How to size an industrial water heater: the four numbers to produce first

Sizing an industrial water heater is one calculation and three checks. The calculation is heat load in kilowatts. It equals flow rate in litres per hour, multiplied by the temperature rise in °C, multiplied by 1.163 and divided by 1,000.

The first check is the real inlet water temperature across the year. The second is the buffer volume that lets a steady-output machine serve a peaky draw. The third is the electrical headroom in your sanctioned load.

We specify Tetra Heat Pump systems from those four numbers. This page shows how to produce them before you ask anyone for a quotation.

What is the heat load, and how do I calculate it?

Heat load (kW) = flow rate (L/hr) × temperature rise (°C) × 1.163 ÷ 1,000.

The 1.163 is the specific heat of water expressed in watt-hours: raising one litre by one degree takes 1.163 watt-hours. It is the same physics as the more familiar 1 kWh = 860 kcal, written the other way round.

Worked example 1. You know the flow and want the load.

A plant draws 5,000 litres per hour of hot water at 80°C, and the water arrives at 25°C.

• Temperature rise: 80 − 25 = 55°C
• 5,000 × 55 = 275,000
• 275,000 × 1.163 = 319,825 watts
• Divided by 1,000: 320 kW

Worked example 2. You know the load and want the flow.

Somebody has told you the duty is 500 kW and you want to know what that means in litres.

• 500 × 1,000 = 500,000 watts
• 500,000 ÷ 55 = 9,091
• 9,091 ÷ 1.163 = about 7,800 litres per hour at the same 55°C rise

Two things fall out of the arithmetic. The load is proportional to the flow, so doubling the draw doubles the machine. The load is equally proportional to the rise, so halving the rise halves the machine at the same flow. Most plants argue about the first and never look at the second.

What inlet temperature should I use?

Use the inlet temperature you measure at the coldest point of the year.

Inlet temperature sets the rise, and the rise sets the load. It is the input most often assumed and the one that most often makes a quotation wrong. A borewell, a municipal main, a rooftop tank and a condensate return are four different inlet temperatures on one site. Each of them moves with the season.

Getting it wrong costs in both directions.

Assume a warmer inlet than you have and the machine is undersized. It runs flat out, never reaches set point on a cold morning, and the boiler you meant to retire stays in service.

Assume a colder inlet than you have and the machine is oversized. It short cycles at part load, where the coefficient of performance is worse, and you have paid for capacity that never runs.

Inlet temperature varies by source and by season, so here is the sensitivity instead of a single figure. The rows use the same 5,000 litres per hour as example 1.

Inlet temperature	Outlet temperature	Temperature rise	Load at 5,000 L/hr
20°C	80°C	60°C	349 kW
25°C	80°C	55°C	320 kW
30°C	80°C	50°C	291 kW
35°C	80°C	45°C	262 kW

Fifteen degrees of inlet temperature is 87 kW of machine on this duty. Put a thermometer on the incoming line for a week in January before anybody quotes.

Which temperature does each process actually need?

The delivery form column is the one that decides whether a heat pump is in the conversation at all. A duty met by a hot water loop feeding existing equipment is a heat pump duty. A duty that needs saturated steam at pressure is not, however close the temperature looks.

Food and beverage

Process step	Temperature	Delivery form	Heat pump suitable?
HTST pasteurisation	72 to 75°C	Hot water loop to the plate exchanger	Yes
LTLT pasteurisation	63°C	Hot water loop	Yes
CIP caustic or detergent wash	70 to 85°C	Hot water loop	Yes
Evaporator first effects	65 to 75°C	Hot water or low pressure steam	Yes, on the first effects
Blanching	To 98°C	Hot water	Yes
Retort sterilisation	121°C	Saturated steam at pressure	No, needs steam
Spray dryer inlet	180 to 200°C	Hot air	No, needs fuel
Frying and baking	160 to 220°C	Direct fired	No, needs fuel

The heat pump never touches product on any of these. It makes hot water, and the hot water feeds the plate exchanger or the jacket that was there before. That keeps the hygiene and HACCP position unchanged. There is more on the sector split on the food and beverage page.

Pharmaceutical

Process step	Temperature	Delivery form	Heat pump suitable?
HVAC reheat after dehumidification	Low grade, continuous	Hot water coil	Yes, and it is the largest load on this list
CIP circuits	To 100°C	Hot water loop	Yes
Purified water and WFI loop sanitisation	80 to 95°C	Hot water	Yes, for sanitisation
Fluid bed drying	60 to 80°C	Hot air via a coil	Yes
SIP and autoclave	121 to 134°C	Pure steam at pressure	No, needs steam
WFI multi-effect still	Above the range	Plant steam	No, needs steam

Two words are doing work in that table. Sanitisation at 80 to 95°C is a hot water duty. Sterilisation at 121°C and above is a steam duty. The pharmaceutical page covers where a first project lands without touching a qualified utility.

Hospitality

Process step	Temperature	Delivery form	Heat pump suitable?
Guest hot water	45 to 70°C	Stored hot water	Yes
Laundry	60 to 85°C	Hot water to the washer extractor	Yes
Dishwash rinse	To 82°C	Hot water	Yes
Pool heating	26 to 30°C	Hot water via an exchanger	Yes
Kitchen cooking and tandoor	Direct flame	Fuel	No

Hotel draw is the peakiest of the four sectors here, so the buffer section below matters more on this duty than on any other. The hospitality page carries the sizing shape for a property.

Automotive and general engineering

Process step	Temperature	Delivery form	Heat pump suitable?
Alkaline degrease	55 to 80°C	Heated bath	Yes
Zinc phosphating	50 to 60°C	Heated bath	Yes
Hot rinses	55 to 80°C	Hot water	Yes
Component washing	55 to 80°C	Hot water	Yes
Electrodeposition cure oven	190°C	Hot air, direct fired	No, needs fuel
Bake oven	140 to 200°C	Hot air, direct fired	No, needs fuel
Powder cure oven	177 to 204°C	Hot air, direct fired	No, needs fuel

Every oven is out. Every bath is in. That split is unusually clean, which is why pre-treatment lines are one of the easier first projects.

How much buffer volume do I need?

The buffer has to cover the gap between what your plant draws in its worst hour and what the machine makes in that hour.

A heat pump is happiest running steadily at a set output. A factory is not steady: a shift change, a CIP cycle or a morning guest peak pulls hot water far faster than the average. Buffer volume is what reconciles a steady maker with a peaky taker. Sizing it is arithmetic.

Four inputs produce it:

1. Peak draw rate, in litres per hour, measured at the worst period of the day.
2. Duration of that peak, in hours.
3. Machine output, in litres per hour at your temperature rise, which is the load calculation run backwards.
4. The gap. Peak draw minus machine output, multiplied by the duration, is the volume the tank has to hold.

Take a plant drawing 8,000 litres per hour for two hours. A machine making 5,000 litres per hour at the same rise leaves it short by 3,000 litres per hour. Over two hours that is 6,000 litres of usable buffer. Add an allowance on top for the water that has stratified below temperature and serves nothing.

Stratification is the reason the tank shape matters. Hot water is less dense and sits above cold. A tall, narrow tank keeps the two layers apart, with the cold feed entering at the bottom and the hot draw leaving at the top. A short, wide tank mixes them, and mixed water at an intermediate temperature serves nothing. Two smaller tanks in series usually outperform one large one for the same reason.

Size the buffer on your own measured peak. A design that works on the daily average and fails at 7 am is the most common fault on an installed system.

Which heat source should the machine work from?

Use the warmest source that is closest to being free.

Source temperature sets the lift, the lift sets the coefficient of performance, and the COP sets the running cost. Independent measurement in Indian conditions puts real-world performance at about 2.26 at 110°C output and about 2.0 at 120°C. Both are the hard end of the curve. A hot water duty at 60 to 80°C is a much smaller lift, so the achievable figure is higher.

We build Tetra Heat Pump units in air source, water source and cascade configurations. So the source is a design decision. Water source machines reach COP 4.5 to 5.2 at 60°C output, which is the rated point for that configuration.

Walk the plant for these streams before anyone assumes ambient air:

• Cooling tower return, which is warm, continuous and already being thrown away
• Refrigeration or chiller condenser rejection, which in a dairy or cold store is warmest when hot water demand peaks
• Compressor jacket and aftercooler heat
• Effluent leaving a wash or dye house
• Condensate return, if the plant still runs steam

The comparison of air source and water source configurations works through what each one costs you in footprint, noise, water treatment and seasonal variation.

What electrical connection will it need?

The electrical input is the thermal duty divided by the COP you expect at your output temperature. That is where the project meets the DISCOM.

A 320 kW thermal duty at a COP of 3.5 draws roughly 91 kW. The same duty at a COP of 2.0, which is what a 120°C output would give, draws 160 kW. Size the connection on the conservative figure for your actual output temperature, not on the rated figure at 60°C.

Then check it against what you are allowed to draw. Pull twelve months of maximum demand off your electricity bills. Compare the highest month against your sanctioned load to see whether the headroom is there. If it is not, an enhancement application adds cost and lead time. Put both in the project plan, so they do not arrive as a surprise three months later.

What about water quality?

The water side is a running cost as well as a capital one, and it is the half of the specification most quotations skip.

Condenser and heat exchanger performance depends on what is flowing through them. Indian borewell supply varies widely in hardness and dissolved solids. Scale on an exchanger surface appears first as a slowly falling COP, not as a fault. Pre-treatment, a strainer regime and a cleaning interval are part of the system.

Three questions settle it at design stage. What hardness and TDS does a test of your water show? What pre-treatment is already installed, and what does it cost to run? What cleaning access does the exchanger have, and how long does a clean take? The limits are then set against your water at design.

Can I retrofit this alongside a running boiler?

Yes, and that is the usual shape of it in India. A plant rarely stops to change how it makes hot water.

The sequence is: build alongside, tie in during a shutdown window you already have, then run hybrid. The heat pump takes the base load. The boiler covers the peak and stays as backup through the first year, so the fuel bill falls without disappearing. Once a season of data says the heat pump holds the duty, the boiler’s role can be reduced.

Integration is into the controls the plant already runs: BMS, SCADA and PLC networks, including Siemens and Allen-Bradley, over Modbus and BACnet. In practice the heat pump appears on the same screen as everything else, and the operator learns no second system. The integration page sets out the interfaces.

Clearances, footprint and weight are set per unit. They depend on the configuration and capacity selected, so they are issued on the general arrangement drawing at quotation.

What to do next

Produce the four numbers first. Flow rate and temperature rise give you the load. The coldest measured inlet temperature tells you whether that load is the right one. The peak draw and its duration give you the buffer. Twelve months of maximum demand against your sanctioned load tells you whether the electrical side is a detail or a project.

With those in hand, the savings calculator puts a running cost against them. The comparison of heat pump and boiler running costs shows the arithmetic behind it. The full temperature envelope covers the duties above the hot water range. When the numbers are ready, send them to us and we will tell you what fits and what does not.