilers need to be sized correctly in order to perform efficiently at these operating points. Opting for boilers with a large turndown ratio is a smart move as the wider the operational range of the boiler plant, the better it can respond to meet fluctuating requirements in the network.


In this scenario, cascades of smaller boilers can be particularly beneficial as the turndown ratio is based across the entire cascade rather than a single boiler. What’s more, selecting multiple boilers in a cascade arrangement ensures even load matching as each boiler will only operate when required.


Good turndown, along with good control, help ensure peak load boilers do not flood a system with thermal energy that could have otherwise been picked up by a renewable or low carbon heat source. Wear and tear is effectively shared between the boilers, enhancing efficiency, extending the operational life of the system, and making the chance of a complete breakdown far less likely.


Rather than designing extra capacity into the system, the CIBSE CP1 Code of Practice suggests that top-up standby boilers can be used for additional capacity when needed


rather than continuously operating a boiler which is too large for the network.


Common issues affecting the sizing of both the network pipework and heat producing plant are inappropriate diversity factors for DHW demand, and over conservative delivery temperatures. Whereas in the past, the use of inappropriate standards has led to significant oversizing, CP1 has cleared up any doubt about diversity standards with guidance to use the Danish DS439.


For delivery temperatures, the guidance contained within HSE ACOP L8 for instantaneously heated hot water, should allow designers to work with delivery temperatures as low as 50°C. These two factors have a significant impact on the overall design which would otherwise distort the network size, efficiency, and cost.


Pipe down


With the goal of reducing network heat loss, pipe work runs need to be carefully considered. It is normal to optimise the pipe runs to reduce cost, but for a heat network, even greater attention is required.


Experience has shown that common issues with poor installation and insulation occur on the lateral runs, leading to some of the most significant heat loss. CP1 gives guidance based on this to consider breaking up the network into more vertical runs, which tend to be better insulated and installed. Where appropriate, smaller pipe sizes, with their reduced surface area, will give lower heat losses; and whilst there is a knock on effect to pump energy use, it can still work out to give the best efficiency overall. The target should be to reduce network heat losses to below 15% with 10% being best practice.


Learn, don’t copy


It’s clear that European and Scandinavian countries lead the way when it comes to well-performing, efficient district heating schemes, with the likes of Denmark using this method to heat the majority of its residential housing. However, it is important that when it comes to designing heat networks in the UK, we consider typical UK operating conditions rather than directly replicating larger projects in other countries. There are many good UK examples of how well designed and operated schemes have significantly reduced users’ bills and the industry must learn from these by using them as a template to inform future designs. Heat networks should offer the potential for energy cost reductions over many years, which means a long term view is necessary. This way of thinking should permeate the design and operation strategy adopted. It is vital that the end user is properly considered from the outset; how they will be metered, billed and pay must be considered early on. The Heat Trust scheme has done terrific work to establish itself quickly as a voluntary consumer protection scheme, giving end users the confidence they need.


uThe efficiency of a heat network is dependent upon correct design procedures from the start.


20 BUILDING SERVICES & ENVIRONMENTAL ENGINEER SEPTEMBER 2017


Ultimately, the efficiency of a heat network is dependent upon correct design procedures from the outset. The UK has the potential to benefit greatly from district heating schemes, particularly local authorities and housing associations which are often faced with tight budgets and high efficiency targets. However, appropriate design is crucial in realising these benefits. Correctly sizing a plant and network according to heat demand and using smaller pipes and less pipework to keep heat loss in a network to a minimum are two key factors to consider.


www.bosch-industrial.co.uk VISIT OUR WEBSITE: www.bsee.co.uk ‘ With the goal


of reducing network heat loss, pipe work runs need to be carefully considered. It is normal to optimise the pipe runs to reduce cost, but for a heat network, even greater attention is required.





Advertising: 01622 699116 Editorial: 01354 461430


DESIGN IS KEY TO EFFICIENCY Cutting heat networks down to size


uThere are many good UK examples of how well designed and operated schemes have significantly reduced users’ bills.


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180  |  Page 181  |  Page 182  |  Page 183  |  Page 184  |  Page 185  |  Page 186  |  Page 187  |  Page 188  |  Page 189  |  Page 190  |  Page 191  |  Page 192  |  Page 193  |  Page 194  |  Page 195  |  Page 196  |  Page 197  |  Page 198  |  Page 199  |  Page 200  |  Page 201  |  Page 202  |  Page 203  |  Page 204  |  Page 205  |  Page 206  |  Page 207  |  Page 208  |  Page 209  |  Page 210  |  Page 211  |  Page 212  |  Page 213  |  Page 214  |  Page 215  |  Page 216  |  Page 217  |  Page 218  |  Page 219  |  Page 220  |  Page 221  |  Page 222  |  Page 223  |  Page 224