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Georg MagerSenior Advisor Evaporative Cooling EquipmentEurovent Association | Ian ButlerM.ScProject / Audit ManagerEurovent Certita Certification Ltd |
The
principle “value for money” is probably as old as human trade and whilst it
sounds simple and straight forward, we consider that in reality it is not
always straight forward for what a customer expects. Indeed, verification of
the real value can be a challenge; verification of quantities, dimensional
data, weight, etc. are comparatively easy to assess, but what about the
performance of a cooling tower operating in a HVAC plant?
Before we
address the problem of performance verification, let us analyse the impact of an underperforming cooling tower
using a numerical example of an industrial HVAC application operating year
round with a load variation from 100% in summer to 80% in winter. The cooling
tower for this application would be selected for a summer condition to cool 52
l/s of water from 32°C to 27°C at an entering wet bulb temperature of 21°C. The
cooling capacity to be rejected would be 1,090 kW in this case.
The cooling
tower delivering the required performance, let us designate it as “Model 100”, would be 3.6 m long, 2.4 m wide and 3.5 m high with an
absorbed fan power of 28.5 kW, a 30 kW fan motor would be installed
and the overall sound power level of “Model 100” being 93 dB(A).
Now let us analyse this model compared to a cooling tower which
would only deliver 80% of the required duty. This cooling tower (we will
designate it as “Model 80”) could be 20% smaller in physical
size or alternatively, it would have the same physical dimensions as “Model 100”,
however the required fan power is only 20 kW and hence the fan motor
installed would only need to be 22 kW. This example focuses on the last
option for ease of comparison. In addition, the declared overall sound power
level for “Model 80” would be 91 dB(A) instead of 93 for “Model 100”.
Also the “Model 80” could be available at a slightly lower price.
The question:
“Which unit gets ordered” is rhetoricunless the customer knows that
“Model 80” underperforms. In order to know that, however, it is not sufficient
to look at dimensional data and face values for fan power and sound.
Before we
discuss how such verification can be achieved, let us see what the effect of an
underperforming “Model 80” provides. What will happen at design conditions and
more importantly, what will be the knock-on economics effect on an annual base?
For the 1,090 kW,
which has to be dissipated at 21°C wet bulb, “Model 80” will supply water 1.2°C
warmer than that designed. It will take a wet bulb of 19.3°C to supply the
required 32°C / 27°C water temperatures. Two deductions can be drawn from that:
·
The
installed chiller will not totally stop due to excessive high pressure; due to
the 1.2°C warmer water the chiller will unload and capacity will suffer,
however it will not fail. Final result will be some loss of comfort or in the case
of industrial applications, some slowdown of the production process will for
sure take place.
·
In
typical Mid European climates there will be less than 100 hours when the
wet bulb temperature is higher than 19.3°C spread over a few summer days.
Based on
those deductions, it is fair to say that on first sight the underperformance
does not create a catastrophe or send alarm bells ringing. In fact, there may
be several years of bad summers, where design water temperature conditions are
never exceeded. So, after all, could it be said that the problem is not so big?
The
magnitude however can only be answered if we look at the annual economic impact.
With the information we have up to now, we can only say: “Yes, you can get away
with offering cooling towers, which deliver only 80% of the required
performance”. The chances that an operational problem occurs due to the
capacity shortage are nil and unless a performance test reveals the true
situation, the chances you getting caught are very small.
Under such
conditions the likelihood that manufacturers may take risks when stating the
performance of their cooling towers is high. Owners may not even challenge
their performance data due to the fact that they say: “We never had a problem
before.” However, we know now that, whilst it may be so that the problem is not
noticed, it does not mean that it is not there!
What we do
not know yet is: What is the magnitude of the problem? As mentioned before, we
can only answer this question, if we look at the economic annual impact of
underperformance. For that we will use the “Models 100 and 80” from the
previous example and the industrial HVAC year round load profile varying from
100% capacity requirement in the summer to 80% in the winter. Both cooling
towers will use variable frequency drives and run with a concentration factor
of 2.5.
The fan kWh
requirement for “Model100” will be 55,540 kWh and for “Model 80” it will
only be 50,800 kWh, due to the smaller fan motor of the underperforming
“Model 80”. However, look at the electrical energy needed for the chiller:
For the “Model 100” we need 1,114,360 kWh, but for “Model 80” the
chiller requirement goes up to 1,178,700 kWh, which is almost 6% more. If
we therefore add up the chiller and fan kWh the “Model 80” still needs 5%
more electrical energy on an annual base. At a typical cost rate of 0.12 €/kWh
this represents an annual operating cost addition of 7,152 €.
In addition
to that, there is more water consumption for “Model 80” because the chiller has
to work harder hence more waste energy has to be dissipated and more water will
evaporate. In our example “Model 80” will consume per annum 500 m³
water more. If we take the very modest cost for water supply, sewage and
chemicals (3 €/m³), this adds another 1,500 € per year.
The total
operating cost for water and electricity for the system with “Model 80” is 8,652 €.
This is probably about half the first cost of the new cooling tower. It is
clear that an initial small price advantage of the “Model 80” which may
exist; melts away those perceived benefits like snow
on a sunny day.
Cumulative 10 yearadditional operating cost for ‘Model 80’ compared to ‘
Model 100’.
Value for
money does not just come by looking at dimensional data and published values of
certain consumables and emissions. What needs to be challenged is the self-declared
thermal performance especially if it has never been independently tested or
certified. An acceptance test according to a recognized standard is the minimum
needed to take out the guesswork in believing the declared thermal performance,
but for that the cooling tower needs to be purchased and installed. What now if
the tower fails in the test? Penalties, compensations? For sure long and
unpleasant discussions, possibly legal action and at the end of all of that the
owner is still stuck with a faulty cooling tower.
The smart
way to handle this problem is to select a cooling tower which has Eurovent Certified Performance (ECP mark) via 3rd party controlled outside or internal lab
testing.
Only then
the owner is sure prior to purchase that they will not have higher operation
costs due to underperformance.
Certified thermal performance testing removes
risk to obtain system economics and removes guess work, it also removes the
problems of litigation, penalties & compensation should an already
purchased product be found to underperform, because by then it’s too late!
Look for the Eurovent Certified Performance mark
to make that intelligent Cooling Tower selection decision.
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