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  1. #14
    Quote Originally Posted by jpsmith1cm View Post

    Now, if I'm reading between the lines well enough, I'm envisioning a water heater type setup and, as was previously mentioned, this isn't uncommon.
    Yes it is a water heater type situation. I have the ability to swap whether it comes before or after the air cooled condenser, depending on the required heat. I'm basically trying to assess what effect all this will have on the performance of the vapour compression cycle.

    Thanks everyone for your replies.

  2. #15
    Join Date
    Dec 2011
    Location
    New Zealand
    Posts
    2,204
    The refrigeration is loop, so any change in the loop will effect the whole loop, any change a new equilibrium point is reached!
    So a very basic example.
    If you have a starting point, you change the refrigeration cond temp by 1C, you will get a change in power consumption of approx 3.5%, and change in refrigeration capacity "how much cooling"
    Again from this starting point, if you change the evap temp by 1C, you will get a change in cooling capacity of about 2.5%.
    But remember this is a loop, so any change with one of the above will still effect the other.
    And of course there are limits on an existing system
    Using the other fine explanations, you may get a feel, or not.

  3. #16
    Quote Originally Posted by barbar View Post
    Using the other fine explanations, you may get a feel, or not.
    Thank you, yes I think I'm getting there. I'm using CoolPack to give a (theoretical) feel for what should happen when the condensing temperature changes.

  4. #17
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,370
    Quote Originally Posted by gateeight View Post
    Yes it is a water heater type situation. I have the ability to swap whether it comes before or after the air cooled condenser, depending on the required heat. I'm basically trying to assess what effect all this will have on the performance of the vapour compression cycle.
    Quote Originally Posted by gateeight View Post
    Yes it is a water heater type situation. I have the ability to swap whether it comes before or after the air cooled condenser, depending on the required heat. I'm basically trying to assess what effect all this will have on the performance of the vapour compression cycle.
    So is this system solely for water heating...like a dedicated heat pump water heater...or are you simply capturing "free" heat from the cooling process? Since you have an air-cooled condenser, I would assume the latter.

    I have a water cooled "condenser" for domestic water heating in the discharge line of my home A/C (before the condenser) which I figure has saved me about $400/yr over the past 10 years or so. In my case, it's primarily a desuperheater, and can maintain 130F+ in the tank even though the condensing temp never get much above 110F.

  5. #18
    Join Date
    Jan 2013
    Posts
    20
    Sounds like the air cooled chillers with chiller barrels. Utilize water source loop then heat exchangers to transfer to the refrigerant side. But not both at once which would work but with todays controls it probably would make no sense. I have never worked on a water and refrigerant system. It sounds like it was before my time.

  6. #19
    Join Date
    Jan 2013
    Posts
    20
    Ok now I see what you are thinking. (Heat Recovery) there are many companies offering refrigeration energy retrofits utilizing the waste condensor heat in a heat recovery strategy. In essence utilizing a water based system to reclaim waste heat at the condensor and then transfering through a heat exchanger to a hydronic system or inline coils in duct to pre-condition supply air. These are mostly utilized in institutional settings with large walk-in in coolers or freezers during energy retrofit projects and typically this strategy offers exceptional payback.

  7. #20
    Join Date
    Apr 2012
    Posts
    15
    Remember head pressure equals condensing temperature. The compressor adds energy to the refrigerant vapor. The temperature will rise to the point where the gas will condense. That temperature is a factor of the ambient temperature if air cooled, and the size and efficiency of your heat exchanger. Heat exchange is a factor of surface area and temperature difference. Your condensing temperature needs to be higher than the medium you are transferring to, ambient temperature + temperature difference required for your heat exchanger.

    Load matters as well. A given heat exchanger will transfer X energy at Y TD. If X goes higher due to high load, Y goes up as well, and visa versa. A dirty condenser coil is less efficient a heat exchanger, so the head pressure rises to compensate, put another way, your TD of the heat exchanger rises.

    A tx valve doesn't control temperature, so let's back up a bit. You have an evaporator of some sort, lets say a coil with air flowing across it. The amount of air, say X lbs of air per minute, at Y temperature crosses the coil. The size of the coil, the design, fins, etc. will determine how much energy can be transferred to the air at Z TD. So you have refrigerant entering the coil after your metering device. It boils, absorbing energy at the rate your heat exchanger allows. The air is cooled. What the tx valve does is senses the temperature of the pipe leaving the evaporator. If the coil design and the air flow is engineered to give you 20 degrees F saturated suction temperature, the tx valve will meter refrigerant so that the leaving pipe is at 20 degrees saturated suction plus superheat, say 7 degrees. What that means is that your serpentine coil with liquid refrigerant entering on one end, flowing through the pipes, evaporating absorbing heat from the air. The liquid refrigerant will all be evaporated before it reaches the end of the pipe to leave the evaporator, long enough for the vapor to absorb another 7 degrees of temperature.

    The pumping capacity of the compressor determines how many pounds of refrigerant vapor are pumped. Pounds of refrigerant pumped is your capacity of the system. Assuming the capacity of the pump is constant at all pressure differentials (they are not, but leave that for now), the pounds of refrigerant evaporated determines the capacity of the evaporator. The condenser needs to reject that much energy plus the heat of compression from the compressor.

    For most systems, and there are exceptions, the suction pressure and the head pressure are determined by the heat exchangers and the medium to which they are transferring energy to or from. If your condenser is in a cool spot, the head pressure will be lower. If the evaporator is loaded, warm, the suction pressure will be higher.

    There are ways to actually control the pressures. To control head pressure you can shut the fan off, which changes the heat exchanger characteristics. Or you can install a device that backs up liquid refrigerant into the condenser coil, decreasing the size of available heat exchange surface area, forcing the temperature higher to exchange the same amount of energy.

    On the suction side it is a bit more complicated, there are valves that hold back the suction pressure, but typically you cycle the compressor to prevent the pressure from going too low.

    Most systems the evaporator, compressor capacity and condenser coil are matched so that the operating pressures are within the normal range for the application. A change in the sizes of any of these three will change the pressures and temperatures in all parts of the system.

    All the rest of the devices in a refrigeration system are about controlling liquid refrigerant, starting and stopping the process, moving the air around, and other bits and pieces to protect the main components from harm. It's all quite simple. The key to remember is that you are moving energy around, so the laws of thermodynamics apply. Surface area, temperature difference, amount of energy.

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