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  1. #53
    Join Date
    Sep 2007
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    NH
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    Balance port valves are pretty much all we have installed for many many years now, and yes it's because they can be used with low liquid pressure. Thats the main reason to use a balance port for valve for the varying load (ie outdoor ambient). I see a lot of new condensing unit coming through with 150psi headmaster's now for 404a, r448a, and balanced port valves seem to do a great job with less than a 100psi diff across the txv.

  2. #54
    Join Date
    Aug 2002
    Location
    Southold, NY
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    22,266
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    Quote Originally Posted by Guyinfield View Post
    Trying to eliminate/mitigate the 10-20# swing in suction pressure after the fan condenser fan cycles off and my head pressure starts to build. I wouldn't imagine a system with a headmaster would necessarily need balanced port due to the steady head pressure.

    Sent from my SAMSUNG-SM-G891A using Tapatalk
    One problem you may have is the current Ambient temperatures and winds (Beyond Design Conditions).....the headmaster may just not be able to bypass enough refrigerant to maintain pressure and flood the condenser. I've found at or below Zero the headmaster has trouble maintaining especially if windy. Block off 1/2 of the condenser and see if things stabilize...........If not i use ICM325 instead of fan cycle. Less swinging.
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  3. #55
    Join Date
    Aug 2018
    Location
    Chicago Suburbs
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    93
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    Thread Starter
    Well... didn't intend my post to become a debate about BP valves. If this was an excersise in confidence I failed miserably.

    Just to rap it up. I did not add in my initial post that the system was short of gas. I did add refrigerant on the initial service call, but wasnt able to identify a leak. On return I found a leak (possibly several) in a rotting section of the evap. Also confirmed headmaster had failed, but was not stuck as I originally wrote up. Would in fact have had to be cut out of system before going forward. Tacking on the age of the equipment owner opted to replace system.

    Basically my concern with the TXV was me weighing my concious vs my company,s will. Not a great spot to be stuck in but only happens every once in a while. Thanks to all who threw in their thoughts and resources.

    Pecmsg, I will look into the icm 325. Have seen them on equipment but never stocked or installed.

    Sent from my SAMSUNG-SM-G891A using Tapatalk

  4. #56
    Join Date
    Oct 2011
    Location
    Chicagoland Area
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    7,546
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    Quote Originally Posted by VTP99 View Post
    Well now, on a small reach-in on the line, you could have a lot of door opening and closing.
    But... those are typically cap tubes....
    Reach in freezers more likely.
    Fractional systems like Federal open air cases all use balanced port because the door is open all the time
    Officially, Down for the count

    YOU HAVE TO GET OFF YOUR ASS TO GET ON YOUR FEET

    I know enough to know, I don't know enough
    Liberalism-Ideas so good they mandate them

  5. #57
    Join Date
    May 2003
    Location
    Utah
    Posts
    876
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    In looking at the common force balance in a TEV, there are typically three forces mentioned:

    (1) Bulb pressure, which will increase/decrease as the bulb temperature increases/decreases. This pressure acts on the top of the element diaphragm, and is the force that drives the valve open.

    (2) Evaporator Pressure, which acts on the bottom of the element diaphragm. This is a closing force.

    (3) Adjustment spring pressure, which also acts on the bottom of the diaphragm. This is also a closing force.

    The evaporator pressure and spring pressure together comprise the total closing force in the valve force balance.


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    The standard TEV has an additional opening force...the liquid pressure acting on the surface area of the TEV piston. the force is determined by the liquid pressure (lbs/sq in) multiplied by surface area of the piston (sq in), and acts in the opening direction. Where this opening force starts to become significant is when the TEV is required at low load conditions. As the load reduces, the valve throttles closed to maintain the superheat set-point. Yet, while it's attempting to maintain the superheat set-point with the valve throttling, there's this large force (liquid pressure) trying to open the valve. The result is that the standard TEV has a finite range that allows it to operate in reduced load conditions, and still maintain the superheat set-point. The standard valve can reasonably maintain its superheat set-point to load conditions that are approximately 50% of the TEV capacity. So, a standard TEV with a 1 ton capacity will maintain the superheat set-point to a point where the load is approximately 1/2 ton. If the load reduced below that, the valve will have difficulty maintaining SH.

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    The balanced ported valves were designed to eliminate the effect of liquid pressure as an opening force. Doing so allows the valve to operate at a lower percentage of its total capacity and still maintain the superheat set-point. This can be critical in preventing floodback, and the resulting compressor damage in VAV air conditioning systems, which might have wild load swings between the dead of summer, and the mild load demands in the spring and fall.

    The O valve utilizes a specially designed piston that (1) has a Teflon piston ring which isolates the bottom half of the piston from the top half of the piston, and (2) two passageways which allow the liquid inlet pressure to pressurize the bottom of the piston.

    There's no way to prevent the liquid pressure from acting on the surface area of the top of the piston. However; this design allows that same pressure to act on the bottom of the piston...thus eliminating the effect of liquid pressure as an opening force.

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    The smaller balanced ported valves for refrigeration achieve this "balancing", or eliminating the effect of liquid pressure as an opening force. They utilize a single pushrod, and the valve seat is integral to the pushrod. The area of the seat that is exposed to the liquid inlet pressure is where the "force" acts upon, and will contribute to the valve's opening force. The shoulder that is machined into the upper portion of the pushrod creates a surface area that allows the liquid pressure to act upon, but in an opposing direction, again eliminating the effect of liquid pressure as an opening force.

    Balanced ported TEVs can effectively operate down to approximately 25% of their rated capacity, and still maintain their superheat set-point.

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    These smaller balanced ported valves were created for a different purpose. As supermarkets systems started to be operated at lower condensing pressures in the cooler months to increase efficiency, it became very apparent that the TEV was the limiting factor in determining how low the condensing pressure could be reduced. The problem is that TEV capacity is in part based upon the pressure drop through the TEV port. As the condensing pressure is reduced, it also reduces the pressure drop through the TEV port. At some point, if the condensing pressure is continuing to be reduced, you'll reach a point where the TEV capacity becomes less than the evaporator load...meaning that the TEV then becomes too small.

    There were two solutions to this problem: (1) limit how low the condensing pressure could be reduced, or (2) develop a valve that could be oversized in the summer months when the condensing pressure is at its peak, yet still have sufficient capacity during the low condensing pressure operation in the colder ambient temperatures in the winter.

    The balanced ported valve achieves the desired result. It can effectively be oversized for operation in the summer months, as it has the ability to operate down to 25% of its rated capacity. And because it is oversized at the design condition, there is sufficient capacity in the lower condensing pressure periods in the winter time.


    One misnomer about the balanced ported valves is that they somehow "either largely reduce or eliminate the effect of pressure drop across the valve port".

    There are two ways in which pressure drop across the TEV port effect the valve: (1) the liquid pressure as an opening force, which is described above. Indeed, the balanced ported valves eliminate the effect of liquid pressure as an opening force. (2) the greater the pressure drop across the TEV port, the greater the capacity. This is true for a standard TEV and a balanced ported TEV. The fact that a valve is balanced ported does not in any way change the fact that increased or decreased pressure drop (which means increased or decreased mass flow) will result in increased or decreased capacity.

    The elimination of liquid pressure as an opening force will result in a more consistent superheat control. But the effect of fan cycling, which results in fluctuations in liquid pressure (meaning fluctuations in the amount of pressure drop across the TEV port), and the resulting fluctuations in TEV capacity, will cause balanced ported valves to be somewhat less consistent in their ability to maintain superheat.

    So, Balanced Ported Valves:

    They eliminate the liquid pressure as an opening force.

    This allows the valve to operate satisfactorily, maintaining its superheat set-point, down to a lower percentage of its rated capacity (approximately 25%).

    It also will result in more stable superheat control, since the liquid pressure is no longer able to influence the valve's opening.

  6. Likes icemeister, Guyinfield, trippintl0 liked this post
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