Snow Melt Control - 3-Way Mixing Valves?

slengy

I'm designing a commercial snowmelt system for a hospital. This consists of multiple zones which will be served by one heat exchanger (hot side 160/140F water, snowmelt side 135/105F glycol). Our zone outputs will be between 150-220btuh/ft depending on the ASHRAE snow accumulation class and location.

I'm wondering what the best option is for controlling multiple zones in a hospital.

Scenario 1: glycol system pump, 3way mixing valve and slab pump for each zone

• 3 way mixing valve and pumps can be placed in manifolds, increasing cabinet size. Worried about coordination in a hospital setting
• mixing valve and pump can be placed in ceiling, reducing serviceability. Worried about this in a hospital setting
• allows for temperature mixing, providing better temperature control and preventing system shock and better control

Scenario 2: glycol system pump, injection loop pump for each zone, circulator slab pumps for each zone

• can be placed in mechanical room. Will need more space to serve multiple zones
• allows for temperature mixing, providing better temperature control and preventing system shock and better control

Scenario 3: glycol system pump, zone valves to each snowmelt zone

• I have not used this design due to worry of slab thermal shock, heat exchanger thermal shock
• hard to control modulation

In the past, I have used Scenario 1. With the 3-way mixing valve and snowmelt pump in a mechanical room close to the heat exchanger. For this job, the snowmelt areas are far away from the heat exchanger.

• In healthcare projects what design have you seen?
• Would you recommend 3-way mixing valves?
• How far can a snowmelt controller monitor a control point? I've been told 150ft but have seen 400ft in literature.
• Would you recommend mixing capabilities?

hot_rod

What type of boiler? How far from the mechanical room to the manifolds?

I have used a mini tube injection on large 20,000 sq ft shops. I locate the manifold central to the shop. Run 2- small, 3/4" in some cases, S&R to the manifold. This supply may need to be 160F or hotter to get you the mixed down temperature. tekmar controls modulate that injection pump at the mechanical room. Basically injection mixing at the manifold location.

I think injection mixing is the simplest, no valves to modulate and worry about sticking. The pump speed just ramps up and down.

Google John Siegenthaler minitube mods for a 2002 article on design.

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slengy

@hot_rod This heat exchanger is being served from the high temperature heating (HTS) plant which provides 160F water to domestic hot water heat exchangers for preheating domestic hot water. We are hooking into this 160/140F loop to serve our snow melt system as well.

it is 4x10,000MBH condensing boilers. Total snowmelt load is 7,500MBH. We have multiple heat exchangers because snow melt areas are on many floors of the building. For ease I only mentioned one HX in my post.

manifolds could be 650ft from the glycol heat exchanger.

• injection mixing means bringing many pipes to and from the mechanical room where injection pumps are housed. For example, one heat exchanger is currently serving 8 zones so 16pipes would be leaving the mech room to go to manifolds. with mixing valves, I planned to have fewer pipes throughout the building, but I'll have same problem if I can't house valve & pump in the manifolds.

hot_rod

Really the selling feature of mini tube is leveraging a high delta, to greatly reduce piping costs, materials labor, etc. Possibly running 3/4 or 1" copper to a manifold instead of 2 or 3" It is great for traveling long distances. Pricing out 3 or 4" pipe and insulation, compared to 1" for example.

3" copper flowing 100 gpm at 20∆ get you 1 million btu/ hr

1-1/2" copper flowing 25 gpm at 80∆ gets you 1 million btu/hr

First you need to define the SIM design that every manifold will see. How many btu at each location, what design SWT temperature, and is 180 SWT an option. Which class SIM, what melt %, etc.

When you mention one HX is serving 8 zones, are those zones at the same manifold? Requiring the same temperature?

If it is 8 zones at 8 scattered manifold locations, the mini tube may not be your best option.

three or 4 way valves are an option, a 4 way gives you two mixing points if you need to watch return temperature for any reason?

Without a dimensioned plan, it's all a guess on my end.

Kaos

This is just mostly thinking out loud. I like the minitube idea and high delta T. Can you size your snow melt to work with 140F-150F glycol straight off the plate HX ?

My modification would be to have use a temperature controlled pump on the glycol loop sensing the outlet temp on boiler side of the plate HX. Essentially run the pump as chiller trying to cool down the building side hot water (to whatever delta you want) with the glycol. You can also do this with a mixing valve on the glycol with a remote sensing bulb if you don't want electronic controls. This keeps all the bits inside the building where it can be easily serviced.