cooling tower w/ water source heat pumps

[swoosh172]

I am working on a water source heat pump installation with a boiler and closed circuit tower. I want to isolate the tower from the building heat pump loop via a plate heat exchanger, so that glycol only has to be in the tower loop outside, and not the entire building loop. My question is if my building loop is variable flow rate, how will this affect the plate heat exchanger characteristics? Does my tower pump have to be variable speed also, so that the flow rate on both sides of the plate exchanger are equal. Or can the tower loop pump be constant speed and the building loop be variable speed?

 

[cdxx139]

I dont see an issue with having different types of flows on either side of the Plate and Frame Heat Exchanger.

What you should be concerned with is the variable flow side of the plate and frame. Having too low of a flow can allow the HX to clog up easier and faster than normal. Talk with the manufacturer.

An option is to have a jockey pump for the building side of the cooling tower, with a 3way valve to bleed the CT water with the building water. Its another piece of equipment but it keeps constant flow into the HX.

Also suggest specifying 20% extra plate capacity for future, and so the HX works when the owner doesnt clean them as often as they should.

Good luck.

knowledge is power

 

[MintJulep]

Why do you hate the building owner?

I've never seen one of these water loop heat pump systems that works well. Never.

The initial installed cost is high. The maintenance costs are high and maintenance is difficult and disruptive to building occupants because the units are scattered throughout the building - often above peoples' desks.

The chemistry in the loop is never maintained properly, so there are always corrosion and fouling problems with the heat exchangers.

But to answer the question, consider variable speed on both the pump and the fan of the cooling tower. Lay out the performance equations for the exchanger and the tower and use them in the controller to obtain the required performance at lowest operating cost.

 

[dpoppeli]

Consider shell & tube heat exchanger to reduce maintenance headaches from small passage plate&frame units. This will require additional space and have higher first cost but the building owner will thank you.

If your glycol/tower side is below freezing, how do you avoid freezing building side?

 

[willard3]

+2 on heat-pump loops.

I have been doing Engineering a long time now, and I have been involved in fixing these systems a lot more than I should and have represented owners in arbitration when they don't work.

Read ASHRAE on water-loop heat-pump systems. The use of the system doesn't meet the needs of most buildings with which I am familiar.

They are cheap, but so is a campfire for heating and a block of ice and fan for cooling

 

[swoosh172]

I am going the WSHP route with tower and boiler because we are renovating a historic building and there is not enough room for duct chases with central air handlers. The floor to floor height in the basement is only 9 feet and I can't fit main trucks thoughout like I planned with a central VAV air handler. The roof is slate and sloped, so no roof top units. There is also not enough space for a geothermal well field, so I am stuck with these WSHP with boiler/tower. Not to mention I have to meet LEED Silver energy savings of 30% or else I'd just do fan coils or something.

The building side will all be heated so no glycol is required. The heat exchanger will be inside the heated mechanical room and seperate the two loops. I guess I will go with a VFD pump on the cooling tower loop unless I hear otherwise.

 

[rmw]

I disagree with CDxx's advice to put 20% extra plates in the Hx for the very reason he/she states in his/her second paragraph

Having too low of a flow can allow the HX to clog up easier and faster than normal. Talk with the manufacturer.

Adding the extra plates will guaranteed lower your flow even before you start cranking down your variable flow rate further compounding the problem.

rmw

 

[cdxx139]

How are you covering ventilation air? DOAS? Natural? Ducting directly to the WSHP? If so, check the mixed air so you dont freeze the coils, especially with no glycol.

Before you "guess" on the VFD, think about your controls, that will tell you if you need to go VFD or not. (You guessed right, I would go VFD anyways, even if it is constant flow, it can be used to better balance the system, for minimal cost).

knowledge is power

 

[cdxx139]

rmv, the thinking behind the extra plates was...
as you lose heat transfer capacity due to a film buildup on the HX tubes (Im assuming the Plate & Frame type), you have the extra plates to offset the loss and still provide design capacity (up until the film buildup offsets the capacity of the extra plates).

I understand it is somewhat of a catch 22, by adding more plates you do lower the flow, but I also assume you would design to a adequate velocity WITH the extra 20% plates.

My next line in my original post, made a recommendation on how to keep a constant flow through the building side HX.

With the constant flow, and correct velocity, I stand by my recommendation for 20% extra plates; otherwise, as soon as you form the slightest film, you no longer will meet a Design Day load.

If thats incorrect, please let me know, I will stop designing in that manner. Thanks.

knowledge is power

 

[CTDon]

If the tower hasn't already been purchased, switch to an open tower. The two steps of heat transfer you propose (at the tower and at the HX)will be a real performance buster/energy waster.
If the tower is old, junk it and get a new open tower.
If its kinda old, consider scrapping the coil casing for a wet deck casing.
With either type tower you will still have to keep the sump from freezing, insulate and heat trace the make-up line and any water filled piping. Ideally, you would find a way for the tower to drain by gravity to an indoor sump eliminating the freeze potential altogether.
If you retain the closed circuit tower idea, remember there is a minimum flow that will be considerably higher than for the HX. If the flow goes laminar, capacity will be near zero. So, speed drive on the tower pump seems a waste. For more design considerations visit my web site

http://www.expertctr.com

 

[cry22]

Historic building and you aim at 30% over ASHRAE with WSHP? Highly questionable, as you stand, you will have a hard time meeting ASHRAE 2007, let alone beating it by 30%.
You did not say if this was residential or commercial, and you did not mention your total load - System choice in Appendix G depends a lot on application and load limits.

I suggest the following:

1. Forget about WSHP, when it gets cold, the electric heat kicks in and takes over your WSHP. You will have a high electric consumption, gooes against LEED.
2. For such an application, I highly recommend that you go Chilled water with MODULAR chillers (there is always space for modular chillers), step down chillers by turning chillers OFF. Nothing beats OFF, you should get around 0.5 KW/ton and a lower IPLV than centrifugal chillers, VFD on CHW, CV on CW to the tower.
3. Why glycol for the tower? just heat the basin.
4. You can even get air-cooled modular chillers in place of the cooling towers, and you will do better and cheaper. The IPLV on air-cooled modular chillers is comparable to centrifugal chillers efficiency.
5, I'd use condensing boilers along for a 4-pipe system.
6. I'd use a total enthalpy wheel for all OA
7, If you can replace the windows with Low-E glass and get some kind of Lutron type dimmers on lighting (total 0.6 W/SF), daylight harvesting, occupancy sensors, DCV, etc. so much the better.

Then you may be NEAR 30% over ASHRAE 2007.

 

[mdub782]

The building is existing 10,000 sf with brick face and block walls, no insulation at all and single pane windows. I get to model the proposed building vs the building as it stands now, except for the baseline building needs to be modeled with single zone ac/gas units. We are putting R19 on walls and R40 on roof with low e windows, so preliminary results are about a 30% savings over the building as it stands now. Going with condensing boilers, vfd loop pumps, and vfd tower. It'll be close.

 

[cry22]

No, it will not be close, it will be far being close, unless you're tweeking the numbers. You need to seriously check your preliminary results being near 30%.

R19 on walls, and R40 on roof do not give you much, because your R-values pre-requisite and fairly stringent (probably in the R13 for walls and R-24 for roofs since you're in the frost belt).
How come you're comparing the model to the existing? aren't you supposed to compared it to the min code required value (ASHRAE envelope)?

Savings from VFD's for such a small building are minimal (very small HP motors, what is it? 3 HP pump motors max?. So small, they don't even warrant VFD's - no payback).

Your Low-E glass will help (depending on extent of glass), but without enthalpy heat recovery and some serious eletrical trade cooperation (eletrical tends to always penalize the EA credit in LEED), you're not likely to get near your target. Check your electrical design compliance with ASHRAE 90.1 and then try to plug in their numbers into your LEED EA template, you'll see how much penalty you will be hit with.

Air side economizer (if it is not required in your baseline) will help if you implement it.

I would not promise the LEED leader more than the two pre-requisite LEED points if I were you.

 

[317069]

- How many floors in your building.
- What is you existing HVAC system?
- What is your load pattern, I mean in winter for example, are there zones needs cooling while other zones needs heating or vise vers in summer.
-How many WSHP are you thinking to use through your building?
-Where are you planing to locat these WSHP's.(noise is one of WSHP's disadvantages(but they still have many advantages)?
- you said that you don't have space in basment for central air handler. so I guess you have to use more than small air handler through your building(let say equal to the number of WSHP's)
- why don't use boilers in basment with those small air handlers for heating and DX units for cooling with VVT duct system instead of chilled water system, in this case you don't need a cooling tower and 4 pipe piping system and I think is less headache.
- you said you don't want to use fan coils or other due to LEED, who said that the fancoil dosen't meet LEED.?

 

[swoosh172]

I am modeling the baseline building as it stands now, with brick face and block wall. That is the way I read Ashrae 90.1 appendix G. Please correct me if I am wrong. The way appendix G reads under the envelope requirement is that the baseline building only needs to meet the minimum stated r values if the building is new, if it is existing then model it as it stands prior to the renovation.

The building is 3 stories with no hvac right now, I'm going the wshp route in lieu of fan coils due to no space or headroom in the basement and client does not want to create a dedicated mechanical room. The WSHP also have less piping, no pipe insulation, and COP in the 5-6 range with Carrier 2 stage HPs. There will be about 10 heat pumps total, each 1hp. Ventilation will be via dedicated unit with VFD and DCV and an energy recovery wheel. The project is in climate zone 6.

 

[cry22]

Sweoosh

Get your code analysis out. looks liek you don't have one.

When performing a major remodel, one has to bring the building up to code, meaning that your base case is ASHRAE 90.1, not the old single pane glass.

That is a question that needs to be run by your LEED leader and/or your code analyst.