HVAC and A/C sizing - Is it accurate?

Discussion in 'Boat Design' started by chrisyk, May 28, 2013.

  1. chrisyk
    Joined: Dec 2012
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    chrisyk Junior Member

    HVAC and A/C sizing - Is it accurate?

    Today's air conditioning market is focusing on efficiency as a means to reduce energy consumption.

    • But how do you size your system?
    • What is the best type of HVAC systems for my vessel?
    • Should I design for the worst case scenario or the most frequent scenario?
    • What impact does this have on my Gen sets?

    These are the questions that builders and designers are faced... but can be sometimes ill advised by the suppliers, who appart from wanting to sell the device to you, may want to be absolutely sure that all scenarios are catered for.

    I am asking this forum therefore, to consider best practice for A/C system sizing & what would be the best approach to selecting and installing the system?
     
  2. groper
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    groper Senior Member

    Question 1 = Size of system (capacity) is given by change in temperature * volume of air. So read off the enthalpy of air and humidity from psychometric chart and the difference from ambient to target temp and humidity, and use teh volume of the space you wish to wish to cool. You also need to consider the heat load acting upon the space, such as windows, insulation on roof, shading of walls etc, and add this to the result. It gets a bit technical but theres info out there for estimating this.

    Question 2 = Define "best"??? Noone can help with this question unless they know what your trying to acheive, best efficiency, easiest maintenance, lowest capitol cost or???

    Question 3 = depends on what you can live with? do you want it to work ok most of time and cant cope with the hottest days?

    Question 4 = Completely depends on how the system is designed, will it be electric powered compressors, or belt driven compressors from the engines? Either way is possible, and the extra load is proportional to the capacity you need. But in electrical terms, if you need 30,000btu of cooling capacity, your AC system will draw around 3-3.5kw of electrical power to achieve it, and this can be in turn be converted into mechanical energy of the same, so it will load your genset by an extra 3-3.5kw engine power / efficiency of driven alternator, to get total engine load.

    You also need to define "best approach" for AC selection and installation... There is no "best", only pros and cons for different approaches. A very energy efficient system will be a variable frequency drive, utilizing keel cooling or perhaps waste heat recovery for hot water system, will be more complex, more difficult to maintain and install, and more expensive to maintain and install. On the other hand a cheap system might be less efficient, maybe not, depending on how its used. You can even get solar assisted units (vacuum tubes) these days, but theres no gain if the unit doesnt get much sun because its shaded by sails all day etc...

    So whats important to you, efficiency or simplicity or capitol costs or maintenance costs or installation costs or ???
     
  3. jonr
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    jonr Senior Member

    Capacity needed is more related to surface area than volume. Also look at air infiltration since this controls the humidity (latent load). The estimation tools typically aren't very accurate. Consider an inverter driven compressor so that startup current is low and efficiency is high.
     
  4. daiquiri
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    daiquiri Engineering and Design

    Personally, I never use suppliers' a/c load estimates and sizing, because I know that it is in their interest to sell an oversize system. This is not to say they are dishonest, but simply that each one is trying to optimize his own cash flow (within limits of technical functionality, budget and decency) and also to safeguard themself from eventual complaints about insufficient a/c plant.

    So I do all the calculations by myself, and after many years of consultancy in the HVAC industry I am very confident that they are almost always spot-on. The number of unsatisfied clients in my 13 years of engineering consultancy business can be counted on the fingers of one hand, which I consider a good track record.

    The base principle is always the same:
    - have a briefing with a client and other designers involved in the project
    - write down a SOR in a collaborative manner with all the parties involved, taking into account the technical regulations
    - get or calculate the necessary thermal characteristics of all the surfaces involved in the heat exchange
    - statistically analyse the weather data in the area of operation
    - take into account eventual special requirements, either by the client, by regulations or from producers of any special equipment inside the air-conditioned ambients
    - determine the design heating and cooling load on the a/c plant and create a preliminary plant layout and functional diagram.
    - have at least another briefing (which usually become several briefings) with all parties involved in the project, during which eventual modifications to each part of the project are decided, in order to ensure that everything will be buildable, maintainable and fully functional.
    - create the final project (usually takes at least 3-4 iterations of the previous step)
    - estimate construction costs of the plant, taking into account all the necessary materials, procedures and difficulties due to particular work ambients.

    If the above procedure is done with dedication and attention, one will hardly miss the target. By comparing the size of same plants designed by myself and proposed by the suppliers, I have noticed that suppliers often tend to oversize by 40-50% (seen some cases of up to 100% oversized plants). And the building, running and maintenance cost (as well as weight) go upwards accordingly.

    Cheers
     
  5. daiquiri
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    daiquiri Engineering and Design

    That will depend on the tools, on the practical experience and on technical competence of the person who uses them. ;)
     
  6. jonr
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    jonr Senior Member

    I agree. One thing the entire hvac industry should do is more measurement and less estimation. For example, one can put in temporary heaters (or AC) and measure the energy needed to maintain the interior at a 10C delta from ambient.
     
  7. daiquiri
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    daiquiri Engineering and Design

    Pretty difficult to do in a reliable manner in practice, because of the thermal inertia (or thermal lag) of walls and various items in the ambient. It would take several hours (or even days) to stabilize the internal temperature to a final value. But in the meanwhile, the outside conditions will have changed completely (day/night cycle, temperature/humidity change, winds, clouds, shadows etc.), turning your best efforts into an endless lagging behind the outside weather... ;)
     
  8. Mark Cat
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    Mark Cat Senior Member

    In addition to what has already been said.

    Is this a retrofit or ground-up design. How large is the vessel?

    For new designs, HVAC is approached as a system.

    For smaller vessel under about 65 Feet the approach for HVAC may be done on a per cabin basis. It really depends on the builders approach and the function of the boat.

    The efficiency of any AC system is in keeping the coils (heat exchangers) clean. Provide an easy way to clean the evaporating coils or coil filters.

    For custom designs, especially for aluminum hulled yachts, the condenser may be hull mounted, meaning Refrigerant to sea water, which means no need to clean the condenser coils.

    Best Regards,

    Mark Cat
     
  9. jonr
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    jonr Senior Member

    It's best to do testing overnight and you can log outdoor temperature and generally account for it. And you still adjust for expected weather and sun. Never perfect, but then we know that other methods are wildly inaccurate (which everyone seems to agree with, except for when they do them :)).
     
  10. daiquiri
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    daiquiri Engineering and Design

    Unfortunately, it is really not so simple. :)
    Fact is - due to thermal inertia and humidity of the walls, the heating load (talking generally, not specifically related to boats) never accurately follows the outdoor temperature. There is always a several-hours lag between the outdoors and inside ambient conditions. It is well-explained and illustrated in this page: http://sustainabilityworkshop.autodesk.com/buildings/thermal-mass

    Even in apparently simple case of an overnight measurement, the outside temperature changes on hourly basis. See, for example, today's data from one meteo station here in Italy: http://www.sardegna-clima.it/collegamenti/nuovestazioni2/capoterra.php#
    As you can notice, the temperature curve never flattens sufficiently to assume it constant. Perhaps it happens to some extent in some particular world areas or for some particular types of constructions, but then you have a measurement method valid for that area only. That's why we need more generalized methods, which are usually of mixed theoretical/empirical nature. If used well, they give sufficiently accurate and reliable results, which are the basis of HVAC engineering.

    However, a low to moderately-isolated boat hull (which might be of interest here) does have a relatively low thermal mass (and hence thermal lag), and hence your proposed measurement method could be used in some cases.

    Cheers
     
  11. philSweet
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    philSweet Senior Member

    I started this post this morning and walked away from it. It goes back to the original question. My basic take is that for a decent sized ship, efficacy comes before efficiency. Global efficiency, while meaningful, is rather elusive. Efficiency is best left to component level selection and much more important is to get the business decisions correct and act on them. Daiquiri's emphasis on the decision process should be taken to heart. I think it often gets short shrift and is where most of the real world savings are to be found.

    Basically, you have to divide your questions into categories as far as who is supposed to contribute the info to make the decisions- the buyer or the designer. The buyer is the one who really needs to know how different onboard comfort levels will affect his income. The supplier needs to be able to relate those comfort levels to system costs and demands on the power distribution system. A consultant ought to have a feel for both and be able to guide the decision process by providing or supporting the other party's info when appropriate.

    The variation in the energy supplies available on boats make all such designs rather particular to that boat and its modes of operation. The buyer needs to be able to categorize these modes accurately if they want an efficient system. This is probably the most touchy issue from a suppliers point of view, since a supplier won't be in business very long if he goes running around saying the buyer gave me bad data on his boat.

    Perhaps Daiquiri can give us a list of the specific objectives that he would like to see met at that first meeting, and a list of who is going to bring what to the second meeting. With HVAC, I think that the desired comfort is the biggest cost determinant, then the GA of the space and it's modes (A/C duty cycle and availability of power sources).

    Another huge factor is who has control/access to the thermostat. Don't laugh, this is actually a rather fundamental question to be resolved in HVAC design. Do you want individual tstat room control by occupant as in a hotel, Air control of individual space as on an airplane, no control as in the cheap cabins on a cruise ship, Occupancy sensors that transfer this control? The larger the space under one control scheme, the easier it is to use energy saving methods such as load anticipation.
     
  12. jonr
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    jonr Senior Member

    > the temperature curve never flattens sufficiently to assume it constant.

    No question that thermal mass, outdoor temp changes and wind cause some inaccuracy. But let's look at numbers. In your data, temperature varies by only 2C over a 6 hour period. Say we didn't account for this in any way and didn't look at any trend curves (as the thermal mass reaches equilibrium). Heat the interior of a low mass structure to a steady 20C above ambient and look at the 7am BTU/hr readings. Now compare this < 10% error to the 40 to 100% error so often seen.

    Air infiltration - don't even try to estimate it.
     
  13. daiquiri
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    daiquiri Engineering and Design

    From my experience, in 90% of cases the biggest and most difficult goal to reach at the first meeting is to obtain necessary space for ductwork, pipework and a/c units. For some curious reason, all to often the space required by these installations is very under-estimated (when not completely neglected) by architects and designers. When that happens, the whole a/c design is compromised from the beginning, because one cannot use the best system but the system which fits the reduced space.

    At the second meeting I expect to see these issues corrected as by previous agreement, so that we can carry on the discussion about operational details of the a/c system.

    Cheers
     
  14. FAST FRED
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    FAST FRED Senior Member

    The most efficient air cond are house systems like the Mitsubishi Mini Split system.

    They air cond a bit better but are usually 400% to 500% better at producing heat , even at below zero temperatures.

    Air operated they only need a smooth surface where there almost foot deep structure will not be in the way.

    Sadly they are house systems so most require 240V and may have a hard time if douched with sea water.

    https://www.goductless.com/Mitsubishi-Air-Conditioners/cat7275.ac

    In time there will be marine versions , but so far , I cant find even one.
     

  15. jonr
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    jonr Senior Member

    > ... most difficult goal to reach at the first meeting is to obtain necessary space for ductwork, ...

    Some cases will require hydronic systems - moving hot/cold water around takes less duct space than moving air.
     
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