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  1. #14
    Join Date
    Nov 2007
    Location
    Sherman, TX
    Posts
    9,441
    Quote Originally Posted by BigBacardi View Post
    Personal pet peeve of mine.........

    negative pressure??????

    I freakin hate that term......negative pressure does not exist...oxymoron...
    I've always wondered why those little round things with the hose to the inducer on a gas furnace were called "pressure switches"....
    Technical incompetence is NOT a sales tool....

  2. #15
    Join Date
    Sep 2004
    Location
    Kansas City, Kansas, United States
    Posts
    13,833
    negative pressure does exist!
    but not in an evaporator or a trap!

    Negative pressures

    While pressures are, in general, positive, there are several situations in which negative pressures may be encountered; When dealing in relative (gauge) pressures. For instance, an absolute pressure of 80 kPa may be described as a gauge pressure of −21 kPa (i.e., 21 kPa below an atmospheric pressure of 101 kPa). When attractive forces (e.g., van der Waals forces) between the particles of a fluid exceed repulsive forces. Such scenarios are generally unstable since the particles will move closer together until repulsive forces balance attractive forces. Negative pressure exists in the transpiration pull of plants, and is used to suction water even higher than the ten meters that it rises in a pure vacuum/ The Casimir effect can create a small attractive force due to interactions with vacuum energy; this force is sometimes termed "vacuum pressure" (not to be confused with the negative gauge pressure of a vacuum). Depending on how the orientation of a surface is chosen, the same distribution of forces may be described either as a positive pressure along one surface normal, or as a negative pressure acting along the opposite surface normal. In the cosmological constant.
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  3. #16
    Join Date
    Mar 2012
    Location
    Connecticut
    Posts
    43
    Isn't the heatstrips used to control the humidity in the ducts too?

  4. #17
    Join Date
    Mar 2007
    Location
    Myrtle Beach, SC
    Posts
    2,919
    Quote Originally Posted by Samshawn View Post
    Isn't the heatstrips used to control the humidity in the ducts too?
    No.

    The humidity coming off the cooling coil will be near 100%. Running the strips with the cooling will ruin the efficiency and capacity in cooling. Strips are used only in heat and defrost modes.
    Remember, Air Conditioning begins with AIR.

  5. #18
    Join Date
    Mar 2012
    Location
    Connecticut
    Posts
    43
    Thank you Kevin O'Neill for the information.

  6. #19
    Join Date
    May 2004
    Location
    Frederick, MD
    Posts
    993

    There are

    Quote Originally Posted by Samshawn View Post
    Isn't the heatstrips used to control the humidity in the ducts too?
    some applications where the customer runs the a/c to de-humidify a space and runs the electric heat strip(s) so the space does not get too cold. These are usually in labs, etc. where they are not concerned about energy consumption, only controlling the climate in the space. But,as was stated, the heating elelments themselves do not control humidity.
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  7. #20
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    Quote Originally Posted by gregp View Post
    some applications where the customer runs the a/c to de-humidify a space and runs the electric heat strip(s) so the space does not get too cold. These are usually in labs, etc. where they are not concerned about energy consumption, only controlling the climate in the space.
    I've set up a number of systems like that for applications where temperature and humidity control were the only concern, and energy efficiency was not a consideration.
    If more government is the answer, then it's a really stupid question.

  8. #21
    Join Date
    Feb 2004
    Location
    British Columbia, Canada
    Posts
    529

    Heat strips, traps, and neg pressure

    It is important to install condensate traps on both blow-through and draw-through systems for two main reasons. The first is that without those few inches (or more depending on system air pressure) of standing water in the trap, you may experience a situation where the airflow (exiting the system with a positive pressure or entering the system with a negative pressure) through the drain causes the condensate water to not flow through the drain opening which in turn will prevent proper drainage and overfilling of the evap condensate pan until the system cycles off. This will cause leakage into the ductwork below the evap pan and in some circumstances, icing up the coil.

    In certain areas where the humidity is not high enough you may not see this problem, giving you the false impression a trap is not needed.
    However, even if you do not have a drainage problem you will still lose efficiency on both draw through or blow through systems as there will be unconditioned air being introduced or conditioned air being forced out of the system as there is no standing water head to overcome those pressures.
    On a draw through negative pressure/drain side system the unconditioned air source will be most likely from the floor drain where the condensate drain line sits. This is a prime source of bacteria and odours in the supply air being distributed throughout the space. It may be necessary to fill the trap with water at times that as it may dry out. That's a whole new pile of info if you want it.

    The heat strips are effective as discharge air reheat on heat pump systems when dehumidifying as without them, overcooling of the space will occur until dehumidification is completed. On a/c systems they may be the only source of heat for the system, depending on how it is set up.

    As for the term "Negative Pressure" check with ASHRAE (American Society of Heating, Refrigeration, and Air-conditioning Engineers, as that is the term they use.

    WWW.ASHRAE.COM

    Happy trails all,




    Op

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