Water to drain

[macmet]

We have a heat exchanger that is water/glycol on one side and domestic water on the other.

The domestic water goes to drain after going through the heat exchanger. The temperature leaving will be 170F. Does anyone know if there are any restrictions on water temperature going to drain? If so, what are they?

I could increase the flow to lower the temperature but the customer is trying to waste as little water as possible. Personally, I don't like the idea of wasting any, but I've been overruled.

The job is in the midwest USA.

 

[PEDARRIN2]

In my neck of the woods (Ohio), where we use the International Plumbing Code, section 701.7 states that nothing above 140 F will be discharged to the sanitary sewer unless approved cooling methods are used.

When we have dishwasher discharges that are too high, we install a type of mixing valve that lets in some domestic cold water and cools the effluent below 140. Not sure if that would work in your case.

 

[ChrisConley]

I agree with Peddarrin, we're not allowed (in Canada) to discharge anything hotter than 140F to the sewer. Some good reasons for that are that PVC starts to soften at those temperatures, don't know what your drainage system materials are, but PVC is pretty common.

Depending on volumes/flows/durations you could also use a storage tank and allow the water to cool to 140 before discharging.

 

[JimFischbeck]

Our local law in NY limits discharge to 150F.
Seems like a huge waste of energy, too.

 

[IRstuff]

You could try running it through a radiator with a fan, which would at least not waste too much more water. You only need to drop it by 30º, so perhaps a mid-sized auto radiator would be adequate.

TTFN

FAQ731-376

 

[macmet]

The fan and radiator was something I suggested, but the cost was apparently too high. I'm not sure how much it's going to cost to essentially just waste all this water, but... well, dont' get me started. At this point there is no flexibility to change the design.

 

[berkshire]

If you have the area to do it, you could just spray the water in the air, then let it drain.
B.E.

 

[ChrisConley]

Back to the issue then, if you can't change the design, what are you going to do about the temperature? A mechanical mixing valve to blend the temperature to 140F would be the cheapest, but would also waste more water.

 

[macmet]

Well I could increase the flow. I have more available if required.

We're going to have a meeting to discuss this next week. Apparently the customer is also beginning to think draining all this water is unnecessary.

 

[macmet]

Just to clarify my first comment in my last post.

I could increase the flow as it's not limited (to a point). But, my goal is to minimize wasted water.

 

[KiwiMace]

You can buy special drain coolers to do this job too. Look at Armstrong Temp-R-Drain or similar. Integrated air break means you can just pipe up and go.

 

[RossABQ]

Is there another place you can send the water, so it is not wasted?
-- domestic hot water preheater?
-- cooling tower basin (to offset blowdown)
-- irrigation??

How much water are we talking about? If it's only a couple gpm, there is no payback to doing anything except tempering and going to drain.

 

[macmet]

I'm sorry, I thought I posted the water flow but I guess I didn't. The flow will be about 10usgpm.

No, they don't have any other significant use of water. Most of our systems do, but not this one.

 

[RossABQ]

Is the flow 24/7? If so, that's a huge amount of water to waste, and a lot of heat to throw away. If it is intermittent, it would be a simple matter to calaculate whether it's worth fretting about.

 

[IRstuff]

The other choice would be some sort of cooling tower deal, so that the water can be recirculated as a closed-loop cooling system, where the only additional water usage would be makeup for evaporation.

TTFN

FAQ731-376

 

[RossABQ]

I think as an engineer that would be ideal, but from a cost perspective it could be difficult to justify. You'd need pumps, chemical treatment, etc. Since no energy is being recovered, it just becomes a capital cost to offset what is probably pretty cheap water.

 

[cry22]

what is the water/glycol side used for? why are you using glycol at temperatures higher than 180F? since your waste is 170F.

Why use glycol and heat exchanger altogether? Where is your primary medium coming from? hot water boiler? could you elaborate on the application itself? what is the high temperature glycol used for?

You could use a steam boiler that would allow you re-use of the high grade heat in the form of condensate. With a shell and tube heat exchanger. Hardly any waste at all.

 

[macmet]

The water/glycol is used to cool part of an incinerator.

This particular job does not involve any boiler. On jobs where we use boilers we do often use the condensate.

 

[DrRTU]

An air to water/EG heat exchanger on your primary loop seems to be the correct application. You should have a quick payback if the process is up and running most of the time - less than 1 year. Based upon your basis of 170 deg. F. LWT @ 10 GPM at an estimated EWT of 65 deg. F. you have a process load of 505 MBH. The payback will be a function of water cost and process use. Can you advise hours per year and water cost? I have used Modine steam/water unit heaters as the heat of rejection.