How much gas does a building need?

How much gas does a building need?

When we start looking at the incoming services to a building, one of the first things we may need to establish is how much gas the building is likely to require.

This will normally happen fairly early in a project, often at feasibility stage, when we are trying to understand what services are already available and whether they are suitable for what we want to do.

I tend to look at these problems using three questions:

What are we trying to establish?

Why do we need to know it?

And how do we calculate it?

What are we trying to establish?

Incoming services are normally considered early enough in a project that applications can be made for new supplies if necessary, or existing infrastructure can be assessed to see whether it is suitable.

That could apply to a completely new building, an extension to an existing building or perhaps a new building being constructed on an existing site.

Sometimes there may already be a gas supply. There might even be a capped connection that has been left for future development.

Whatever the situation, somebody needs to establish whether what is there is suitable or whether a new or altered supply is required.

For gas, the figure we are ultimately interested in is the approximate maximum gas consumption of the building, normally expressed in cubic metres per hour — m³/h.

Why do we need to know?

Before getting too far into the detailed design, there are some fairly fundamental questions that need answering.

a)      Is there already gas on the site?

b)      What capacity does the existing service have?

c)      Is the existing meter suitable?

d)      Will the proposed plant significantly increase the existing demand?

e)      Do we need to approach the gas network about an alteration or new connection?

f)        If an existing site is being extended, do we increase the existing supply, retain it and provide a second supply, or replace the arrangement altogether?

There can be cost and programme implications attached to all of these decisions.

The time to discover that the incoming gas supply is inadequate is not three months into the detailed design.

This is feasibility engineering. We are trying to put sensible numbers against the problem early enough to make useful decisions.

How much gas will the building need?

At this stage we may have very little information.

For the example here, let us assume we have a proposed office building. We know its approximate gross internal floor area — GIFA — but we do not yet have detailed floor plans or heating calculations. We also know that the building is going to be heated using gas-fired boilers. That last point is important.

Before calculating anything, ask:

What is the gas actually being used for?

A building could be predominantly heated by air conditioning or another electrical system, with gas only serving a commercial kitchen.

A school might have science laboratories using gas.

Another building might use gas for heating, hot water and catering.

So before putting numbers into a calculation, we need to understand something about the building and how it is intended to operate.

For this example, though, we will keep things simple:

5,000 m² office building, heated by gas-fired boilers.

Establishing an initial W/m² figure

We now need an approximate gas-fired heating plant allowance expressed in watts per square metre.

There are a couple of ways of obtaining a useful starting figure.

One is published rule-of-thumb information such as BSRIA guidance.

Another — and one I think is particularly useful — is your own previous project experience.

Go back through several similar projects in your office.

Look at the heating schematics, gas schematics and O&M information. Find the total boiler capacities (If you are using boiler capacities, check whether the figures shown are input or output ratings. For this exercise we ultimately need the gas input, so the distinction matters) or known incoming gas requirements and compare those figures with the GIFA of the buildings.

If you do that for ten similar office projects, you will begin to build up your own set of benchmarks.

But don't simply take the numbers at face value.

Ask why one building might be different from another.

Does it have considerably more glazing?

Is there a huge glazed atrium?

Is part of the building unheated?

Does it contain a commercial kitchen?

Were the boilers significantly oversized?

Is there additional gas-fired equipment that the other buildings didn't have?

Rules of thumb and previous projects give us a starting point, not an answer.

For our simple example, let us use:

100 W/m²

The calculation is then:

5,000 m² × 100 W/m² = 500,000 W

Dividing by 1,000 gives:

500 kW

There is an important qualification here.

A calculated building heat loss, the useful output from a boiler and the gas input to a boiler are not necessarily the same thing. Boiler efficiency, design margins, plant selection and redundancy all need consideration as the design develops.

For this simplified feasibility example, however, we are going to assume that the 500 kW represents the total gas input requirement of the plant.

That allows us to concentrate on the incoming gas calculation itself.

Converting kW into m³/h

Gas consumption is normally expressed volumetrically in m³/h, so we now need to convert our 500 kW gas input into a gas flow rate.

We need the gross calorific value — GCV — of the gas.

Calorific value is simply a measure of the energy available from a given volume of gas and is expressed in MJ/m³.

For UK natural gas, a useful working range is approximately 38–41 MJ/m³. GOV.UK gives this as the range gas transporters are required to maintain for this purpose.

The calculation is:

Why?

A kilowatt is a kilojoule per second.

Dividing by 1,000 converts kJ/s into MJ/s.

Dividing that by MJ/m³ leaves us with m³/s.

Multiply by 3,600 seconds per hour and we arrive at m³/h.

Remember:

So dividing by 1,000 converts kJ/s to MJ/s:

We can now cancel the units that appear in both the numerator and denominator:

 The MJ cancels with MJ, and the seconds cancel with seconds, leaving m³/hour.

For our 500 kW example, using a GCV of 38 MJ/m³:

At 41 MJ/m³:

So, depending on the calorific value of the gas, our indicative consumption is around:

44–47 m³/h

Using 39.5 MJ/m³ gives:

The simple way I remember it

For feasibility work, there is a much easier approximation:

Take the gas input in kW and divide by 10.

So:

500 kW ÷ 10 = 50 m³/h

It isn't exact and it doesn't need to be.

Across the calorific-value range we've just looked at, it deliberately gives us a slightly higher gas flow than the calculated figure.

So at an early design-team meeting, I would be perfectly comfortable saying:

Based on a GIFA of 5,000 m², an initial allowance of 100 W/m² and the assumption that the building is gas heated, I would allow approximately 50 m³/h for the incoming gas requirement at this stage.

Notice the wording.

I haven't said that the building will consume exactly 50 m³/h.

I've explained what I've based the figure on.

That matters.

Always record what you based the calculation on

This is one of those habits that becomes increasingly important the longer you work in design.

Write down your assumptions.

In this case I might record:

GIFA: 5,000 m²

Initial allowance: 100 W/m²

Calculated gas input: 500 kW

Feasibility gas allowance: 50 m³/h

Purpose: Initial incoming-services assessment

Basis: Early-stage feasibility calculation pending detailed design

Because six months later somebody may ask:

“Where did the 50 m³/h come from?”

You don't want to have to reverse-engineer your own thinking.

A calculation is only as useful six months later as the assumptions you wrote beside it.

And that is something I will return to throughout these articles:

What did I base the calculation on?

Once we have an indicative figure for the building, we can start looking at what already exists.

Perhaps there is no gas supply and an application needs to be made.

Perhaps there is an existing service and meter that now need their capacities confirming.

Perhaps there is a shortfall that affects the proposed design.

Those are then engineering and project decisions that can be made while there is still time to do something about them.

But at feasibility stage, we've achieved what we set out to do.

We have taken a building about which we knew very little and put a sensible initial number against its likely incoming gas requirement:

approximately 50 m³/h.

 At feasibility stage, we are not trying to produce the final gas design. We are trying to understand the scale of the requirement well enough to make the next decision.

In this example, 50 m³/h gives us that starting point. We can now compare it with any existing service, speak to the relevant gas provider if necessary, and identify any likely cost or programme implications before the detailed design gets too far ahead.

As more information becomes available, the figure can be refined. The important thing is that we know where the number came from, what assumptions sit behind it, and what it was intended to be used for.

That, really, is the purpose of feasibility engineering: not to know everything yet, but to know enough to ask the right questions and make the next sensible decision.